Illili i : HHI S ru l o .a M. B. L LIBRARY - WOODS HOLE, MASS. EVOLUTION AND ANIMAL LIFE 2 PLATE I. Four species of American orioles : 1, Nelson's oriole, Icterus nelsoni ; 2, orchard oriole, /. spurius; 3, Baltimore oriole, I. gal- bula ; 4, Bullock's oriole, /. bullocki. (From specimens ) EVOLUTION AID ANIMAL LIFE AN ELEMENTARY DISCUSSION OF FACTS, PROCESSES, LAWS AND THEORIES RELATING TO THE LIFE AND EVOLUTION OF ANIMALS BY DAVID STARR JORDAN PRESIDENT OF LELAND STANFOR^^fuNIOR UNIVERSITY AND VERNON LYMAN KELLOGG PROFESSOR OF ENTOMOLOGY, AND LECTURER IN BIONOMICS IN LELAND STANFORD JUNIOR UNIVERSITY Time, whose tooth gnaws away everything else, is powerless against Truth. HUXLEY. NEW YORK D. APPLETON AND COMPA 1907 t. B. L LIBRARY COPYRIGHT. 1907, BY D. APPLETON AND COMPANY Published September, 1907 PREFATORY NOTE IN the present volume the writers have tried to give a lucid elementary account, in limited space, of the processes of evo- lution as they are so far understood. We have turned to ani- mals for illustrative purposes, nearly to the exclusion of refer- ences to plants, simply because both authors are zoologists and have made use of the facts most familiar to them. The book is composed primarily of the substance of a univer- sity course of elementary lectures delivered jointly by the authors each year to students representing all lines of college work. This fact, and the desirable limiting of the book to a convenient size for the general reader and student, account for the extremely laconic treatment of various important moot points concerning the evolution mechanism, and for the omission of certain discussions which otherwise might well have been included. But on the whole the authors feel that the interested general reader will find this small volume a fairly comprehensive introduction to our present-day knowledge of the factors and phenomena of organic evolution. To the general reader we may perhaps with propriety ad- dress the following words, used to the students in the opening lecture of the course: We cannot talk long without saying something others do not believe. Others cannot talk long without saying something we do not believe. We wish you to accept no view of ours unless you reach it through your own investigation. What we hope for is to have you think of these things and find out for yourselves. D. S. J. V. L. K. LELAND STANFORD JUNIOR UNIVERSITY, March 30, 1907. i. TABLE OF CONTENTS CHAPTER I. EVOLUTION DEFINED. Organic evolution and bionomics, 1; Meaning of evolution, 2; Encasement theory, 2; Theory of epigenesis, 3; Evolution of the species or transmutation, 3; Cosmic evolution, 4; Spencer's form- ula of evolution, 5; Biologic evolution and cosmic evolution not the same, 6; Usefulness of the term bionomics, 7; The flux of na- ture, 7; Comprehensiveness of the science of organic evolution, 8; The immanence and permanence of law, 9; Evolution not neces- sarily progress, 10; Theory of descent, 10. CHAPTER II. VARIETY AND UNITY IN LIFE. Range of variety, 12; Meaning of species, 13; Number of species, 14; Extinct species, 16; Changes of species with time and place, 18; Variety in life a factor in the history of the globe, 21; Unity in life, 22. CHAPTER III. LIFE, ITS PHYSICAL BASIS AND SIMPLEST EXPRES- SION. Live things and lifeless things, 25; The basic distinction between life and non-life, 26; Protoplasm, 26; Chemical make-up of proto- plasm, 27; Physical make-up of protoplasm, 28; The cell, 30; The simplest animals, 32; Differentiation and animal types, 32; The genealogical tree, 36; Primary conditions of life, 38; Origin of life, 41: Spontaneous generation, 42; Where did life begin on the earth, 47. CHAPTER IV. FACTORS AND MECHANISM OF EVOLUTION. The fact of descent, 48; Darwinism not synonymous with de- scent, 49; Factors in descent, 49; Variation, 50; Selection, 51; Prodigality of production, 52; Heredity, 53; Isolation, 53; Muta- tion, 54; Orthogenesis, 55; Lamarckism and inheritance of ac- quired chaiacters, 55; Adaptation, 56. vii viii TABLE OF CONTENTS CHAPTER V. NATURAL SELECTION AND STRUGGLE FOR EXIST- ENCE: SEXUAL SELECTION. Natural selection the chief determining agent in adaptation, 57; Adaptation to conditions of life, 58: The crowd of animals, 59; Reproduction by multiplication, 59; Numbers of individuals al- most stationary, 60; Struggle for existence, 60; Discriminate death, 61; Natural selection, 62; Interdependence of species, 63; Animal and plant invasions, 64; Doctrine of Malthus, 67; Limits to the capacity of natural selection, 68; Survival of the existing, 69; Actual standing of Darwinism, 70; Secondary sexual dif- ferences, 71; Classification of secondary sexual characters, 72; Theory of sexual selection, 75; Criticisms of the theory, 77; The sexual selection theory largely discredited, 78. CHAPTER VI. ARTIFICIAL SELECTION. Natural selection and artificial selection, 80; Steps in the pro- duction of new races, 81; Selected traits quantitative, 81; Race traits qualitative, 84; Hybridization, 88; Plant amelioration, 90; Work of Luther Burbank, 90; Panmixia, or cessation of selection, 104; Reversal of selection, 104; Transmission and heredity, 105; Artificial selection and natural selection, analogous processes, 106; Race-forming by sports, 107. CHAPTER VII. VARIOUS THEORIES OF SPECIES-FORMING AND DESCENT CONTROL. Segregation of isolation, 108; Geographic and physiologic iso- lation, 109; Romanes's championship of physiologic isolation, 109; The Lamarckian theory of species-transformation, 111; Ortho- genetic evolution, 112; Species-forming by mutation, 114; The un- known factors of evolution, 115. CHAPTER VIII. GEOGRAPHIC ISOLATION AND SPECIES-FORMING. Migration and faunal distribution, 117; Closely related species not found in the same region, but in contiguous regions, 120; The American warblers, 120; Barriers, 122; The Hawaiian Drepanida?, 124; Adaptive and non-adaptive characters, 127; The American orioles, 128; Species traits not necessarily useful, 129; The persist- ence of the sufficiently fitted, 130. CHAPTER IX. VARIATION AND MUTATION. Actuality and extent of individual variation, 131 ; Darwin's laws of variation, 137; Quetelet's determination that fluctuating variation follows the law of probabilities. 140; Discontinuous varia- TABLE OF CONTENTS IX tion, 141; Discontinuous and continuous variation, 141; Congeni- tal and acquired variation, 142; Determinate variation, 150; The causes of variation, 154, 156; Variation as related to amphimixis and parthenogenesis as mutations of de Vries, 154. CHAPTER X. HEREDITY. Hereditary variancy defined, 163; Atavism or reversion, 166; Telogony, 166; Prenatal influences, 167; Not all transmission is heredity, 168; Determination of sex, 170; Homologies and analo- gies, 172; Vestigial organs, 174; Significance of vestigial organs, 181; Heredity and its "laws," 181; Galton's law of ancestral in- heritance, 184; Mendel's law of alternative inheritance, 187; Modification of Mendelism, 188. CHAPTER XL INHERITANCE OF ACQUIRED CHARACTERS. The Lamarckian principles of evolution, 196; Neo-Lamarckism and Neo-Darwinism, 197; Acquired characters, 198; Effects of use and disuse, 199; Environmental modifications not inherited, 200; Examples of non-inheritance of acquired characters, 201 ; Heredity unproved, 203; Convergence of characters and parallelism, 204; Actual effects of environment, 205; Ontogenetic species, 206. CHAPTER XII. GENERATION, SEX, AND ONTOGENY. Generation and ontogeny, 211 ; Spontaneous generation or abio- genesis, 212; Simplest modes of generation, 213; Parthenogenesis, 215; Differentiation of reproductive cells, 217; Simplest many- celled animals, 218; Effects of sex, 220; Sex dimorphism, 221; The life cycle, 223; The egg, 224; Numbers of young, 225; Em- bryonic and post-embryonic development, 227; Developmental stages, 229; Continuity of development, 231; Metamorphosis or apparent discontinuity, 234; Significance of facts of develop- ment, 234; Divergence of development, 234; The duration of life, 240; Death, 241. CHAPTER XIII. FACTORS IN ONTOGENY, AND EXPERIMENTAL DEVELOPMENT. Processes in ontogeny, 244 ; Extrinsic and intrinsic factors, 245 ; Mechanism versus vitalism, 246; Functions of protoplasm, 247; Ultimate structure of protoplasm, 248; Theories of organic units, 250; Cell division, 251 ; Mitosis, or karyokinesis, 252; Somatic and germ tissues, 257; Reproduction in protozoa, 260; Maturation, 264; Fertilization, 267: Cleavage, 269; Reduction of the chromo- somes, 269; Preformation versus epigenesis, 276; Examples giving X TABLE OF CONTENTS evidence for each, 278; Mechanism versus vitalism, 281 ; Artificial parthenogenesis, 283; Regeneration and regulation, 285. CHAPTER XIV. PALEONTOLOGY. Fossils and theL* significance, 289; Fossil-bearing rocks and their origin, 292; Geological epochs, 296; Conditions of extinct life, 297; Divergent types and synthetic types, 299; Parallelism between geologic and embryonic series, 300; Orthogenesis, 301; Significance in evolution of the facts of paleontology, 301 ; Dur- ation in time of species, 302; History of the vertebrates, 305; Man, 307. CHAPTER XV. GEOGRAPHICAL DISTRIBUTION. Zoogeography, 309; Relation of species to geography, 311; Laws of distribution, 314 ; Species debarred by barriers, 315 ; Species debarred by inability to maintain their ground, 315; Species altered by adaptation to new conditions, 315; Effects of barriers, 316; Faunas and faunal areas, 316; Remains of animal life, 322; Subordinate remains of provinces, 323; Faunal areas of sea, 323; Analogies between language and fauna, 325; Geographic distribution and the theory of descent, 326. CHAPTER XVI. ADAPTATIONS. The principle of fitness and general adaptations, 327; Origin of adaptations, 327; Types and classification of species adapta- tions, 328; Adaptations for food-securing, 329; Adaptations for self-defense, 330; Adaptations brought about by rivalry, 331; Adaptations for defense of young, 338; Special adjustments to surroundings, 343. CHAPTER XVII. PARASITISM AND DEGENERATION. Parasitism defined, 347; Kinds of parasitism, 348; Simple structure of parasites, 350; Gregarina, 351; Parasitic hemospor- idia: the cause of malarial fevers, 351 ; Tapeworm and other flat worms, 354; Trichina and other round worms, 355; Sacculina, 358; Parasitic insects, 359 ; Parasitic vertebrates, 361 : Parasitic plants, 362; Degeneration through quiescence, 363; Degeneration through other causes, 363; Immediate causes of degeneration, 366; Ad- vantages and disadvantages of parasitism and degeneration, 367. CHAPTER XVIII. MUTUAL AID AND COMMUNAL LIFE AMONG ANI- MALS. Man not the only special animal, 369; Animal societies, 369; Commensalism, 370; Symbiosis, 373; Symbiosis between animals TABLE OF CONTEXTS x\ and plants, 376; Social life, gregariousness, 380; Solitary and com- munal bees and wasps, 383; The honey-bee community, 387; Ants, 391; Termites, 394; Division of labor the basis of communal life, 395; Advantages of communal life, 397. CHAPTER XIX. COLOR AND PATTERN IN ANIMALS. Color among animals, 398; Protection by color, 400; Protection of color, 402; Significance of color and pattern, 404; Table of in- sect colors, 405; General protective resemblance, 406; Variable protective resemblance, 407; Special protective resemblance, 411; Warning colors, 416; Terrifying appearances, 418; Directive col- oration, 419; Recognition marks, 420; Mimicry, 421 ; Criticism and general considerations of the theory of protective and mimicking color pattern, 424. CHAPTER XX. REFLEXES, INSTINCT, AND REASON. Irritability, 426; Nerve cells or fibers, 427; Brain or sensorium, 427; Mechanical reflexes, 428; The tropism theory, 429; The theories of the method of trial and error, 429; Instincts, 430; In- stincts of feeding, 432; Instincts of self-defense, 433; Instinct of play, 435; Climatic instincts, 436; Environmental instincts, 438; Instincts of courtship, 438; Instincts of reproduction, 439; In- stincts concerned with the care of the young, 439; Variability of instinct, 442; Reason, 443; Mind, 448. CHAPTER XXL MAN'S PLACE IN NATURE. Post-Darwinian conception of humanity, 452; Man's place among the other animals, 453; Classification of the primates, 455; Evidences from comparative anatomy of man's relation to lower animals, 456: Special physiological evidence, 457; Evidence from embryology, 460; Evidence from paleontology, 461; Conclusions from ethnology, 462; The earliest man, 464' The genealogy of man, 466; Theology and Darwinism, 467. EVOLUTION AND ANIMAL LIFE CHAPTER I EVOLUTION DEFINED Grau, theurer Freund, 1st alle Theorie, Und grim des Lebens gold'ner Baum. GOETHE. Men of science repudiate the opinion that natural laws are rulers and governors of nature, looking with suspicion on all " necessary " and universal laws. BROOKS. THIS volume treats of the elements of the science of Organic Evolution. To this science belongs the consideration of the forces which govern the changes in organisms. It includes the influences which control development in the individual and in the species which is the succession of individuals, together with the laws or observed sequences of events which development exhibits. From another point of view, this is the science of life adaptation. The term Bionomics (/3i'6s, life, vo/xos, order or custom), first suggested by Prof. Patrick Geddes, is essen- tially equivalent to the older term Organic Evolution, the science of the facts, processes, and laws involved in the mutation of organisms. For many reasons, this new name, Bionomics, with its technically exact meaning, should be preferred to the phrase Organic Evolution, as, unlike the latter, it involves no philosophic assumptions. That organs and organisms do change from day to day, and place to place, and from generation to generation is an observed fact, which now admits of no doubt. The orderly arrangement of our knowledge of this process constitutes a branch of science. To use the word evolution in regard to this process is to use a philosophic term in connection with a group of scientific facts. For the word evolution means unrolling. It carries the thought 1 2 EVOLUTION AND ANIMAL LIFE that something which was previously hidden is now brought to light. This leads naturally to the philosophic suggestion that whatever is evolved must be previously involved. This may be true as a matter of words, but not necessarily so as a matter of fact, unless we reduce these words to the simple meaning that the actual now must have been the possible before; what- ever actually takes place was a possibility before it happened. The word evolution, then, belongs to philosophy rather than to science. In the philosophy of nature the idea that present conditions are brought about through unrolling or unveiling has had a long existence. The word evolution has been fre- quently applied to the process of growth and maturity of the individual animal or plant, and again to the process of deriva- tion of species from ancestral organisms, and again to the pro- gressive changes in the forms of inorganic bodies, as planets or mountains. Each one of these meanings is essentially dis- tinct from the others, and each is distinct from the theory of evolution which existed in the dawn of biological science. When men first began to notice the changes in the animal embryo, through which, from the formless egg, little by little, the individual was built up, becoming at each stage of the process larger, more specialized, and more like the parent from which it sprang, it was natural to regard this process as an unrolling. It was natural, too, to suppose that the egg was not really formless, but that the beginnings of each part of the final organism existed within it in fact, if we could see them. Hence evolution took the form of a theory of encasement. Men imagined that the egg of the chicken con- tained a minute chicken, and that within this chicken were the germs of the eggs the future hen would bear; and again, that encased within each of these eggs was an endless series of the eggs and chickens of all the future. In like fashion, men conceived that in the small human egg were the bodies and embryos of countless future generations. In some theories, this idea of encasement was applied not to the egg, but to the male germ, the homunculus or minute man in whom the gener- ations of the future were enfolded and from which they un- rolled. The perfection of the microscope as an instrument of pre- cision did not verify these theories of encasement. The egg still appeared essentially formless, a mass of undifferentiated EVOLUTION DEFINED 3 protoplasm, or at least without traceable lineaments of the future embryo. It was a single cell, apparently essentially like any other cell, a single one of the units of structure of which living organisms are made. Thence arose the theory of upbuilding or epigenesis (e-n-i, upon, yeVetm, birth) of organisms, by the addition of cell upon cell, to the original germ or egg. Each egg cell by segmentation divides into two daughter cells, and these, through the influence of heredity, naturally arrange themselves so that a new organ- ism is formed similar to the parent organism. It was recognized that the form was predetermined by the ancestry, but no longer that the embryo was literallv released from encasement O / */ within the structure of the egg. The evolution of the individual is thus conceived as the realization of an hereditary tendency. But "hereditary tendency' is again a metaphorical ex- pression. In biology, we know no "influence" or "tendency' which is not localized somewhere. Any act or modification of an act is a function of some particular organ. To account for the likeness involved in the facts of heredity, we must expect to find some form of organic mechanism. Such mechanism must exist within the germ cell itself, and its existence as the "physical basis of heredity' is now well established. In a later chapter we shall discuss the nature of this physical basis, the structures within the nucleus of the germ cell which control or preside over the development of the individual. From our knowledge of the operation of the cell in heredity we recognize the facts of epigenesis, and with these a theory of individual evolution, much more subtle than the / old theory of encasement. We may therefore still imagine the maturing of the individ- ual organ as a process of evolution, or unrolling, of the hereditary plan as hidden in the structure of its cells. We may also speak of the same process as a development. To envelop is to make snug. Development is its opposite. To develop is to make free or independent. From the evolution of the individual it is natural to extend the use of the word evolution or the word development to the changes which characterize the history of a species or other group of animals or plants, a process which has also been called transformism or transmutation. This word transmutation de- scribes the process more literally than either evolution or 4 EVOLUTION AND ANIMAL LIFE development. That species do change their structure with time or with space is a matter of common scientific observation. With the lapse of time, generation following generation, direct- ive influences combine to modify the line of descent. With the separation of individuals by barriers of land and water and varying climate, differing lines of descent are brought into existence. The fact of descent with modification large or small is a matter of common knowledge in the biology of to-day, veri- fied in the hundreds of thousands of species of organisms now known and classified. To call this transmutation of species is but to state the fact. To call it evolution is to suggest a theory that all these changes are but the unrolling of the plan a move- ment toward some predetermined end. That this is true we have no means of knowing, and the results as they appear to us seem to be determined by proximate causes alone. Among these proximate causes are differences in structure and in degrees of adaptability among individuals, the operation of the rule of the survival of the best adapted, the inheritance by individuals of the traits of the immediate ancestry, and the effects of cli- matic changes, and of migrations hampered and unhampered by the presence of physical barriers. The effects of influ- ences like these are considered by most writers as the es- sential elements in "organic evolution." But a few writers give external influences a secondary place, confining the term evolution solely to the results of causes resident within the individual. Speaking broadly we find as a fact that transmutation of species through the geologic ages has been accompanied by increasing divergence of type, by the increased specialization of certain forms, and by the closer and closer adaptation to conditions of life on the part of the forms most highly special- ized, the more perfect adaptation and the more elaborate specialization being associated with the greatest variety or variation in environment. Accepting for this process the name of organic evolution, Herbert Spencer has deduced from it the general law that as life endures generation after generation, its character, as shown in structure and function, undergoes con- stant differentiation and specialization. In this view, the transmutation of species is not merely an observed process, but a primitive necessity involved in the very organization of life itself. EVOLUTION DEFINED 5 A process of orderly mutation is observed not only in living things but in inanimate objects as well. The features of the surface of the earth pass through a slow process of unrolling- from primitive chaos to the diversified earth of to-day. Mani- festly we cannot imagine a homogeneous earth which could forever retain its homogeneous condition. At least our universe and our earth have not done so. A cooling earth must lose its perfect rotundity, its surface must become diversified, its relation to the sun must cause its equator to differ from its poles. A single homogeneous form of life on this earth could not remain uniform because it would be thrown under varying conditions. It could not be the same under the tropical sun as under the arctic cold, and the individuals adapted to either would tend to reproduce individuals likewise adapted. There must, then, exist in all things a " tendency " to become special- ized and differentiated. In accordance with this tendency, it is conceived that nebulous masses have been concentrated into planets and the generalized creatures of geologic time have been succeeded by variant and specialized forms, their lineal de- scendants. The universal formula of the process of evolution is com- pactly stated by Herbert Spencer in these famous words: "Evolution is a continuous change from indefinite incoherent homogeneity to a definite coherent heterogeneity of structure and function, through successive differentiations and integrations. In its physical aspect evolution is further an integration of matter with concomitant dissipation of motion." This formula applies more or less to all forms of orderly change, that is, change due to a persistent cause, a continuous force. Thus solar systems are conceivably formed from nebulae. Thus continents and mountain chains, islands and river basins are shaped. Thus organisms are derived from parent organisms. Thus all the variant chemical elements may have been (hypo- thetically) derived through influences as yet not even imagined, from the unknown and probably unknowable primitive element, protyl. The general movement is from the simple to the complex, from the homogeneous to the heterogeneous, from the inexperienced to the experienced, from the undivided to the divided, from the inchoate to the integrated. Whatever 6 EVOLUTION AND ANIMAL LIFE happens in time or is encountered in space promotes evolution. But the kind of evolution thus produced is very different in different kinds of objects. Biological evolution and cosmic evolution are not the same. From the biological side a certain objection must be made to this philosophical theory of universal or cosmic evolution. In organic and inorganic evolution there is much in common so far as conditions and results are concerned; but these likenesses belong to the realm of analogy, not of homology. They are not true identities because not arising from like causes. The evo- lution of the face of the earth forces parallel changes in organic life, but the causes of change in the two cases are in no respect the same. The forces or processes by which mountains are built or continents established have no homology with the forces or processes which transformed the progeny of reptiles into mammals or birds. Tendencies in organic development are not mystic purposes, but actual functions of actual organs. Tendencies in inorganic nature are due to the interrelations of mass and force, whatever may be the final meanings attached to these terms or to the terms matter and energy. It is not clear that science has been really advanced through the conception of the essential unity of organic evolution and cosmic evolution. The relatively little the two groups of processes have in common has been overemphasized as compared with their fundamental differences. The laws which govern living matter are in a large extent peculiar to the process of living. Processes which are functions of organs cannot exist where there are no organs. The traits of protoplasm are shown only in the presence of protoplasm. For this reason we may well separate the eA^olu- tion of astronomy, the evolution of dynamic geology and of physical geography, as well as the purely hypothetical evolu- tion of chemistry, from the observed phenomena of the evolution of life. To regard cosmic evolution and organic evolution as identical or as phases of one process is to obscure facts by verbiage. There are essential elements in each not shared by the other or which are at least not identical when measured in terms of human experience. It is not clear that any force whatever or any sequence of events in the evolution of life is homologous with any force or sequence in the evolution of stars and planets. The unity of forces may be a philosophical necessity. A philosophical necessity or corner in logic is un- EVOLUTION DEFINED 7 known to science. We can recognize no logical necessity until we are in possession of all the facts. No ultimate fact is yet known to science. For reasons indicated above the term evolution is not wholly acceptable as the name of a branch of science. The term bionomics is a better designation of the changing of organisms influenced through unchanging laws. It is a name broader and more definite than the term organic evolution, it is more euphonious than any phrase meaning life adaptation, it involves and suggests no theory as to the origin of the phenom- ena it describes. It is a matter of common observation that organisms change from day to day, and that day by day some alteration in their environment is produced. It is a conclusion from scientific investigation that these changes are greater than they appear. Not only do they affect the individual animal or plant, but they affect all groups of living things, classes or races or species. No character is permanent, no trait of life without change; and as the living organism and groups of organisms are un- dergoing alteration, so does change take place in the objects of the physical world about them. "Nothing endures/' says Huxley, "save the flow of energy and the rational order that pervades it." The structures and objects change their forms and relations, and to forms and relations once abandoned they never return; but the methods of change are, so far as we can see, immutable. The laws of life, the laws of death, and the laws of matter never change. If the invisible forces which rule all visible things are themselves subject to modification and evolution we have not detected it. If these vary, their aberrations are so fine as to defy human observation and com- putation. In the control of the universe we find no trace of "variableness nor shadow of turning." But the objects we know do not endure. Only the shortness of human life allows us to speak of species or even of individuals as permanent entities. The mountain chain is no more nearly eternal than the drift of sand. It endures beyond the period of human observation; it antedates and outlasts human history. So may the species of animal or plant outlast and antedate the lifetime of one man. Its changes are slight even in the lifetime of the race. Thus the species, through the persistence of its type among its changing individuals, has come to be regarded 8 EVOLUTION AND ANIMAL LIFE as something which is beyond modification, unchanging so long as it exists. "I believe," said the rose to the lily in the parable, "that our gardener is immortal. I have watched him from day to day since I bloomed, and I see no change in him. The tulip who died yesterday told me the same thing." As a flash of lightning in the duration of the night, so is the life of man in the duration of nature. When one looks out on a storm at night he sees for an instant the landscape illumined by the lightning flash. All seems at rest. The branches in the wind, the flying clouds, the falling rain, are all motionless in this instantaneous view. The record on the retina takes no account of change, and to the eye the change does not exist. Brief as the lightning flash in the storm is the life of man com- pared with the great time record of life upon earth. To the untrained man who has not learned to read these records, species and types in life are enduring. From this illusion arose the theory of special creation and permanence of type, a theory which could not persist when the facts of change and the forces causing it came to be studied in detail. But when men came to investigate the facts of individual variation and to think of their significance, the current of life no longer seemed at rest. Like the flow of a mighty river, ever sweeping steadily on, never returning, is the movement of all life. The changes in human history are only typical of the changes that take place in all living creatures. In fact, human history is only a part of one great life current, the movement of which is everywhere governed by the same laws, depends on the same forces, and brings about like results. Organic evolution/ _or bionomics, is one of the most com- prehensive of all the- sciences, including in its subject matter not only all natural history, not only processes like cell division and nutrition, not only the laws of heredity, variation, segre- gation, natural selection, and mutual help, but all matters of human history, and the most complicated relations of civics, economics, and ethics. In this enormous science no fact can be without a meaning, and no fact or its underlying forces can be separated from the great forces whose interaction from moment to moment writes the great story of life. And as the basis to the science of bionomics, as to all other science, must be taken the conception that nothing is due to EVOLUTION DEFINED 9 chance or whim. Whatever occurs comes as the resultant of moving forces. Could we know and estimate these forces, we should have, so far as our estimate is accurate and our logic perfect, the gift of prophecy. Knowing the law, and knowing the facts, we should foretell the results. To be able in some degree to do this is the art of life. It is the ultimate end of science, which finds its final purpose in human conduct. "A law," according to Darwin, "is the ascertained sequence of events." The actual sequence of events it is, in fact, but man knows nothing of what is necessary, only of what has been ascertained to occur. Because human observation and logic can be only partial no law of life can be fully stated. Because the processes of human mind are human, with organic limita- tions, the study of the mind itself becomes a part of the science of bionomics. For it is itself an instrument or a combination of instruments by which we acquire such knowledge of the world outside of ourselves as may be needed in the art of living, in the degree in which we are able to practice that art. The necessary sequence of events exists, whether we are able to comprehend it or not. The fall of a leaf follows fixed laws as surely as the motion of a planet. It falls by chance because its short movement gives us no time for observation and calcu- lation. It falls by chance because, its results being unim- portant to us, we give no heed to the details of its motion. But as the hairs of our head are all numbered, so are numbered all the gyrations and undulations of every chance autumn leaf. All processes in the universe are alike natural. The creation of man or the growth of a state is as natural as the formation of an apple or the growth of a snowbank. All are alike super- natural, for they all rest on the huge unseen solidity of the universe, the imperishability of matter, the conservation of energy, and the immanence of law. We sometimes classify sciences as exact and inexact, in / accordance with our ability exactly to weigh forces and results. The exact sciences deal with simple data accessible and capable of measurement. The results of their interactions can be reduced to mathematics. Because of their essential simplicity, the mathematical sciences have been carried to great com- parative perfection. It is easier to weigh an invisible planet than to measure the force of heredity in a grain of corn. The sciences of life are inexact because the human mind can never 10 EVOLUTION AND ANIMAL LIFE grasp all their data. The combined effort of all men, the flower of the altruism of the ages, which we call science, has made only a beginning in such study. But, however incomplete our realization of the laws of life, we may be sure that they are never broken. Each law is the expression of the best possible way in which causes and results can be linked. It is the necessary sequence of events, therefore the best sequence, if we may imagine for a moment that the human words "good' and "bad' are applicable to world processes. The laws of nature are not executors of human justice. Each has its own operation and no other. Each represents its own tendency toward cosmic order. A law in this sense cannot be "broken." "If God should wink at a single act of injustice," says the Arab proverb, "the whole universe would shrivel up like a cast-off snake skin." The laws of nature have in themselves no necessary principle of progress. Their functions, each and all, may be defined as cosmic order. The law of gravitation brings order in rest or motion. The laws of chemical affinity bring about molecular stability. Heredity repeats strength or weakness, good or ill, with like indifference. The past will not let go of us ; we cannot let go of the past. The law of mutual help brings the perpetua- tion of weakness as well as the strength of cooperation. Even the law of pity is pitiless, and the law of mercy merciless. The nerves carry sensations of pleasure or pain, themselves as indif- ferent as the telegraph wire, which is man's invention to serve similar purposes. Some men who call themselves pessimists because they cannot read good into the operations of nature forget that they cannot read evil. In morals the law of compe- tition no more justifies personal, official, or national selfishness or brutality than the law of gravitation justifies the shooting of a bird. The science of bionomics centers about the theory of descent, the belief that organs and species as we know them are derived from other and often simpler forms by processes of divergence and adaptation. According to this theory all forms of life now existing, or that have existed on the earth, have risen from other forms of life w r hich have previously lived in turn. All characters and attributes of species and groups have developed with changing conditions of life. The homologies among animals are the results of common descent. The differences EVOLUTION DEFINED 11 are due to various influences, one of the leading forces among these being competition in the struggle for existence between individuals and between species, whereby those best adapted to their surroundings live and reproduce their kind. This theory is now the central axis of all biological investi- gation in all its branches, from ethics to histology, from anthro- pology to bacteriology. In the light of this theory every peculiarity of structure, every character or quality of individual or species, has a meaning and a cause. It is the work of the investigator to find this meaning as well as to record the fact. "One of the noblest lessons left to the world by Darwin/' says Frank Cramer, "is this, which to him amounted to a profound, almost religious conviction, that every fact in nature, no matter how insignificant, every stripe of color, every tint of flowers, the length of an orchid's nectary, unusual height in a plant, all the infinite variety of apparently insignificant things, is full of significance. For him it was an historical record, the revelation of a cause, the lurking place of a principle." It is therefore a fundamental principle of the science of bionomics that every structure and every function of to-day finds its meaning in some condition or in some event of the past. CHAPTER II VARIETY AND UNITY IN LIFE "L'espece, c'est im etre qui dans ses generations successives presente toujours les memes caracteres d'organisation ; il faut ajouter dans les memes localites, et les memes circonstances exterieures." RAMBUR, 1842. " THAT mystery of mysteries as it has been called by one of our greatest philosophers " this is Darwin's phrase regarding the problem before us, the origin of species the origin or cause of variety in the life of the globe. That variety exists, that there are many kinds and types, grades and grada tions in animal and vege- table life is evident to all. Birds and trees, beetles and butterflies, fishes and flowers, ferns and blades of grass, all these are objects of constant recognition. The green cloak which covers the brown earth is the shield under which myriads of organisms, brown and green, carry on their life work, and still farther below the level of our ordinary notice exists a range of life scarcely less 1 Th\s figure and the others in this chapter are introduced simply to illustrate graphically the variety of animal form. 12 FIG. 1. Long-horned boring beetle from Central America (one-half natural size). 1 VARIETY AND UNITY IN LIFE 13 varied. Pasteur has defined fermentation as "life without air." A host of chemical changes in organic matter, fermentation, putrefaction, infection of disease all these are the work of minute organisms none the less real because invisible and as varied in form and structure as in the differing effects their presence may produce. Each kind of animal or plant, that is, each set of forms which in the changes of the ages has diverged tangibly from its neighbors, is called a species. There is no absolute definition for the word species. The word kind represents it exactly in common language, and is just as susceptible to exact definition. Fi<;. 2. Kangaroo rat from the Calif ornia-Mojave desert (one-half natural size). The scientific idea of species does not differ materially from the popular notion. A kind of tree or bird or squirrel is a species. Those individuals which agree very closely in structure and function belong to the same species. There is no absolute test, other than the common judgment of men competent to decide. Naturalists recognize certain formal rules as assisting in such a decision. A series of fully intergrading- forms, however varied at the extremes, is usually regarded as forming a single species. There are certain recognized effects of climate, of climatic iso- lation, and of the isolation of domestication. These do not usually make it necessary to regard as distinct species the extreme forms of a series concerned. In the words of the entomologist Rambur, " A species is a group of beings which in successive generations show the same characters of organization, unchanged so long as the locality and external conditions remain unchanged." 14 EVOLUTION AND ANIMAL LIFE The number of species actually existing is far beyond ordi- nary conception. The earliest serious attempt to catalogue the species of animals and plants was made by Linnaeus. In the tenth edition of his " Systema Nature " in 1758, in the 823 pages f L_ FIG. 3. Brittle or serpent stars species undetermined. (Natural size.) devoted to animals, he describes and names some four thousand different kinds. Great as this number seemed, Linnaeus ven- tured to suggest that probably his pages did not include half of those kinds of animals actually existing. To-day our records contain descriptions of more than one hundred and fifty times as many kinds of animals as were known to Linnaeus and all his predecessors and all his associates of a century and a half ago. Each year, since 1864, there has been published in London a volume called the "Zoological Record/' Each of the volumes larger than the whole " Systema Naturae" -contains the names of the animals new to science which have been added to the system in the year of which it treats. In the VARIETY AND UNITY IX LIFE 15 record of each year we find about twelve thousand species, about three times as many animals as in the whole " Systema Naturae." Yet the field shows no signs of exhaustion. As the volumes of the " Zoological Record " stand on the shelves, it is easy to see that the later volumes are the thickest; and those of the new century, with a general revival of interest in systematic zoology U: FIG. 4. California quail, Lophortyx californicus. (Two-thirds natural size.) and the study of geographical distribution, are the thickest of all. The depths of the sea, the jungles of the tropics, the crev- ices of the coral reefs, the tundras of the north, the limbs of 16 EVOLUTION AND ANIMAL LIFE trees, the hair of mammals, the feathers of birds, the body tissues of mosquitoes, all places where animal life is found, are being examined with an eagerness not less than that of the early explorers, while the investigators of to-day are armed with every appliance that science can devise. Yet now, as in Lin- nseus's time, it is certain that not half of the number of species of animal organisms is yet known. The 600,000, more or less, -SSS FIG. 5. Diamond rattlesnake, Croialus adamanteus. (Photograph by W. K. Fisher.) on our registers to-day are certainly far less than half of the > */ millions which actually exist. In botany we find the same conditions. There are fewer known species of plants than animals by half, and they are more easily preserved and handled, while the work of collection and investigation proceeds on a scale even more extensive, yet it would be a bold statement to say that we know to-day half the species of plants that exist. All this refers to the forms now living, without reference to the host which composes their long ancestry, extending back- ward toward the dawn of creation. The species have come down through the geological ages, changing in form and func- tion to meet the varying needs of changing environment. This VARIETY AND UNITY IN LIFE 17 enumeration takes no account of the still vaster myriads of forms almost endlessly varied which have perished utterly in the pressure of environment, leaving no trace in the line of descent. tf o. rt t-i 5 o -*j o s o 00 rt Of these extinct forms of animals and plants we know a few, one here and another there: here a bone, there a tooth, here a 18 EVOLUTION AND ANIMAL LIFE mass of shells, there a piece of petrified wood, an insect in the marl bed or a leaf preserved flat in the shale. Each of these fossils is a record of past life, true beyond impeachment, but the fragments are so few, so scattered, so broken, as to give only hints of the history they represent. Moreover, as we extend our studies of species we find that they change with space as w r ell as with time. These changes FIG. 7. Common lizard or swift, Sceloporus undulatus. (Photograph by W. K. Fisher.) are in large degree a response to external conditions. As conditions change, so do forms change to fit their surroundings. A movement over the surface of the earth, any movement in space, brings organisms in contact with barriers. A barrier means a change in conditions of life. As distance in space brings barriers, so does the passage of time bring events which are barriers also. Time brings new events; events mean changes in conditions, and change brings about divergence. Neither time nor space flows evenly. Variations in turn become greater with lapse of time and VARIETY AND UNITY IN LIFE 19 space, for these again bring other events and disclose other barriers. A closer observation will show us that the range of variety is far greater than is indicated by the number of species. There is not one blade of grass in the meadow exactly like any other blade. There is not a squirrel in the forest like any other squirrel, not a duck on the pond like any other duck in every detail of its structure. If we compare two rose leaves we shall find differences in size, in serration of the margin, in the length of the stalk, in the hairs on the surface, in the intensity of the green, in the number of breath- ing pores on the lower side. In every structure and function where difference is possible variation will appear. The squirrels or the ducks will differ in shade of color, in dis- tinctness of marking, in length of limb, in breadth of organ, in every way in which there is play for in- dividualism. Nor are these differences limited to matters of color or form. There are like variations in function, in tendency, in disposition, in endur- ance. No two men ever bore the same features, no two ever held the same character, no two ever lived the same life. The traits of the in- dividual, however small, appear on every hand. It is by little traits of emphasis that we recognize our friends. The same individu- alism is possessed by the lower animals and by plants, though the differences in stress and emphasis in color and figure are most marked in creatures of the most highly specialized organi- zation. In all animals and all plants like differences obtain. No two individuals of any species are ever quite the same. No two germ cells of the same parent are ever quite alike. No two cells in the body are ever exactly identical. Among plants of the same kind in the field, some are cut down by frost while others persist; some are destroyed by drought while others en- dure; some are immune to attacks of rust \vhile others are ex- Fio. 8. Sea encumber, Cucu- maria, sp. (Natural size.) 20 EVOLUTION AND ANIMAL LIFE FIG. 9. Blunt-nosed salamander, A mblystoma opacum. (Photograph by W. K. Fisher.) FIG. 10. White pelicans, Pelecanus erythrorhynchus, and whooping-crane, Grus ameri- cana. (Photograph by W. K. Fisher.) VARIETY AND UNITY IN LIFE 21 terminated by such parasites. Fill a bottle with flies. All in time will die of suffocation, but a certain few will outlast the many. Bring in a number of wolf cubs. Some will become relatively tame some will remain wolves, and between the most fierce and the most docile we shall find all ranges of variation. "What is one man's food is another man's poison." This proverb is a recognition of the principle of individuality which accompanies everywhere the formation of species, and being everywhere present, it must be an integral part of the FIG. 11. Silver fox, Vulpes pennsylranicus argentatus. (Photograph by W. K. Fisher.) process. Such differences are not matters of structure alone. Psychological differences, differences in instinct, in adapta- bility, in rate of nerve processes are just as marked as differ- ences in anatomy. They may separate one species from an- other. They may be just as decided within the limits of the species itself. Moreover, the beginning of variation is not with the individual organisms. No two cells are absolutely alike, and in the variance of the germ cells, from which individuals spring, all the elements of their future variation are involved. Without further discussion, it is evident that variety in life is a factor in the history of our globe, that it may be expressed in terms of number of species, but that the actual range of varia- tion is far greater than the number of species, and that if causes are to be judged by range of effects, we must find in the origin of 3 22 EVOLUTION AND ANIMAL LIFE species the operation of world-wide forces, the cooperation of great influences, far-reaching in time and space, as broad as the surface of the globe and as enduring as its life. To consider these causes so far as known is the purpose of this work. Our problem is no longer the "mystery of mysteries/' for in a large way by the work of Darwin and his successors the influences promoting variety in life are already known. We know many of the different factors which produce divergence in form and adaptation to conditions. But the relative value of these factors is less certain, and from time to time other and more FIG. 12. Lizard walking. (After Marey.) subtle factors are brought to light, or the great forces them- selves are analyzed into finer component elements. But with all that we may say of the universality of variation and the prevalence of individualism, we are equally impressed with the underlying unity. There are only a few types of structure among animals, and in these few the beginnings in development are the same. The plants show similarly a few modes of development, and all the range of families and forms is based on the modification of a few simple types. Moreover all living forms, plants and animals alike, agree in the funda- mental elements. All are made of a framework of cells, each cell a source of energy, containing in all cases a semifluid net- work of protoplasm, which is found wherever the phenomena of life appear. In all the cells is the mysterious nuclear sub- stance which seems to direct the operations of heredity. The same laws or methods of heredity, variability, and 'response to. VARIETY AND UNITY IN LIFE 23 outside stimulus hold in all the organic world. We call animals and plants "organic' because they are made up of organs, cells, and tissues so grouped that like structures perform like functions. We could not use a generic term like organic, were it not for the structural resemblances existing in each individual in great groups of organisms. All organisms have the impulse to repro- duction. All are forced to make concession after con- cession to their surroundings and in such concessions prog- ress in life consists. At last each organism or each alliance of organisms is dis- solved by the process of death. The unity in life is then not less a fact than the diversity. However great the emphasis we lay on in- dividuality or diversity, the essential unity of life must not be forgotten. Whatever solution we may find to the problem of the origin of species, must also explain why species and individuals may be so much alike in all large details of structure. To know the origin of species we must also know why species admit of natural classification. Why is variety in life based on essential unity? From the fundamental unity of the species of to-day, we may infer the similar unity of species in past time. From the knowledge of variety in unity comes the likening of species of FIG. 13. Starfish walking. Marey.) (After 24 EVOLUTION AND ANIMAL LIFE animals or plants to the separated twigs of a tree, of which the trunk is more or less concealed. " We can only predicate and define species at all," says Dr. Elliott Cones, "from the mere circumstance of missing links. Our species are twigs of a tree separated from the parent stem. We name and arrange them arbitrarily, in default of a means of reconstructing the whole tree, in accordance with nature's ramifications." To continue FIG. 14. Heron flying. (After Marey.) the figure, in our studies of the origin of the twigs of the tree, the existence of the trunk must not be forgotten. In the life of the earth variety in unity, unity in variety are nowhere separated. Another equally striking simile is this: A species is an island, a genus, an archipelago, in a sea of death. The species is clearly definable only as its ancestors and cousins have dis- appeared, only in the degree that the stages in its development are unrepresented in our records. The genus is a group of species, an archipelago of islands, and there may be every conceivable degree of width or breadth of channel which seems to separate one island or group of islands from another. CHAPTER III LIFE, ITS PHYSICAL BASIS AND SIMPLEST EXPRESSION There can be little doubt that the further science advances the more extensively and consistently will the phenomena of nature be represented by mathematical formulae and symbols. But the man of science who, forgetting the limits of philosophical inquiry, slides from these formulae and symbols into what is commonly understood by materialism, seems to me to place himself on a level with the mathe- matician who should mistake the x's and y's with which he works his problems for real entities, and with this further disadvantage as compared with the mathematician, that the blunders of the latter are of no practical consequence, while the errors of systematic materi- alism may paralyze the energies and destroy the beauty of a life. HUXLEY. IN practice the distinction between a live thing and a lifeless one is usually of the simplest, but to define this distinction in terms so precise that the definition may be used as an invariable criterion is a problem of considerable difficulty. The sheep grazing in the field and the soil under its feet; the grass and flowers on the one hand, and the stones on the other hand, in the same pasture; there are no difficulties in the distinction here. Nor, indeed, even when we come to consider the simplest kinds of organisms, the tiny one-celled plants and animals that teem in stagnant waters of the wayside puddle. As we examine a drop of this water under the microscope we know without question what in it is alive and what in it is dead. But let us attempt to put into words, into definite declaratory phrases, the characteristics of organisms and we find ourselves curiously impotent. When we come to study analytically organic nature and inorganic nature, things animate and 25 26 EVOLUTION AND ANIMAL LIFE things inanimate, we find structures and behavior among inor- ganic things which cannot be readily distinguished in defining words from structures and behavior that are usually taken as characteristic of organisms. On the other hand we shall find in organic nature the very same chemical elements, and for the most part the same combinations of elements, that we find in the great mass of inorganic world substance. So that some biologists by a detailed and keen, if somewhat sophisticated, analysis of the alleged differences between animate and inani- mate matter show that these differences are not absolute, and leave you with a stone in one hand and a grasshopper in the other logically unable to define the fundamental difference between the two, and yet morally certain of this absolute difference. As a matter of fact there is one distinction between living matter and non-living matter which even the cleverest of the modern physicochemical school of biologists has as yet been unable to explain away. And that is the inevitable presence in living matter and the inevitable absence in non-living matter of certain highly complex chemical combinations of carbon, hydrogen, oxygen, nitrogen, and sulphur, called proteids or albuminous compounds. The actual presence of these chemical substances in living matter is made manifest to us by the physicochemical behavior of these substances: that is, by our observation or recognition of their peculiar attributes. This behavior or these peculiar attributes or activities are those fascinating ones which we are accustomed to call the essential life processes. What these activities are we indicate in a not very precise way by the words organization, assimilation, growth, reproduction, motion, irritability, and adaptation. These essential life processes we have come by constant experience to associate always and only with a substance called protoplasm. Huxley long ago called protoplasm, therefore, the physical basis of life. But protoplasm we have found to be a complex of substances or chemical compounds. Of these, a certain few are indispen- sable and fundamental, while others may be absent or present without affecting the particular capacities which make proto- plasm the physical basis of life. This protoplasm too must be organized in a particular way in order that life may persist in the organism. It must appear in two conditions, and proto- LIFE, ITS PHYSICAL BASIS AND SIMPLEST EXPRESSION 27 plasmic stuff representing these two conditions must be disposed in certain definite relations. Protoplasm must occur as a cell or cells to be capable of performing the necessary activities of life. Hence we must consider at the very beginning of any dis- cussion of life the tw r o things, protoplasm and the cell. The elements that compose protoplasm are the familiar ones, carbon, nitrogen, hydrogen, oxygen, sulphur, phosphorus, potassium, sodium, etc.; but these elements, or some of them, are found in protoplasmic cells in certain complex combinations which arc not found elsewhere in nature, and which therefore actually and absolutely distinguish chemically living proto- plasm from all lifeless matter. These particular combinations are certain albuminous compounds or proteids, composed of C, H, O, N, and S, and their complexity is extreme: the atoms in a single molecule often number more than a thousand. The molecules also are very large, which is probably the reason of their characteristic nondiffusibility through animal membranes or artificial parchment. In addition to these characteristic albuminous compounds and various derivatives of them, protoplasm usually contains certain native albumins and certain other characteristic com- pounds known as carbohydrates and fats (which differ essen- tially from the albuminous substances in lacking nitrogen as a composing element). There are also various salts and gases and always water to be found in living substances. Water is absolutely necessary to the physical condition of half fluidity which gives to protoplasm its essential capacity for motion on itself. The commoner salts found in living substances are compounds of chlorine as w r ell as the carbonates, sulphates, and phosphates of the alkalies and alkali earths, especially common salt (sodium chloride), potassium chloride, ammonium chloride, and the carbonates, sulphides, and sulphates of sodium, potas- sium, magnesium, ammonium, and calcium. The gases found in living matter are oxygen and carbon dioxide. These, when not in chemical combination, are almost always dissolved in water, although rarely they may be in the form of gas bubbles. To sum up the relation of living matter to chemistry we may say that life is always associated with protoplasm, and that this protoplasm is made up of a few familiar inorganic elements, particularly those of lowest atomic weight; that it does not include any special so-called vital or life element, that is, any 28 EVOLUTION AND ANIMAL LIFE elementary substance other than occurs in the inorganic world. These elements are combined in protoplasm into certain most extremely complex compounds, which are always present where life is, and never elsewhere, and hence the essential chemical characteristic of living matter is the presence of these complex as yet unanalyzed, albuminous compounds. It is obvious that this chemical half-knowledge of proto- plasm makes no satisfying revelation to us explanatory of the qualities of this life stuff. How is it then with the physical structure of protoplasm? We know that many simple chemical substances put together in particular physical relationship to each other will give a capacity of performance or function quite different from and beyond that which they possess when simply brought together without definite order or arrangement. Is protoplasm a machine with a capacity for doing extraordi- nary things, with its powers due primarily to its physical make-up? Unfortunately we have no satisfying answer to this question. While chemists are balked in their analysis of the protoplasmic make-up by the complexity of the compounds they meet, a complexity too much for their present technic to resolve, physicists are similarly balked in their attempt to re- solve and expose the ultimate physical structure of protoplasm. This ultimate structure of protoplasm is ultramicroscopic, and its study is checked by the limitations of microscopes. When we examine protoplasm with the highest powers of the microscope we see plainly that it is not as it appears under lower powers, structureless and homogeneous. On the contrary it reveals an apparent granular or fibrillar or alveolar or reticu- lar structure. We find that protoplasm varies in its physical make-up at different times or in different cells. We also find that the difficulties of interpreting just what one sees when using the highest microscopic powers make it impossible to be really certain of understanding what is seen. But however various our interpretations of the finer structure of protoplasm, they agree that any bit of protoplasm is a viscous colloidal mass composed of at least two substances of somewhat different phys- ical make-up. One of these substances is evidently denser than the other and is arranged in the form of grains, rods, threads, or droplets scattered through a ground mass. Concerning this dimorphic condition of protoplasm practically all biologists are agreed. The names, hyaloplasm, paraplasm, or others of sim- LIFE, ITS PHYSICAL BASIS AND SIMPLEST EXPRESSION 29 ilar significance are applied to the viscous hyaline ground substance, while the denser parts are variously called micro- somes, granules, fibrils, spongioplasm, etc. The important part of all this is the fact that all the biologists are not agreed on any certain kind of intimate structure of B iS2^&9 ruiWt HV%Sn> ijfe : ' -"V -' ; A -. ^,, .' V<~< i -U- r- '--w-: v ^'. "\ /v^v^is t ^'V\.;.X ir^s^jOri) &S ;-,-. "^^--. ,^w w> -;%"' ^Hii!;!!^ FIG. 15. Different types of cells composing the body of the squirrel or other highly developed animal : A, liver cell; /, food materials; n, nucleus; B, complete cell; C, nerve cell, with small part of its fiber; D, muscle fiber; E, cells lining the body cavity; F, lining of the windpipe; G, section through the skin. (Highly magnified.) protoplasm as revealed by the highest powers of the microscope, but they all agree that there is a fine and real structural organ- ization of what at first glance appears to be homogeneous structureless life stuff. That is, as Delage expresses it, it is seen that protoplasm is not simply an organic chemical compound, but that it is an organized substance; that is, it possesses a structure of a higher order than the automatic structure of those chemical molecules which compose non-living so-called organic substances. But at the same time we are deceived if we expect 30 EVOLUTION AND ANIMAL LIFE to be able to find in this physical organization of protoplasm any satisfactory explanation of its wonderful properties. We have said that it should always be held clearly in mind that the full life capacity of protoplasm is realized only when it is in that differentiated and organized condition typical of the structural unit or cell. The essential thing about the cell is not that it has a definite shape or size or that it is truly cell- or saclike, but that it is a tiny mass of protoplasm with various FIG. 16. Amoeba, showing different shapes assumed by it when crawling. Verworn.) (After substances secreted by or held in it. The protoplasm itself is differentiated into at least two parts, an inner, denser, smaller part called the nucleus, and an outer surrounding, usually larger, portion called the cytoplasm. Such a differentiated or organized protoplasmic unit can perform all of the essential functions of life and persist in this performance indefinitely unless destroyed by extrinsic causes. The cell itself may not have any indefinite existence as a unit, but it will be the progenitor of an indefi- nitely prolonged series of cells. A single part of this cell, that is, a bit of protoplasm either of the nucleus or the cytoplasm, or the whole of either can perform for a while most of the activities of life; but such a part always lacks the capacity for reproduction, that is, for persistence as living matter. Thus it is obvious that LIFE, ITS PHYSICAL BASIS AND SIMPLEST EXPRESSION 31 FIG. 17. Amoeba eating a microscopic one-celled plant. (After Verworn.) FIG. 18. ^ poJypodia in six successive stages of fusion. The dark white-margined spot in the interior is the nucleus. (After F. E. Schultze.) 32 EVOLUTION AND ANIMAL LIFE if such protoplasmic cells, composed of nucleus and cytoplasm, exist singly they form living units. And we have actual ex- emplifications of this condition in the structure and life of the simplest organism. The simplest organisms are independently living, single proto- plasmic cells (Figs. 16-21). There are thousands of kinds of CJik IT H FIG. 19. Plasmodium of a slime mold on wood, Trichia fai-aginea: A, plasmodium X 2; B, spores; C, spore with contents escaping; D, ciliated swarm spore, showing flagellum, /, and nucleus, n; E, two amoeboid swarm spores; F, part of plasmodium under glass slide; G, a part of F, more highly magnified. (After Campbell.) these single-celled organisms recognizably different by charac- teristics of shape and size, habit and habitat. We try to distin- guish them as single-celled animals (Protozoa) and single-celled plants (Protophyta) , on the basis of alleged differences in their habit of food-taking and general nutrition. This distinction is often most arbitrarily made, and botanists and zoologists are constantly claiming the same organisms as belonging to their respective fields of study. Many naturalists, conspicuously Haeckel, have repeatedly suggested the convenience and even the necessity of grouping most of these unicellular organisms into a phylum or kingdom to be called the Protista, the members LIFE, ITS PHYSICAL BASIS AND SIMPLEST EXPRESSION 33 of which shall not be recognized as sufficiently specialized to be called either plants or animals, but simply organisms. But this suggestion seems to meet with little practical favor from students of systematic biology. For a basis, therefore, of any study of the evolution of life, an acquaintanceship with the life and struc- ture of the simplest organisms is a necessity. As the authors have already tried in another book ("Animal Life") to present a simple account of this life together with an account of certain less simple or slightly complex or- ganisms (Figs. 22-26) whose physiology and structure reveal successive stages in organic complexity and specialization, and as the space in this book is limited, the authors must refer their present readers to chapters I, II, and III of " Animal Life ' for an account of the life of the simplest and slightly com- plex organisms. The differentiation and growing com- plexity of the body of those many-celled animals which differ from and are, we may say, beyond and higher than the simple many- celled forms, are by no means always along the same line (Figs. 27-37). It is familiar knowledge that animals can be classified or grouped into a number of great divisions called branches or phyla. For example, the starfishes, sea urchins, sea cucumbers, etc., constitute one phylum, the Echinodermata; the crustaceans, insects, spiders, etc., con- stitute another phylum, the Arthropoda, and all the animals with a backbone or with a notochord constitute another, the Chordata. Now for each of these phyla there is a fundamental or type structure (Fig. 27). All of the Echinodermata, for example, are built on the radiate plan. They recall the starfish with its five or more arms radiating from a central disk. The Arthropods are all animals with a body composed fundamentally of a series of successive segments, some or all of these segments bearing pairs of jointed appendages; and so on. We need not pursue FIG. 20. Paramce- cium aurelia. At each end there is a contractile vac- uole, and in the center is one of the nuclei. (After Verworn.) 34 EVOLUTION AND ANIMAL LIFE FIG. 21. A group of stalked one-celled animals, Carchesium, sp. (Adapted from Davenport, from a photograph of the liv- ing animals.) grouped into two regions and the appendages limited to the anterior one of these two. The Myriapods, which are also Arthropods, have a structure more in conformity with what may be called the racial or typical plan for the whole phylum; that is, the body is made up of a se'ries of many successive similar segments, each segment bear- ing a pair of jointed ap- pendages. In that general line of descent to which man belongs, and which is distinguished by the name of the phylum Chordata, there are of course various subordinate lines which we recognize under the names further the general classifi- cation of animals into phy- la. Nor need we explain in any detail the structural types or fundamental struc- tural plans which distin- guish the various principal lines of descent in the animal kingdom. Branching out from each of the principal lines are hosts of subordinate lines. Some of the Arthropods, as the insects, have their body segments grouped into three regions and their jointed appendages confined to the anterior two of these re- gions. Others, as the spiders, have the body segments B FIG. 22. Gonium pectorale, a colonial pro- tozoon: A, seen from above; B.seen from the side. (After Stein.) LIFE, ITS PHYSICAL BASIS AND SIMPLEST EXPRESSION 35 FIG. 23. Pandorina sp., a colonial protozoon. (.Highly magnified.) of fishes, amphibians, reptiles, birds, and mammals. In all the subdivisions of the main groups there are also to be recognized differentiated and di- vergent lesser lines of descent, and within these still lesser ones. While, as already noted, the main divisions of the animal kingdom are called phyla and the divisions of the phyla, classes, the subdi- visions of the classes are usually called orders. The next subdi- vision is that into families, each in. turn being a cluster of genera. The genera are composed of species and the species finally of sub-species, varieties, and individuals. Each one of these names refers pri- marily to a special line or mode of differentiation and at the same time refers to the fact that the members of each of these differe n t i a t e d groups are genet- ically related to each other, that is, related by blood, by actual ancestral descent. All these differ- entiated groups indicate diverging lines of evolution, some of them short, and but FIG. 24. A fresh-water polyp, Hydra vulgaris: A, in ex- slightly divergent tended condition and in contracted condition; B, cross from the main section of body, showing the two layers of cells which , . 1 make up the body wall. lme fr m which 36 EVOLUTION AND ANIMAL LIFE they arise; others, on the contrary, long, important, and widely divergent. The traditional tree which is drawn to explain animal classification illustrates at the same time the two fundamental facts upon which this classification is based, namely, differentiation of struc- ture, and corresponding divergence of descent. All the branches of this gene- alogical tree lead back, as they do in a real tree, to its trunk, and the trunk of this tree springs from the simplest of the many-celled animals, namely, from those primitive forms which resemble in essential characters animals like the FIG. 25. Longitudinal section through the body of a sea anemone: oe., oesophagus; m.f., mesen- terial filaments; r., reproductive organs. FIG. 26. One of the sim- plest sponges, Calcoh/n- thus primigenius. A part of the outer wall is cut away to show the inside. (After Haeckel.) simpler polyps. Indeed it seems certain that this tree trunk can be traced farther back; that it must spring in the begin- ning from forms essentially like the lowest organisms that we know to-day, namely, single, simple cells living independently. From the Amoaba to Man; that is the history of descent, or ascent if one prefers. The course has been a continuous one, both in point of time and in point of gradual transformation. LIFE, ITS PHYSICAL BASIS AND SIMPLEST EXPRESSION 37 FIG. 27. Diagram showing fundamental structure of types of several animal phyla: 1, sea anemone; 2, starfish; 3, worm; 4, centipede; 5, clam; 6, honeybee; 7, sala- mander. In each figure the central nervous system is indicated by the black lines. (After Haeckel.) But great periods of this time are shut away from us without record of their duration, and long series of the gradually changing forms are lost to us without hope of discovery. And yet in its large outlines we know the history of all this time and the character of all these graded series. 38 EVOLUTION AND ANIMAL LIFE We should give at least brief attention to what may be called the primary, or necessary, conditions of life. We know that fishes cannot live very long out of water and that birds cannot live in water. These, however, are conditions which depend on the special ecological habits of these two particular kinds of animals. The necessity of a constant and sufficient supply of oxygen is a necessity common to both. It is one of the primary conditions of their life. All animals must have air. Similarly */ both fishes and birds and all other animals must have food. This, then, is an- other of the pri- mary conditions of animal life. If water be held not to be included in the general con- ception of food, then special men- tion must be made of the necessity of water as one of the primary condi- tions of life. Proto- plasm, the basis of life, is a fluid, although thick and viscous. To be fluid its components must be dissolved or suspended in water. In fact, all of the really living substance in an animal's body contains water. This water, so necessary for the animal, may be derived from the general food, all of which contains water in greater or less quantity, or it may be taken apart from the other food by drinking or by absorption through the skin. We know, too, that if the temperature is below a certain minimum point or above a certain maximum, these points vary- ing for different animals, death takes the place of life. It is familiar knowledge that many animals can be frozen without being killed. Insects and other small animals may lie frozen through winter and resume active life again in the spring. An experimenter kept certain fishes frozen in blocks of ice at a tem- perature of -15 C. for some time and then gradually thawed them out unhurt. There is no doubt that every part of the body, all of the living substance, of these fish was frozen, for specimens at this temperature could be broken and pounded up FIG. 28. The fiddler crab, Gehisimus. (Photograph by Miss Mary Rathbun.) LIFE, ITS PHYSICAL BASIS AND SIMPLEST EXPRESSION .'J9 into fine icy powder. But a temperature of -20 C. killed the fish. According to L. J. Turner, the "Alaska mud-fish (Dallia), was fed frozen to Esquimaux dogs. One of these thawing in the stomach of the animal made its escape alive. Frogs lived after being kept at a temperature of -28 C., centipedes, at FIG. 29. The piddock, Zirphnca crispata, a rock-boring mollusk. (Natural size, from life.) a temperature of -50 C., and certain snails endured a tempera- ture of - 120 C. without dying. At the other extreme, instances are known of animals living in water (hot springs or water gradually heated with the organ- isms in it) of a temperature as high as 50 C. Experiments with Amoebae show that these simplest animals contract and cease active motion at 35 C., but are not killed until a temperature of 40 to 50 C. is reached. Variations in pressure of the atmosphere also constitute 40 EVOLUTION AND ANIMAL LIFE conditions which may determine the existence of life. The pressure or weight of the atmosphere on the surface of the earth is nearly fifteen pounds on each square inch. This pressure is exerted equally in all directions so that an object on the earth's surface sustains a pressure on each square inch of FIG. 30. Cephalopoda. Lower figure, the devil-fish or octopus, Octopus punctatus. The upper figure represents the squid, Loligo pealii, swimming backward by driving a stream of water through the small tube slightly beneath the eyes. (From life, one-third natural size.) its surface of fifteen pounds. That is, all animals living on the earth's surface or near it live under this pressure and under no other condition. The animals that live in water, however, sustain a much greater pressure, this pressure increasing with distance. Certain ocean fishes live habitually in great depths, at from two to nearly five miles, where the pressure is equivalent to that of many hundred atmospheres. If these fishes are brought to the surface their eyes bulge out, their scales fall off because of the great expanse of the skin, and the stomach is thrust wrong side out. Indeed the body itself sometimes bursts. On LIFE, ITS PHYSICAL BASIS AND SIMPLEST EXPRESSION 41 the other hand if an animal which lives normally on the surface of the earth is taken up a very high mountain or is carried up in a balloon to a great altitude where the pressure of the atmos- phere is much less than at the earth's surface, serious conse- quences may ensue, and if too high an altitude is reached, death occurs. Some animals require certain organic salts or compounds of lime to form bones or shells, etc. These salts may be re- garded as necessary articles of nutrition, though their function is not that of ordinary food. These are peculiar demands of special kinds of animals. There might also be included among primary life conditions such necessities as the light and heat of the sun, the action of gravitation, and other physical conditions FIG. 31. Long-horned boring beetle, Ergates sp. larva, pupa and adult insect. without which existence of life of any kind would be impossible on this earth. Finally we may refer briefly to the "grand problem" of the origin of life itself. Any treatment of this question is bound to be wholly theoretical. We do not know a single positive thing about it. We have some negative evidence. That is, we have 42 EVOLUTION AND ANIMAL LIFE no recorded instance and men have searched diligently for examples of spontaneous generation. No protoplasm has been seen, or otherwise proved, to come into existence except through the agency of already existing protoplasm. All life comes from life. All those former beliefs of spontaneous appearance of bees from the carcasses of oxen, flies from decaying flesh, hair worms from horse tail hairs in water troughs, and bacteria and infusoria in infusions of beef or hay have been shown on scientific investi- gation to be utterly with- out basis of fact. But if protoplasm and life do not appear, are not being generated spontane- ously in this earth epoch, may they not have been in earlier ages? Geologists and biologists attempt to explain most of the things that happened in earlier geologic ages by what they observe to be happening now. They would answer, on this basis, that what evidence we now have should lead us to believe that the generation of life has never occurred. But there must have been a beginning. Life has not always been. The ac- cepted geological theory of the making of our earth precludes the existence of life on it until the globe was cool enough for organisms to exist. We know that there is a maximum of temperature beyond which protoplasm inevitably coagulates. When and where was this beginning of life? The biologist can- not admit spontaneous generation in the face of the scientific evidence he has. On the other hand he has difficulty in under- standing how life could have originated in any other way than through some sort of transformation from inorganic matter. As a matter of curiosity we may glance at a few of the FIG. 32. Ascidian or sea squirt. LIFE, ITS PHYSICAL BASIS AND SIMPLEST EXPRESSION 43 FIG. 33. Blacksnake, Buscanion constrictor. (Photograph by W. K. Fisher.) FIG. 34. Hawkbill turtle, Eretmochelys imbricata. 44 EVOLUTION AND ANIMAL LIFE speculations that biologists have allowed themselves concerning the origin of living substance on the earth. A speculation that is interesting only because it was suggested by a great scientific FIG. 35. Golden eagle, Aquila chrysaetus. man a physicist, however, not a biologist is Lord Kelvin's theory that living substance was brought to this earth from celestial regions by meteorites. A more acceptable theory is that at some earlier geologic age the conditions of earth, atmos- phere, temperature, etc., were at one time of such a favorable LIFE, ITS PHYSICAL BASIS AND SIMPLEST EXPRESSION 45 nature that just that fortunate coincidence of all necessary con- ditions and elements occurred which allowed C, H, O, N, to unite in those great, almost infinitely complex, molecules which com- pose the albuminous compounds whose existence is the only real chemical characteristic peculiar to living matter. But we have FIG. 36. African or two-toed ostrich, Struthio camelus. (Photograph by William Graham.) 46 EVOLUTION AND ANIMAL LIFE already indicated that the production of such compounds would not necessarily be the production of protoplasm. What of the complex definitive physical organization of protoplasm on which we predicate so much of its capacity? The botanist Schaffhausen believes that water, air, and the necessary mineral substances have been directly combined under the influences of life and heat and have given birth to an FIG. 37. Opossum, Didelphys virgininnn. (One-tenth natural size; photograph by W. K. Fisher.) uncolored protococcus which next became Protococcus viridis. Delage asks : " If the thing is so simple why does not the author produce one of these protococci in his laboratory? On lid ferait grace de la chlorophylle ! ' Nageli holds that when the albumi- nous compounds had their birth in an aqueous liquid, as they were not soluble in water, they were precipitated. This pre- cipitate was formed of minute particles, a sort of crystal which he calls micellae. These micellae are the materials from which organisms were formed. An inorganic crystal deposited in a saturated solution of the same nature determines a deposit on its surface in the form of tiny crystals, by which means it LIFE, ITS PHYSICAL BASIS AND SIMPLEST EXPRESSION 47 increases in size. In the same way, when some of these albu- minous micellae are formed anywhere, they facilitate further precipitation within their sphere of influence in such a way that the formation of other micella?, instead of going on uniformly in the liquid mass, is localized at certain points. Thus are found aggregates of an albuminous nature which constitute the primitive protoplasm. This is Nageli's suggestion, and Nageli is one of the most thoughtful biologists who has ever lived ! Granting that protoplasm must have had a natural, spon- taneous beginning on this earth, being neither brought to it from other worlds nor created extranaturally on this world, biologists indulge in some speculations as to the probable whereabouts of this first appearance of life, and as to whether living substance was formed spontaneously but once only or several times, and perhaps in several places. It is not necessary here to follow up such speculations. The only one of them with any scientific evidence at all for it is the theory that life began at the poles or perhaps particularly at the north pole. The evidence for this is based, first, on the fact that in accordance with the cosmic theory of world evolution, the poles of the earth must have been first in a condition under which life might exist, and, second, on facts revealed by the study of the geo- graphical distribution of living and fossil organisms. There seems to be some slight scientific foundation for the claim that the first organisms lived in polar regions. CHAPTER TV FACTORS AND MECHANISM OF EVOLUTION Even in the latest and maturest formulations of scientific research, the dramatic tone is never lost. The causes at work are conceived in a highly impersonal way, but hitherto no science has been content to do its work in terms of inert magnitude alone. Activity continues to be imputed to the phenomena with which science deals, and activity is, of course, not a fact of observation but is imputed to the phenomena by the observer. Episternologically speaking, activity is imputed to phenomena for the purpose of organizing them into a dramatically consistent system." -THORSTEIN VEBLEN. THERE is to-day no doubt in our minds of the truth, the actuality, of descent. It is not the theory of descent: it is the fact, the law, of descent, of which we talk and write. Organ- isms are blood-related: they are transformed, descended from one another. This, which is the common knowledge of present- day post-Darwinian science, was the belief of many naturalists even before the days of Darwin. " From the Greeks to Darwin ' was not all darkness nor complete freedom from taint of the "pernicious evolution doctrine." Goethe, Erasmus Darwin, Lamarck, to mention only familiar names, were evolutionists: they believed in the transmutation of species, believed in descent. But it was Darwin who gave the waiting naturalists substantial and satisfactory reasons for the beliefs that were in them; w r ho gave them strength to have the courage of their convictions. While Darwinism, in our present-day use of the name, is not synonymous with descent and evolution, but is the name of a causomechanical explanation of it, or a group of causal factors, yet it might justifiably be more broadly used, and held still to mean, what it certainly did to the world generally for a good many years after the "Origin of Species" appeared, the 48 FACTORS AND MECHANISM OF EVOLUTION 49 general theory of organic evolution and the particular doctrine of the descent of man from the lower animals. For it was Darwin who really proved these things to be realities. But in biology to-day Darwinism is the name which refers to certain particular causal factors or determining agents in the actual production and control of the transmutation of species and the progress and direction of the lines of descent. And the modern scientific adverse criticism of Darwinism which is beginning to find its echoes in popular literature must never be mistaken to be disparagement or adverse criticism of the doctrine of descent, the law of organic evolution. So we are not in this book dis- cussing the probabilities of the truth or untruth of evolution nor presenting evidences or argument to justify a belief in the doctrine. As the days have long passed when the shape of the earth, or the behavior of the members of the solar system, was a fit subject for debate, so the days are now by when the truth or falsity of the law of organic descent is a debatable thesis. The earth is subspherical, the planets revolve about the sun, and species of organisms descend from other species. But in what particular way, or as the effect of what particular causal factors, this descent or transformation of species, that is, kinds of organisms, comes about, here there is unlimited field for debate and polemic, for hypothesis and investigation, for deduction and determination. It is the factors of organic evolution, the factors of each of the particular phases or aspects of evolution phenomena, that are the subject of present-day biological study and discussion. The mechanism and method of evolution is' a subject, with its score of moot questions, its enormous opportunity and inspiration, for fact gathering, fact arranging, arid fact interpreting. So in biological science to- day, no less but even more than in those first exciting days after the "Origin of Species/' the subject and problems of evolution are the inspiriting and absorbing matters which chiefly occupy the attention of biologists. What these factors are that compose the chief subject of present-day evolution study and discussion may be summarily set out in the present chapter. In the first place it is obvious that there can be no transfor- mation or change of species unless there is an ever-present actual variation. By variation is simply meant, in the larger sense, that no two individual organisms in the world, nor for that matter any two that have ever been in the world, are exactly 50 EVOLUTION AND ANIMAL LIFE alike. This refers not only to individuals of different species of plants and animals, but to individuals of the same species and even (and this in a way is most important of all) to indi- viduals born of the same parents. It is indeed this last condi- tion that is the actual basis and fundamental beginning for species change. That this variation does exist is absolute fact, and there is no discussion of it. To what extent or degree, what parts of an organism are chiefly affected, whether or no this variation shows a regularity in its occurrence or a determinateness of tendency or direction. / whether or no this variation is based on inheritance and if so in what degree of similarity or identity all these and a dozen other questions are the moot problems in connection with the great factor variation. These are undecided things, which means, on the whole, that variation, apart from the observed and admitted actuality of the occurrence, is itself a great evolution problem. The variation alone, however, presumably does not make new species nor maintain lines of descent. If this variation is, as it seems to be, almost unlimited in its range of appearance, then as species are of definite character and number and as lines of descent are even more definite and more limited as to number, there must be some factor which determines what kinds or lines of variation may or shall persist and what shall be extinguished. Is there something incident to the causes of variation that determines what lines of descent shall be established by it or ts based on it, or is there some added factor which, having no control over the initial appearance of variation, has absolute control over its persistence and headway? Darwin's factors of selection, more particularly natural selection, is the explanation of this control offered in the famous " Origin of Species.' 3 And natural selection has been in the minds of biologists until to-day, at least, undoubtedly that, factor in evolution which has been believed to have the chief control in the forming of species and the direction of descent lines. But in reference to this particular factor three schools of biologists have gradually grown up; namely, first the school headed by Weismann, who has believed and contended that natural selection is almost the only factor which, on a basis of fortuitous, that is, uncontrolled, variation, has produced the species and lines of descent as we know them; second, the FACTORS AND MECHANISM OF EVOLUTION 51 school which holds that natural selection has practically nothing to do with species-forming but only, and in a large general way, with the control of descent; and, third, the compromise school, which attributes to natural selection an important part in both species-forming and control of general descent lines, but recog- nizes the simultaneous existence and the considerable im- portance of several other species-forming and descent-modifying factors. In addition to these three schools one must note that a number of active working biologists repudiate the factor of natural selection entirely, holding it to be a vagary and an artifact of logic. Associated with natural selection in the general theory of selective action is Darwin's conception of sexual selection. This factor was presumed by Darwin to play a part only in the forma- tion and control of those often very obvious but never well- understood characteristics of a secondary sexual character which distinguish the sexes in many species of animals. Let one recall these characters in the pea fowl, the bird of paradise, the pheasant, some of the butterflies, the lamellicorn beetles, many fishes, and so on. According to the theory of sexual selection the females have chosen for their consorts those males best endowed by variation with these ornamental character- istics, so that by this selection there has come about a gradual cumulation of the characteristics culminating in such bizarrerie as we are familiar with in numerous living animals. The word selection will certainly bring to the mind of the reader also a third kind of selective process, namely, that called artificial selection, and this kind of selection is, of course, a factor, and an important one, and has been such for some eighty centuries, in the modification of plant and animal forms. But however widely differing and extraordinarily modified culti- vated and domesticated kinds of animals and plants may be, these different kinds are not looked on by biologists as having the validity, that is, the stability and characteristics of origin, that the different species of animals and plants found in nature have. All the different kinds of pigeons, for example, are known to be due primarily to the artificial modification of a single wild kind, the rock dove of Europe, and all of these different artifi- cially produced kinds agree in an important physiological charac- teristic, namely, that of being able to mate freely with each other and with their common ancestor. As this physiological char- 52 EVOLUTION AND ANIMAL LIFE acteristic is precisely one of the criteria largely used in deter- mining species limits in nature, naturalists call the artificially produced kinds by another name than species; they call them races or varieties, meaning by this to indicate obvious struc- tural and functional differences. Thus artificial selection, while a factor in determining the extent and character of the modifi- cation of many kinds of animals and plants, is not considered a factor in the determination of natural lines of descent. Its value in this regard lies in the clew it gives to natural processes of the same kind. Selection by nature among the variations which appear is made possible only by several other factors or actually existent conditions. One is the "prodigality of production' or the constant tendency to overpopulation due to reproduction by multiplication or in a geometrically progressive ratio. Every mature female or hermaphroditic plant or animal produces, at least in the condition of eggs or germ cells, more than one new individual like itself. (There are a very few exceptional cases, compensated for, however, in other ways.) Most produce many new individuals and some reproduce enormously. Cer- tain fishes lay millions of eggs; so do certain oysters; many insects produce thousands of young; many plants produce myriads of seeds. But not all can grow up: there is neither room nor food for all. There must inevitably be a selection by active or passive, guided or fortuitous, means. It is a necessary assumption, for the effectiveness of the natural selection factor, that this selection is actually based on the fitness or advantage of some of the variations as compared with others. The trying out or determination of the advantage of these variations comes about as an inevitable active or passive competition for life among the overabundant^ appearing new individuals. This is the "struggle for existence," and the " survival of the fittest ' ' is the expression of the assumed fact of the success of the individuals advantageously (i. e., most fitly) varying. The unfit and the less fit are assumed to com- pose the thousands and hundreds of thousands who must die where only tens or hundreds can live at one time. But if natural selection, which is, so far, obviously one of but individuals alone, is to produce new species and control descent lines, it has to depend on a further factor, one named by a familiar word, but not at all explained by it, namely, the factor FACTORS AND MECHANISM OF EVOLUTION 53 heredity. Although we can rely in our theory building on the fact that no two individuals are exactly alike, yet we can equally certainly rely on the fact that the offspring of any individual will be much more like other individuals of the species to which the parent belongs than like individuals of other species, and also, in the main, more like the parent than like other indi- viduals of the same species. Heredity is the name we use for expressing this fact of likeness of young to parent. Some biologists seem to mean by heredity a force or dominat- ing influence which brings about this likeness; while others use the word heredity to name rather the processes which are gone through with by the young in becoming, in its total develop- ment, like the parent. The essential connotation of the word is, however, simply the fact that this likeness does exist and that we may rely on its continuing to occur. So that when the struggle for existence weeds out, if it does, those individuals of a too abundant population which possess variations of disadvantage or of no special advantage, leaving those to survive and produce offspring which do possess specially advantageous or fit variations, the fact of heredity permits us to assume the almost certain perpetuation of these advan- tageous variations by insuring their reappearance in the off- spring of the "saved' individuals. Thus while we may liken the causes that produce ever-appearing variations to a centrif- ugal force making for difference and instability, heredity (if used as the name for the causes that produce likeness) may be conceived as a centripetal force, making for stability and sameness. But at least one other factor seems to be necessary in species-forming and that is the factor of isolation, separation, or segregation, as it is variously named. By this is meant that those individuals showing similar variations must in some way be segregated, made to live and breed together, in order that the particular variations (which from the point of view of the student of species-forming may be called also the particular varietal differences that are to become in time so developed and fixed as to be true species differences) may be maintained. For it is obvious that if an individual possessing certain particular variations mate with another of its species possessing different variations, the offspring of this union will likely not possess in pure form the variations of that particular parent 5 54 EVOLUTION AND ANIMAL LIFE we are for the moment interested in. The offspring may show a blend of the different characters of the parents, or a mosaic of them, or may show the characters of either one alone, or, indeed, characters of wholly new type. The important thing is, however, that there is no certainty indeed there is almost certainty of the opposite that any particular variation will be fostered and fixed if miscellaneous interbreeding is allowed. So that a segregation of individuals having certain common variations or varietal characters is necessary for the perpetua- tion of these characters. Now the most usual way, probably, in which this segregation or isolation is brought about is by topographic or geographic barriers ; a group of individuals gets isolated from others of their species by some physical barrier, and the variations that appear among them, due often to some cause incident to the special locality and hence common to all of them, are readily preserved and fostered by the enforced breeding among themselves. But such an isolation may conceivably be brought about in several other ways, and observation has shown that probably in some cases so-called biologic isolation occurs, that is, that a restriction of miscellaneous interbreeding among individuals of one species, and an enforced selective breeding among certain ones possessing certain variations or differences in common, does really obtain. Such isolation is also called physiologic, or sexual, isolation. Many biologists, and the number of them has increased rapidly in the last few years, due primarily to the activity and leadership of the botanist de Vries (Amsterdam), believe that species-forming is achieved without the aid of the selection factor; that the actual production of species is a function of variation ("mutation' the special kind of variation efficient in species-making is called), and that the influence of selection is only of a more remote and generally restraining, and thus directive, nature. Such biologists may be said to believe in species-forming by heterogenesis or saltation, as contrasted with species-making by slow, gradual transmutation. And de Vries and his followers have adduced a few apparently undeniable examples of species-forming by heterogenesis. At least this influence seems to have produced forms to all in- tents and purposes apparently similar to natural species. So the particular kind of variation called mutation, which is the FACTORS AND MECHANISM OF EVOLUTION 55 basis of this sort of species-making, must be added to our list of evolution factors. Some other biologists, of whom the botanist Nageli, the zoologist Eimer, and the paleontologist Cope are representa- tives (all three of these men, however, having evolution theories and beliefs distinct and peculiar to each), believe in what may be called orthogenetic evolution. That is, that the lines of descent are determined by the appearance of certain special determinate lines or tendencies of variation or change, this non- fortuitous and determinate variation being itself determined by certain causes either (in Nageli 's belief) inherent in life, or (in Eimer's belief) extrinsic to life but imposed upon it, as for example the influence of climate, etc. So that orthogenesis or determinate variation should also find a place in any list of assumed evolution factors. While it is apparent that variation is ever present and also apparent that heredity or the fact of likeness is always ever to be relied on, the exact relationship or correlation of these two evolution factors is not so apparent. That heredity often preserves or perpetuates variations after they have occurred is well proved, but it is also proved that some variations appearing in the parent are not handed on to the parent's offspring, nor indeed to any future generations of the line. And the general answer to the natural query raised by this condition is that variations which are congenital or blastogenic, that is, are determined at birth for it (although they appear of course only after development), are heritable (that is, will be passed on from parent to offspring); but that variations or modifications acquired during the lifetime of the individual, that is, those which are impressed on it by extrinsic influences during its "growing up " or development, will not be heritable. Thus such modifica- tions in body parts as may be produced by use or disuse, or by other functional stimulation or lack of it, changes caused by mutilation or disease, etc., are believed by most biologists to be non-heritable. Hence it is that only the congenital variations are looked on by these biologists as of importance in the matter of species-forming. Yet the whole pre-Darwinian evolution theory of Lamarck was founded on the assumption that the modifications in individuals due to use, disuse, and other func- tional stimulation, in a word that all body change and adapta- tion, all characters acquired during the lifetime of an individual, 56 EVOLUTION AND ANIMAL LIFE can be, in some degree at least, handed on by inheritance to the offspring. And there are to-day many Lamarckian evolu- tionists. So that in our list of possible evolution factors the so-called Lamarckian factor should not be omitted. And in connection with it may be considered, by and large, the imme- diate influence or non-influence on individuals and on species of all environmental conditions; and particularly the results of such influence during development. In fact the study of development has come largely to be a study of the actual in- fluences or factors that determine and guide growth, instead of one purely descriptive and comparative as in the older days of embryological study. Some of these factors are apparently strictly inherent in the protoplasmic germ cells and in the embryo substance: others are as obviously extrinsic or epigenetic. And the determination of the relative influence and power of these two sets of developmental factors and of the various members of each set is one of the most eagerly worked-at problems of modern biological study. Finally, the general term adaptation should be mentioned in any list of evolution factors; although it is more usually looked on, not as a factor, but as an evolution problem and indeed one of the greatest of the problems. Adaptation is precisely one of the things evolutionists are trying to find the causes or causal factors of. But nevertheless the adaptability of life stuff, its plasticity and capacity of advantageous reac- tion, is, to many biologists, a fundamental fact in organic nature, like gravitation or chemical affinity in inorganic nature: a thing basic and inexplicable, and in itself a factor whose con- sequences are to be determined but not further to be ques- tioned as to their cause. CHAPTER V NATURAL SELECTION AND THE STRUGGLE FOR EXISTENCE; SEXUAL SELECTION The tendency to regard natural selection as more or less unnecessary or superfluous which is so characteristic of our day, seems to grow out of reverence for the all-sufficiency of the philosophy of evolution, and pious belief that the history of living things flows out of this philosophy as a necessary truth or axiom. BROOKS. La selection naturelle est un principe admirable et parfaitement juste. Tout le monde est d'accord aujourd'hui sur ce point. Mais ou Ton n'est pas d'accord, c'est sur la limite de sa puissance et sur la question de savoir si elle peut engendrer des formes specifiques nouvelles. II semble bien demontre aujourd'hui qu'elle ne le peut. DELAGE. OF all the various factors of organic evolution the one which has been most relied on as the great determining agent is that called Natural Selection, the survival of the individuals best fitted for the conditions of life, with the inheritance of those species-forming adaptations in which fitness lies. The primal initiative is not in natural selection, but in variation, germinal and individual. This may be slight variation (fluc- tuation) or large deviation (saltation), but in any case all difference in species or race must first be individual. The impulse to change, once arisen, is continued through heredity. From natural selection arises the choice among different lines of descent, the adaptive tending to exclude the non-adaptive, while traits which are neither helpful nor hurtful, but simply indifferent, may be borne along by the current of adaptive characters. Finally separation or isolation tends to preserve a special line of heredity from being merged in the mass which constitutes the parent stock or species. Without individual variation, no change could take place; all organisms would be identical in structure. Without heredity, 57 58 EVOLUTION AND ANIMAL LIFE if we could conceive such a condition , no change would persist. Without selection, there would be no premium placed on adaptive characters, and organisms would persist in every degree of variance with their surroundings. Without some degree of isolation, every change would be lost by cross-breeding with the mass. In a world of varying conditions with varying organisms, it is not conceivable that species should, through all their generations, undergo no change. Nor in the changes of any species is it possible that any one of the factors or con- ditions named above should be wholly absent. But the effects of each one may show themselves in many different w r ays, and each may be modified by other facts or conditions. We have compared the history of species to the flow of a river. A single rock may change the course of a stream. In like manner incidental circumstances may determine the evolution of a species. Or using a different metaphor we may compare the course of a species with that of a glacier. The movement of a glacier depends on the law of gravitation "resident' within its molecules. Its course is determined by the topography of its bed. To this bed it is perfectly fitted, but the condition of its surface depends on circumstances related neither to the law of gravitation nor to the form of its bed. A species of animal or plant is well fitted to its conditions in life. This natural selection rigidly enforces ; but its surface characters, which are not essential to its life, are determined by other influences, and in this both selection and environment play but a minor part. All animals feed upon living organisms or upon that which has been living. Hence each animal throughout its life is busy with the destruction of the other organisms or with their removal after death. If these creatures, animals, or plants on which animals feed, are to hold their own, there must be an excess of birth and development to make good the drain upon their numbers. If the plants did not restore their losses the animals that feed on them would perish. In like fashion flesh- eating animals are dependent on those which feed on plants. But throughout nature there is a vast excess in the process of reproduction. More plants sprout than could find standing room were all to grow. More seeds are developed than can find place to sprout. More animals are born than can possibly survive. The process of increase among animals is rightly called multiplication. Each species tends to increase in geo- NATURAL SELECTION; SEXUAL SELECTION 59 metric ratio, but as it multiplies it finds the world already crowded with other multiplying species. A single pair of any species whatsoever, if not checked by adverse conditions, would soon fill the whole earth with its progeny. An annual plant producing two seeds only would have 1,048,576 descendants in twenty-one years, if each seed sprouted and matured. But most plants produce hundreds or thousands of seeds. The ratio of increase is a matter of minor importance. It is the ratio of increase above loss which determines the fate of species. Those species increase in numbers in which the gain exceeds the rate of destruction through the influence of other species or the adverse conditions of life. Where few enemies exist the ratio of increase need not be large. One of the most abundant of birds is the fulmar petrel of the mid- Pacific. It lays but one egg yearly, but it has few enemies and the low rate of increase suffices to cover the sea with fulmars within the region it inhabits. It is not easy to realize the inordinate numbers any species would attain were it not for the checks produced by the presence of the activity of other organisms. Certain protozoa, at their normal rate of increase if none were devoured or destroyed- i night fill the entire ocean within a very short time. It is said that the conger eel lays 15,000,000 eggs yearly. If each hatched and the conger grew to maturity, in a few years there would be no room for any other kind of fish in the sea. The codfish has been known to produce 9,100,000 eggs each year. If each egg were to develop, in ten years the sea would be solidly full of codfish. The female quinnat salmon of the Columbia, Oncorhynchus tchawytscha, ascends the river at the age of about four years, and lays 4,000 eggs, after which she dies. Half these eggs develop into males. If each female egg came to maturity, we should have at the end of fifty years 8,000,000,000,000',000 r 000,000,000,000,000,000,000,000,000 female salmon and as many males as the offspring of a single pair. It takes about one hundred of these salmon to weigh a ton. Could all these fishes develop, in a very short time there would be no room for them in all the rivers of the North, nor in all the waters of the sea. If each egg of the common house fly should develop and each of the larvae should find the food and temperature it needed, 60 EVOLUTION AND ANIMAL LIFE with no loss and no destruction, the people of the city in which it happened would suffocate under the plague of flies. When- ever any species of insect develops a large percentage of the eggs laid, it becomes at once a plague. Thus originate plagues of locusts, grasshoppers, and caterpillars. But the crowd of life renders these plagues rare. Scavenger-beetles and bacteria destroy the decaying flesh where the fly would lay its eggs. Minute creatures, bacteria, protozoa, other insects, are parasitic within the larva itself. Millions of flies starve to death. Mil- lions more are eaten by birds and predaceous insects. The final result is that from year to year the number of flies does not increase. Linnaeus once said that "three flies will devour a dead horse as quickly as a lion." Quite as soon would three bacteria with their descendants reach the same result. "Even slow-breeding man," says Darwin, "has doubled in twenty-five years. At this rate in less than a thousand years there literally would not be standing room for his progeny. The elephant is reckoned the slowest breeder of all animals. It begins breeding when thirty years old and goes on breeding until ninety years old, bringing forth six young in the interval and surviving to be a hundred years old. If this be so, after about 800 years there should be 19,000,000 elephants alive descended from the first pair." A few years of still further multiplication without check, and every foot of the earth would be covered by elephants. Similar calculations may be made in regard to any species of animal or plant whatsoever. Each one increases at a rate which without checks would make it soon cover the earth. Yet the number of individuals in a state of nature in any species re- mains about stationary. With the interference of man, in many species the numbers slowly diminish; very few increase. There are about as many squirrels in the forest one year as another, as many butterflies in the field, as many frogs in the pond. Wolves, bears, deer, ducks, singing birds, fishes, all suf- fer from man's attacks or man's neglect and grow fewer year by year. It is manifest that the tendency to reproduce by geometric ratio meets everywhere with a corresponding check. This check is known as the Struggle for Existence. The struggle for existence is threefold: (a) Among individuals of one species, as wolf against wolf or sparrow against sparrow; (b) between individuals of different species, as rabbit with wolf or blue-bird with sparrow; (c) with the conditions in life as NATURAL SELECTION; SEXUAL SELECTION 61 the necessity of the robin to find water in summer or to keep warm in winter. All three forms of the struggle for existence, intraspecific, interspecific, and environmental, are constantly operative and with every species. In some regions or under some conditions the one phase may be more destructive, in others another. Any one of these may be in various ways modified or ameliorated. When the conditions of life are most easy, as with most species in the tropics, there the conflict of individuals and the conflict of species is most severe. It is not possible to say that any one of these three forms of struggle and selection is more potent than the others. In fact, the first and the second are in a sense forms of the third. All struggle is, strictly speaking, with the conditions of life. Those individuals FIG. 38. Praying mantis, eating a grasshopper. (Adapted from photograph from life by Slingerland.) which endure this struggle survive to reproduce themselves. The rest die and leave no progeny. Because of the destruction resulting from the struggle for existence, more individuals in each species are born than can mature. The majority fail to reach maturity because for one reason or another they cannot do so. All live that can. Each animal tries to feed itself: many try to take care of their young. But in self protection and in propagation of the species very few individuals succeed in comparison with the vast number which the process of reproduction calls into being. The destruction in nature is not indiscriminate. In the long run and for the most part, those creatures least fitted to resist are the first to perish. It is the slowest animal which is soonest overtaken by the pursuers. It is the weakest which is 62 EVOLUTION AND ANIMAL LIFE crowded aside or trampled on by its associates. It is the least adaptable which suffers most from extremes of heat and cold. By the process of Artificial Selection the breeder improves his stock, destroying his weakest or least comely calves, reserving the strong and fit for parentage. In like fashion, on an in- conceivably large scale, the forces of nature are at work modify- ing and fitting to the demands of their surroundings the different species of animals. Because the processes and results of the struggle for existence seem parallel with those of artificial selection, Darwin suggested the name of Natural Selection for the sifting process as seen in nature. To the general re- sult of natural selection, Herbert Spencer has applied the term FIG. 39. The Australian ladybird, Vedalia cardinalis, feeding on cottony cushion scale, Icerya purchasi. (From life.) Survival of the Fittest. By fitness in this sense is meant only adaptation to surrounding conditions, for the process of natural selection has no necessary moral element, nor does it necessarily work toward progress among organisms. With changing con- ditions species undergo change. Some individuals, by the possession of slight advantageous variations of structure or of instinct, meet these new demands better than others. These survive, the others die. The survivors produce young sharing in part, at least, their own advantages, and with renewed selec- tion the degree of adaptation increases with successive genera- tions. To the process of natural selection we must, in most cases, probably ascribe the adjustment of species to surroundings. NATURAL SELECTION; SEXUAL SELECTION 63 Natural selection does not create species, it enforces adaptation. If a species or a group of individuals cannot fit itself to its environment, it will be crowded out by others which can do so. It will then either disappear entirely from the earth, or it will be limited to that region or to those conditions to which it is adapted. A partial adjustment tends to become more perfect, for the individuals least fitted are first destroyed in the struggle for existence. Very small variations may sometimes, therefore, lead to great changes. A side issue apparently unimportant may perhaps determine the fate of a species. Any advantage however small may possibly turn the scale of life. "Battle within battles must be continually recurring, with varying suc- cess, yet in the long run the forces are so nicely balanced that the face of nature remains for a long time uniform, though assuredly the merest trifle would give the victory to one organic being over another." Darwin says: " I have found that the visits of bees are necessary for the fertili- zation of some kinds of clover; for instance, twenty heads of white clover (Trifolium repens) yielded two thousand two hundred and ninety seeds, but twenty other heads protected from the bees produced not one. Again, one hundred heads of red clover (Trifolium pmtense) produced two thousand seven hundred seeds, but the same number of protected heads produced not a single seed. Humble-bees alone visit red clover, as other bees cannot reach the nectar. . . . Hence we may infer as highly probable that, if the whole genus of humble-bees became extinct or very rare in England, the heartsease and red clover would become very rare or wholly disappear. The number of humble-bees in any district depends in a great measure on the number of field mice, which destroy their combs and nests; and Colonel Newman, who has long attended to the habits of humble-bees, believes that more than two-thirds of them are thus destroyed all over England. Now the number of mice is largely dependent, as everyone knows, on the num- ber of cats; and Colonel Newman says: 'Near villages and small towns I have found the nests of humble- bees more numerous than else- where, which I attribute to the number of cats that destroy the mice.' Hence it is quite credible that the presence of feline animals in large numbers in a district might determine, through the intervention first of mice and then of bees, the frequency of certain flowers in that district." 64 EVOLUTION AND ANIMAL LIFE Huxiey carries this calculation still further by showing that the number of cats depends on the number of unmarried women. On the other hand, clover produces beef, and beef strength. Thus in a degree the prowess of England is related to the number of spinsters in its rural districts! This statement would be true in all seriousness were it not that so many other elements come into the calculation. But whether true or not, it illustrates the way in which causes and effects in biology become intertangled. There was introduced into California from Australia, on young lemon trees, twenty-five years ago, an insect pest called the cottony cushion scale (Icerya purchasi). This pest in- creased in numbers with extraordinary rapidity, and in ten years threatened to destroy completely the great orange orchards of California. Artificial remedies were of little avail. Finally, an entomologist was sent to Australia to find out if this scale insect had not some special natural enemy in its native country. It was found that in Australia a certain species of ladybird beetle attacked and fed on the cottony cushion scales and kept them in check (Fig. 39). Some of these ladybirds (Vedalia cardi- nalis) were brought to California and released in a scale-infested orchard. The ladybirds, having plenty of food, thrived and produced many young. Soon they were in such numbers that many of them could be distributed to other orchards. In two or three years the Vedalias had become so numerous and widely distributed that the cottony cushion scales began to diminish perceptibly, and soon the pest was nearly wiped out. But with the disappearance of the scales came also a disappearance of the ladybirds, and it was then dis- covered that the Vedalias fed only on cottony cushion scales and could not live where the scales were not. So now, in order to have a stock of Vedalias on hand in California, it is necessary to keep protected some colonies of the cottony cushion scale to serve as food. Of course, with the disappearance of the pre- daceous ladybirds the scale began to increase again in various parts of the State, but with the sending of Vedalias to these localities the scale was again crushed. How close is the inter- dependence of these two species! There is little foundation for the current belief that each species of animal has originated in the area it now occupies, for in many cases our knowledge of palaeontology show's the reverse of this to be true. Even more incorrect is the belief that each NATURAL SELECTION; SEXUAL SELECTION 65 species occupies the district or the surroundings best fitted for its habitation. This is manifested in the fact of the extraor- dinary fertility and persistence shown by many kinds of animals and plants in taking possession of new lands which have become, through the voluntary or involuntary interference of man, open to their invasion. Facts of this sort are the "enormous in- crease of rabbits and pigs in Australia and New Zealand, of horses and cattle in South America, and of the sparrows of North America, though in none of these cases are the animals natives of the countries in which they thrive so well ' (Wal- lace). The persistent spreading of European weeds to the exclusion of our native plants is a fact too well known to every farmer in America. The constant moving westward of the white weed and the Canada thistle marks the steady deteriora- tion of our grass fields. The cockroaches in American kitch- ens represent invading species from Europe. The American cockroaches live in the woods. Perhaps a majority of the worst insect pests of the United States are of European or Asiatic origin. Especially noteworthy are cases of this type in Australia and New Zealand. In New Zealand the weeds of Europe, toughened by centuries of selection, have won an easy victory over the native plants. Dr. Hooker states that, in New Zealand " the cow grass has taken possession of the roadsides; dock and watercress choke the rivers ; the sow thistle is spread all over the country, growing luxuriantly up to 6,000 feet; white clover in the mountain dis- tricts displaces the native grasses." The native Maori saying is: " As the white man's rat has driven away the native rat, as the European fly drives away our own, and the clover kills our fern, so will the Maoris disappear before the white man himself." Prof. Sidney Dickinson gives the following notes on the rabbit and other plagues of Australia: "The average annual cost to Australasia of the rabbit plague is 700,000, or nearly $3,500,000. The work which these enormous figures represent has a marked effect in reducing the number of rabbits in the better districts, although there is little to suppose that their extermina- tion will ever be more than partial. Most of the larger runs show very few at present, and rabbit-proof fencing, which has been set around thousands of square miles, has done much to check further inroads. Until this invention began to be utilized it was not uncommon to find 66 EVOLUTION AND ANIMAL LIFE as many as a hundred rabbiters employed on a single property whose working average was from three hundred to four hundred rabbits per day. As they received five shillings a hundred from the station owner, and were also able to sell the skins at eight shillings a hundred, their profession was most lucrative. Seventy-five dollars a week was not an uncommon wage, and many an unfortunate squatter looked with envy upon the rabbiters, who were heaping up modest fortunes, while he himself was slowly being eaten out of house and home. "The fecundity of the rabbit is amazing, and his invasion of remote districts swift and mysterious. Careful estimates show that, under favorable conditions, a pair of Australian rabbits will produce six litters a year, averaging five individuals each. As the offspring them- selves begin breeding at the age of six months, it is shown that, at this rate, the original pair might be responsible in five years for a progeny of over twenty millions. That the original score that were brought to the country have propagated after some such ratio, no one can doubt who has seen the enormous hordes that now devastate the land in certain districts. In all but the remoter sections, the rabbits are now fairly under control; one rabbiter with a pack of dogs supervises stations where one hundred were employed ten years ago, and with ordinary vigilance the squatters have little to fear. Millions of the animals have been killed by fencing in the water holes and dams during a dry season, whereby they died of thirst, and lay in enormous piles against the obstructions they had frantically and vainly striven to climb, and poisoned grain and fruit have killed myriads more. A fortune of 25,000 offered by the New South Wales Government still awaits the man who can invent some means of general destruction, and the knowledge of this fact has brought to the notice of the various colonial governments some very original devices. "Another great pest to the squatters is developing in the foxes, two of which were imported from Cumberland some years ago by a wealthy station owner, who thought that they might breed, and give himself and friends an occasional day with the hounds. His modest desires were soon met in the development of a race of foxes far surpassing the English variety in strength and aggressiveness, which not only devour many sheep, but out of pure depravity worry and kill ten times as many as they can eat. When to these plagues is added the ruin of thousands of acres from the spread of the thistle, which a canny Scot brought from the Highlands to keep alive in his breast the memories of Wallace and Bruce; the well-nigh resistless inroads of furze; and, in New Zealand, the blocking up of rivers by NATURAL SELECTION; SEXUAL SELECTION 67 the English watercress, which in its new home grows a dozen feet in length, and has to be dredged out to keep navigation open, it may be understood the colonials look with jaundiced eye upon suggestions of any further interference with Australian nature. "Not to be outdone by foreign importations, the country itself has shown in the humble locust a nuisance quite as potent as rabbit, fox, or thistle. This bane of all men w r ho pasture sheep on grass has not been much in evidence until within the last few years, when the great destruction of indigenous birds by the gun and by poisoned grain strewn for rabbits has facilitated its increase. The devastation caused by these insects last year was enormous, and befell a district a thousand miles long and two thousand wide. For days they passed in clouds that darkened the earth with the gloomy hue of an eclipse, while the ground was covered with crawling millions, devouring every green thing and giving to the country the appearance of being carpeted with scales. It has been discovered, however, that before they attain their winged state they can easily be destroyed, and energetic measures will be taken against them throughout all the inhabited districts of Australia whenever they make another appearance." The conditions of the struggle for existence are not neces- sarily felt as an individual stress to the individuals which sur- vive. The life they lead is the one for which they are fitted. The struggle is painful or destructive only to those imperfectly adapted. Men in general are fitted to the struggle endured by their ancestors as they are adapted to the pressure of the air. They do not recognize the pressure itself but only its fluctua- tions. Hence many writers have supposed that the struggle for existence belongs to animals and plants and that man is or should be exempt from it. Competition has been identified with injustice, fraud, or trickery, and it has been supposed that it could be abolished by acts of benevolent legislation. But competition is inseparable from life. The struggle for existence may be hidden in social conventions or its effects more evenly distributed through processes of mutual aid, but its necessity is always present. Competition is the source of all progress. The first suggestion of the doctrine of natural selection came to Darwin through the law of population as stated by Thomas Malthus. The law of Malthus is in substance as fol- lows: Man tends to increase by geometrical ratio that is, by multiplication. The increase of food supply is by arithmetical 68 EVOLUTION AND ANIMAL LIFE ratio that is, by addition; therefore, whatever may be the ratio of increase, a geometrical progression will sooner or later outrun an arithmetical one. Hence sooner or later the world must be overstocked, did not vice, misery, or prudence come in as checks, reducing the ratio of multiplication. This law has been criti- cised as a partial truth, so far as man is concerned. This means simply that there are factors also in evolution other than those recognized by Malthus. Nevertheless, Malthus's law is a sound statement of one great factor. And this law is simply the ex- pression of the struggle for existence as it appears among men. The doctrine of organic evolution was first placed on a firm basis by Darwin, because Darwin was the first who clearly defined the force of natural selection. Darwin, however, rec- ognized other factors, known or hypothetical, and was inter- ested more in showing the fact of descent and one cause of modification than in insisting on the all-sufficiency of the cause especially defined by himself. In later times, Weismann and his followers haveMaid more exclusive stress on natural selection and its Allmacht or ex- clusive power in bringing about organic evolution. This view is known as Neo-Darwinism and the school of workers who profess it as Neo-Darwinians. Few investigators question the far-reaching influence of natural selection, but there are many phases in organic evolution which cannot be ascribed to it. Hence the search for other factors has been assiduously prosecuted, and doubts of Darwinism have been widely ex- pressed; but this doubting has been thrown not so much on the Darwinism of Darwin, nor, as a rule, on the law of natural selection, but rather on the Allmacht claimed for it by Weis- mann and his associates. Without attempting any elaborate discussion of questions still far from settled we may venture these suggestions : 1. Given the facts of individual variation, of inheritance, and some check to freedom of migration, natural selection would accomplish some form of organic evolution; species would be formed by the survival of the adapted, adaptations would be perpetuated, and minor differences would develop in time into deep-seated differences. 2. With natural selection alone, however, the actual facts in organic evolution as we know them would apparently not be achieved. NATURAL SELECTION; SEXUAL SELECTION 69 3. In other words, while natural selection furnishes the motive force of change, other influences, extrinsic and intrinsic, help to direct the channels in which life runs. It is necessary to consider other causes for the great body of indifferent characters or traits not produced by adaptation, and apparently not yield- ing either advantage or disadvantage in the struggle for life. 4. The formation of species of animals and plants through natural selection finds an analogy in the formation of rivers through gravitation. Gravitation is the motive power carrying the waters from the uplands to the sea. The courses of streams are determined by a number of minor influences acting in con- currence with gravitation, the final result far more complex than the single cause would produce. 5. In like fashion, w r hile natural selection is the motive element in descent or evolution, the total result is due to a concurrence of causes, and is too complex to be explained by natural selection, by the principle of utility, or the survival of the fittest alone, and the varying effects must be ascribed to a variety of causes. Certain minor traits, as color patterns, relative proportions of parts, survive apparently without special utility, but because these traits were borne by some ancestors or group of ancestors. This has been called the Survival of the Existing. In making up the fauna or flora of any region those organisms actually present when the region is first stocked must leave their qual- ities as an inheritance. If they cannot maintain themselves their breed disappears. If they maintain themselves in iso- lation their characters remain as those of a new species. In hosts of cases, the survival of characters rests not on any special usefulness or fitness, but on the fact that individuals possessing these characters have inhabited or invaded a certain area. The principle of utility explains survivals among com- peting structures. It rarely accounts for qualities associated with geographic distribution. The nature of the animals which first colonize a district must determine what the future fauna shall be. From their actual specific characters, largely traits neither useful nor harmful, will be derived for the most part the specific characters of their successors. It is not essential to the meadow lark that he should have a black blotch on the breast or the outer tail feathers white. Yet all meadow larks have these marks, as all shore larks possess 6 70 EVOLUTION AND ANIMAL LIFE the tiny plume behind the ear. Any character of the parent- stock, which may prove harmful under new relations, will be eliminated by natural selection. Those especially helpful will be intensified and modified. But the great body of characters, the marks by which we know the species, will be neither helpful nor hurtful. These will be meaningless streaks and spots, variations in size of parts, peculiar relations of scales or hair or feathers, little matters which can neither help nor hurt, but which have all the persistence heredity can give. In regard to natural selection our knowledge seems positive. In regard to most other factors of organic evolution we have to deal so far not with clearly demonstrated facts but with "probabilities of a higher or lower order," their value to be ultimately shown by experiment. In this connection the following words of Dr. Edwin Grant Conklin are very pertinent : "On the whole, then, I believe the facts which are at present at our disposal justify a return to the position of Darwin. Neither Weis- mannism nor Lamarckism alone can explain the causes of evolution. But Darwinism can explain those causes. Darwin endeavored to show that variations, perhaps even adaptations, were the result of extrinsic factors acting upon the organism, and that these variations or adap- tations were increased and improved by natural selection. This is, I believe, the only ground which is at present tenable, and it is but another testimony to the greatness of that man of men that, after exploring for a score of years all the ins and outs of pure selection and pure adaptation, men are now coming back to the position outlined and unswervingly maintained by him." Finally we ought not to suppose that we have already reached a satisfactory solution of the evolution problem, or are, indeed, near such a solution. "We must not conceal from ourselves the fact," says Roux, "that the causal investigation of organisms is one of the most difficult, if not the most difficult, problems which the human intellect has at- tempted to solve, and that this investigation, like every causal science, can never reach completeness, since every new cause ascer- tained only gives rise to fresh questions concerning the cause of this cause.' NATURAL SELECTION; SEXUAL SELECTION 71 In order to explain certain important phenomena outside the apparent range of natural selection, a theory of another sort of selective activity is recognized by many biologists. This is the theory of Sexual Selection first propounded by Darwin. FIG. 40. Male and female humming bird; showing sex dimorphism. (After Gould.) Differences between male and female individuals of the same species are the rule rather than the exception (Fig. 40). Many of these differences are what might be called the necessary ones due to the particular functions assumed by each individual in this differentiation of sex. Of this nature are, besides those funda- mental ones of the primary reproductive ones, such others as 72 EVOLUTION AND ANIMAL LIFE those specially connected with the care and rearing of the young; as the mammae of female mammals, the brood pouches of the female kangaroos and opossums, etc. But a moment's reflection calls to mind the existence of a host of other differ- ences between males and females of the same species which plainly have no such immediate relation to the distinct functions or duties assumed by each in the business of production and care of young. For example, the long plume feathers of the male bird of paradise, the curious chitinous horns of the male leaf-chafer beetles (Fig. 41), the brilliant plumage of many male birds as contrasted with the sober dress of the females, and a host of other distinguishing characteristics of the sexes in many animal species. Now these differences are all conveniently named by the phrase " secondary sexual differences," and the explanation of their origin has come to be one of the most FIG. 41. Male and female Scarabeid beetles, Phnneus mexicanus, showing sex dimor- phism; the male with prominent dorsal horn on head. (From specimens.) puzzling of biological problems. The most familiar and, for many years, a widely accepted solution of this problem, is that embraced in the theory of sexual selection proposed and fought for by Darwin and Wallace, but later discarded by the latter of these great naturalists. Before taking up the sexual selection explanation of dis- tinguishing sex characters, it is well to pay a little further attention to the characters themselves. And for this purpose a rough grouping or classification may be attempted. The characters may be of special use to the possessor (male or female) or for the benefit of the young, such as weapons of offense and defense (antlers of male deer, stings of female bee and wasp, tusks of male swine, etc.), or special organs for mat- ing (seizing and holding organs of certain male crabs, suckerlike holding pads on the feet of male water beetles (Fig. 42), or special locomotory organs (presence of wings in the male and their NATURAL SELECTION: SEXUAL SELECTION 73 absence in the female in numer- ous insect species), or special sense organs (the much more expanded antennae of male cecro- pia, promethea, polyphemus, and other bombycine moths, as com- pared with those of the female), or special structures for the care of the young (milk glands of female mammals, brood pouches of female marsupials, pits on the back of the male of the frog Pipa (Fig. 43), for carrying the eggs, etc.), or recognition marks (the eye spots, collars, wing bands, tail blotches, and such other con- spicuous color spots and mark- ings possessed by the males and wanting in the females of various bird species), or, finally, char- acters connected with special habits of one sex differing from those of the other (the pollen baskets and wax plates of the worker female honey bees, the winglessness of certain female parasitic insects, the males being nonparasitic and winged, etc.). The special characters may be apparently for the purpose of attract- ing or exciting the other sex, as the brilliant colors, markings, and other ornamentation of many male birds, some mammals, and some reptiles and very many fishes, and the cries and songs, special odors, and curious antics or dancing of the males of various animals (mammals, birds, spiders, insects, etc.). In many of these cases the special secondary sexual characters appear only during -A male frog, Pipa ih breeding sea son; in others they americana, carrying eggs in pits on its back. (After Darwin.) are persistent. FIG. 42. Fore leg of male water beetle, Dyticus, showing special suckerlike expansion of the leg. (After Miall.) 74 EVOLUTION AND ANIMAL LIFE The characters may also be of the type called reciprocal, t hat is, organs which exist in functional condition in one sex, but in the other appear in rudimentary and often nonfunctional forms, as the reduced horns of female antelopes and goats, the undeveloped stridulating organs of female crickets and katydids, small spurs on the female pheasant, reduced mamma? of male mammals, undeveloped mimicry of male butterflies, etc. FIG. 44. Male (A) and female (B) of the fly, Culotarsa insignia Aid., showing secondary sexual characteristics on the feet of the male. (After Aldrich.) Finally the characters may be indifferent, that is, without any apparent utility; as the reduced wings of numerous female insects, the rudimentary alimentary canal of the male Rota- toria, absence of antlers of female deer, loss of wings in insect females, small differences in size and markings between males and females, slight differences in wing form in hummingbirds, dragon flies, and butterflies, differences in number of tarsal and antennal segments in insects, etc. The explanation of these various differences between males and females plainly cannot be a single one. The extreme vari- ety of the secondary sexual differences of itself makes it neces- sary to find more than one explanation for their existence. To take the most obvious case, it is apparent that the useful characters, such as the fighting antlers of the male deer, can be explained probably by natural selection. At least these char- acters fall readily into line with precisely that type of useful specialization for whose explanation we rely on natural selec- tion. So practically all those secondary sexual characters of our first category, namely, those obviously useful to the pos- sessor or to its young, such as organs of offense and defense, brood pouches, food-producing or gathering organs, special NATURAL SELECTION; SEXUAL SELECTION 75 means of locomotion, etc., may be considered to offer no special problem. Although indeed the reason why these useful char- acteristics should be possessed by but one sex is by no means always, or perhaps even often, plain to us. But the real problem presented by secondary sexual char- acters is that thrust on us by the nonuseful and even appar- ently disadvantageous differences. Why the male bird of para- dise should be decked out in a plumage certain to make it a conspicuous object to every enemy it has, and of a weight and difficulty of manipulation that must mean a constant demand on the strength and attention of the bird, is a question that demands a special answer. In the same case with the bird of paradise are the peacock, the gorgeous male pheasant (Fig. 45) , FIG. 45. Male and female argus pheasant; the male is shown in characteristic "courting attitude." (From Tegetmeier's " Pheasants.") many hummingbirds (Fig. 40), etc. Now to explain these ex- traordinary secondary sexual differences the theory of sexual selection has been devised. This theory, in few words, is that there is practically a competition or struggle for mating, and that those males are 70 EVOLUTION AND ANIMAL LIFE successful in this struggle which are the strongest and best armed or equipped for battle among themselves, or which are most acceptable by reason of ornament or other attractiveness to the females. In the former case mating with a certain female depends upon overcoming in fight the other suitors, the female being the passive reward of the victor; in the second case the female is presumed to exercise a choice, this choice depend- ing upon the attractiveness of the male (due to color, pattern, plumes, processes, odor, song, etc.). The actual fighting among males, and the winning of the females by the victor is an ob- served fact in the life of numerous animal species. But a spe- cial sexual selection theory is hardly necessary to explain the development of the fighting equipment, antlers, spurs, claws, tusks, etc. This fighting array of the male is simply a special phase of the already recognized intraspecific struggle; it is not a fight for room or food, but for the chance to mate. But this chance often depends on the issue of a life and death struggle. Natural selection would thus account for the development of the weapons for this purpose. For the development, however, of such secondary equal char- acters as ornament, whether of special plumage, color, pattern, or processes, and song, and special odors, and "love dancing/ 7 the natural selection theory can in no way account; the theory of sexual selection was the logical and necessary auxiliary theory, and when first proposed it met with quick and wide acceptance. Wallace in particular took up the theory and applied it to ex- plain many cases of remarkable plumage and pattern develop- ment among birds. Later, as he analyzed more carefully his cases, and those proposed by others, he became doubtful, and finally wholly skeptical as to the theory. The theory as proposed by Darwin was based on the follow- ing general assumptions, for the proof of each of which various illustrations w r ere adduced. First, manv secondary sexual / / / characters are not explicable by natural selection; they are not useful in the struggle for life. Second, the males seek the females for the sake of pairing. Third, the males are more abundant than the females. Fourth, in many cases there is a struggle among the males for the possession of the females. Fifth, in many other cases the females choose, in general, those males specially distinguished by more brilliant colors, more conspicuous ornaments, or other attractive characters. Sixth, NATURAL SELECTION; SEXUAL SELECTION 77 many males sing, or dance, or otherwise draw to themselves the attention of the females. Seventh, the secondary sexual characters are especially variable. Darwin believed that he had observed certain other conditions to exist which helped make the sexual selection theory probable, but the conditions noted are sufficient if they are real. Exposed to careful scrutiny and criticism, the theory of sexual selection has been relieved of all necessity of explaining any but two categories of secondary sexual characters; namely, the special weapons borne by males, and special ornaments and excitatory organs of the males and females. For examination has disclosed the fact that males are not alone in the possession of special characters of attraction or excitation. Regarding these two categories Plate in his able recent defense of Darwinism, says "the first part of this theory, the origin of the special defensive and offensive weapons of males through sexual selec- tion, is nearly universally accepted. The second part of the theory, the origin of exciting organs, has given rise to much controversy. Undoubtedly the presumption that the females compare the males and then choose only those which have the most attractive colors, the finest song, or the most agreeable odor, presents great difficulties, but it is doubtful if it is possible to replace this explanation by a better." Some of these diffi- culties may be briefly enumerated. The theory can be applied only to species in which the males are markedly more numerous than the females, or in which the males are polygamous. In other cases there will be a female for each male whether he be ornamented or not; and the unor- namented males can leave as many progeny as the ornamented ones, which would prevent any accumulation of ornamental variations by selection. As a matter of fact, in a majority of animal species, especially of the higher vertebrates, males and females exist in approximately equal numbers. Observation shows that in most species the female is wholly passive in the matter of pairing, accepting the first male that offers. Note the cock and hens in the barnyard, or the fur seal in the rookeries. Ornamental colors are as often a characteristic of males of kinds of animals in which there is no real pairing, as among those which pair. How explain by sexual selection the remark- able colors in the breeding season of many fishes, in which the 7S EVOLUTION AND ANIMAL LIFE female never, perhaps, sees the male which fertilizes her dropped eggs? In many fishes the spring ornamentation of the males is just as marked and just as brilliant as in the birds or other animals of much higher intelligence and corresponding power of choice. Witness the horned dace, chubs, and stone rollers in any brook in spring. Choice on a basis of ornament and attractiveness implies a high degree of aesthetic development on the part of the females of animals of whose development in this line we have no other proof. Indeed, this choice demands aesthetic recognition among animals to which we distinctly deny such a development, as the butterflies and other insects in which secondary sexual characters of color, etc., are abundant and conspicuous. Sim- ilarly with practically all invertebrate animals. Further, in those groups of higher animals where aesthetic choice may be presumed possible, we have repeated evidence that preferences vary with individuals. Certainly they do with men, the animal species in which such preferences certainly and most conspicu- ously exist. In some human races hair on the face is thought beautiful; in others, ugly. Besides even if we may attribute fairly a cer- tain amount of aesthetic feeling to such animals as mammals and birds, is this feeling so keen as to lead the female to have preference among only slightly differing patterns or songs? Yet this assumption is necessary if the development of ornament and other attracting and exciting organs is to be explained by the selection and gradual accumulation through generations of slight fortuitously appearing fluctuating variations in the males. There are actually very few recorded cases in which the ob- server believes that he has noted an actual choice by a female. Darwin records eight cases among birds. Since Darwin, not more than half a dozen other cases, all doubtful, have been noted. Also a few instances, all more illustrative of sexual excitation of females resulting from the perception of odor or actions, than any degree of choice on their part, have been listed. In numerous cases the so-called attractive characters of the males, described usually from preserved (museum) specimens, have been found, in actual life, to be of such a character that they cannot be noted by the female. For example, the brilliant colors and curious horns of the males of the dung beetles are, in NATURAL SELECTION; SEXUAL SELECTION 79 life, ahvavs so obscured bv dirt and filth that there can be no . */ question of display to the female eye about them. The dancing swarms of many kinds of insects are found to be composed of males alone w r ith no females near enough to see; it is no case of an excitatory flitting and whirling of many males before the eyes of the impressionable females. Of many male katydids singing in the shrubbery will not for any female that particular song be loudest and most convincing that proceeds from the nearest male, not the most expert or the strongest stridulator? Simi- larly with the flitting male fireflies; will not the strongest gleam be, for any female, that from the male which happens to fly nearest her, and not from the distant male with ever so much better, stronger light? Even in the human species, propin- quity is recognized as the strongest factor in the choice of mates. Several other serious objections can also be urged against the sexual selection theory, but the most important one of them all is that all the evidence (though it is little in quantity as yet, although of good quality) based on actual experiment, is strongly opposed to the validity of the assumption that the females make a choice among the males based on the presence in the males of ornament or attractive colors, pattern, or special structures. Such experiments have been undertaken by Diiri- gen and Douglas w r ith lizards, and by Mayer with moths. It must be said, however, in closing this brief discussion of the sexual selection theory, that no replacing or substitute theory of anything like the same plausibility has yet been offered to take its place. There is no question that, in many cases, brilliancy of breeding colors, development of processes, and the like, is often correlated with superior vigor. This is especially true among fishes and birds. This reason could, however, not at all account for such structures as the highly specialized stridu- lating organs of certain insects. The problem of the secondary sexual characters, especially of those which seem to stand in opposition to the natural selection theory, is one of the most pressing in present-day biology. CHAPTER VI ARTIFICIAL SELECTION We can command Nature only by obeying her laws. This prin- ciple is true even in regard to the astonishing changes which are super- induced in the qualities of certain animals and plants in domestication and in gardens. LYELL. VARIETIES are the product of fixed laws, never of chance. With a knowledge of these laws we can improve the products of nature, by employing nature's forces in ameliorating old or producing new species and varieties better adapted to our necessities and tastes. Breeding to a fixed line will produce fixed results. There is no evidence of any limit in the production of variation through artificial selection; especially if preceded by crossing. LUTHER BURBANK. THE name Selection has been long used for the process by which breeds or races of domestic animals or plants have been formed in the past, and for the process by which the skill- ful breeder can develop new forms at will. This latter proc- ess, called by Youatt "the magician's wand/' by which the breeder can summon up any form of animal which may meet his needs or please his fancy, has been especially designated as Artificial Selection. By it we have derived all of our famil- iar hosts of varieties of domesticated animals and plants. The similar process in nature was accordingly designated by Darwin, Natural Selection. It refers to the development or increase of traits adaptive or advantageous in the life of a species, through the survival for reproduction of a greater proportion of individuals possessing the characters in question than of those which do not. In any race, it is the individual which succeeds in reaching maturity which determines the future of the race. The qualities of the multitude which die prematurely are naturally not repeated in heredity. In general, the forms pro- 80 ARTIFICIAL SELECTION 81 *. duced in artificial selection are not those which could arise or even exist in nature. In nature, hardiness or power of resistance in competition or the struggle for existence is all important. In artificial selection stress is laid chiefly on char- acters useful or attractive to man. From the standpoint of self dependence, the improvements due to artificial selection constitute a sort of retrogression. In general, the production of a new race of animals or plants in domestication is the outcome of the work of "~1 a number of factors, in which human or artificial selection plays a leading part, a part which in- creases in importance with the degree of intel- ligent choice concerned in it. In the formation of a new race of animals or plants, we may have the following stages or fact- ors : 1. Unconscious se- lection with more or less complete isolation. 2. Conscious selec- tion of the most desira- ble individuals. 3. Conscious selec- tion directed toward definite or special ends. 4. Crossing with other races or with other species (known as hybridizing), in order to increase the range of variation, or to add or combine certain specific desirable qualities or to elimi- nate those undesirable, this accompanied by conscious selection directed toward definite ends. On this series of processes breeding as a fine art must depend. Taking as an illustration some of the breeds of medium wool sheep found in Southern England: we have (1) the domes- tication of sheep in each of the different counties or natural FIG. 46. White-crested black Polish cock. (After photograph.) 82 EVOLUTION AND ANIMAL LIFE areas. In the beginning men are satisfied with sheep as sheep. Little attention is paid to the distinction among individuals. Those which are feeble, ill nourished, untamable, scant-fleeced, or otherwise unfit will be eliminated, a process which will tend to improve the stock, without giving the race distinctive quali- ties, except as compared with the wild original. To form dis- tinct races, the factor of isolation must enter. Those in one county, for example, will be, at the beginning, somewhat different from those in an- other. Each herd will show its own traits in time, these due primarily to differences in the original stock, secondarily to the pre- dominance of one form of variation over others. Ex- changes of sheep will, by cross-breeding, tend to unify the type of sheep in some one county, or on some side of a barrier across which sheep are not driven. With this, there will be also variations in the character of the unconscious selec- tion. One type of sheep will flourish in a meadow county, another on a moor, and still another on the rocky hills. At any rate, as the environment varies, so will the character of the selection. Thus as a final result, in Southern England, the Southdown sheep of Sussex have tawny faces and legs; the sheep of Hampshire have black faces, ears, and legs, with a black spot under the tail; this black spot is lacking in the sheep of Devon. In the Cheviot sheep the face and ears are white, the head free from wool, while the ears, unlike those of most of the others, stand erect. In the dun-faced Shropshire sheep, the faces are more or less covered by wool. All these are hornless, while the more primitive Dorset sheep with white face and ears FIG. 47. Silver-laced Wyandotte cockerel. (After photograph.) ARTIFICIAL SELECTION 83 have almost always small curved horns which are white, not black, as in the still more primitive Irish breed. Most of these distinctive traits offer neither advantages nor disadvantages either to the sheep or its owner. They are nonadaptive or N FIG. 48. Typical Dorset ewe, horned. (After Shaw.) FIG. 49. Polled Welsh sheep, a primitive type, lean and scant wooled. (After Youatt.) indifferent characters. These characters are therefore asso- ciated with the hereditary traits of the original stock. They are preserved through segregation and they are lost when herds from different counties freely intermingle. Free interbreeding would give a new and relatively uniform race of sheep over the whole area occupied by these separate breeds. 84 EVOLUTION AND ANIMAL LIFE At this point we may conceive that (2) conscious selection of the more desirable individuals appears. Through its agency, Hampshire, Shropshire, Cheviot, and Southdown sheep alike, and the others in their degree, tend toward larger size, more wool, plumper bodies, earlier maturity, greater docility, greater fertility, or whatever virtues the average shepherd may prize in a sheep. While in race traits, the breeds (uncrossed) tend to diverge from one another, in these adaptive qualities, their tendency is to run parallel or even to converge toward greater resemblance. With conscious selection (3), there is first a tendency to emphasize the qualities of desirable breeds. If, for example, FIG. 50. Typical Southdown ewe. (After Shaw.) the Hampshire is a favorite breed, the individuals showing most distinctly black ears, legs, and face will be preferred by breeders to those having these parts pale. Again, new points of special excellence will appear in the breed and these will be deliberately emphasized, and perhaps by continuous selection a new breed will be formed having one or more of these as a distinctive trait. According to Somerville, one may chalk out on a wall any form or type of sheep he may like, and then in time reproduce it through selective breeding. In Nova Scotia, Mr. A. Graham Bell has developed a new breed of sheep by selection, its distinctive character being in the increased milk flow, with an increased number of teats. ARTIFICIAL SELECTION 85 At Chillenham, in England, is still preserved a herd of the original wild white English cattle, from which most or all of the British breeds are said to be descended. It is stated that Lord Cawdor has offered to reproduce this herd, by selection alone, in three or four generations, using the relatively primitive Welsh cattle as his base of operations. In general, those characters which are usually affected by selection, whether natural or artificial, are characters of degree. They are matters of more or less, a greater or less degree of strength, swiftness, size, endurance, fertility, capacity to lay FIG. 51. Typical American merino ewe, a highly specialized breed with fine close-set wool. (After Shaw.) on fat, docility, intelligence, or of whatever it may be. Under ordinary conditions these characters selected are not traits of quality. They do not represent a new thing, a new acquisition, but a different degree of development of an old one, or, at most, a change in their relative arrangement, an alteration of bio- logical perspective. The characters which distinguish true breeds as well as true species are not of this order. They are in their essence quali- tative and not quantitative. They are not, as a rule, adaptive. One set of species or race traits is as good as another, if the good qualities or adaptive qualities are represented in an equally 7 FIG. 52. Heads of various British breeds of domestic cattle, showing variations in shape of head and condition of horns. (After Romanes.) FIG. 53. Various races of pigeons, all probably descended from the European rock dove, Columba lima. (After Haeckel.) EVOLUTION AND ANIMAL LIFE high degree. The Southdown sheep are valued not for their Southdown traits, but for the excellence of their mutton, a trait with which middle length of wool, tawny legs, naked faces, drooping ears, and absence of horns have nothing necessarily to do. We value these race traits only for the other qualities FIG. 54. Skulls (in longitudinal section) of two breeds of domestic fowl, showing the large modification in the cranium: upper figure, Polish cock; lower figure, Cochin cock. (After Darwin.) which have been in a high degree associated with them in the heredity of the race. Under crossing and selection, much bolder attempts are possible. When parents widely divergent are crossed, many very different results are attained. In general the progeny, at least after the first generation, diverge very widely from one another. Some will have the good traits of both parent stocks ; some will have the undesirable ones; some will show a mosaic of parental characters; some a more or less perfect blend of char- acters, this blend being definable as a finer type of mosaic. Some will diverge widely from either stock, often showing traits either remotely ancestral or wholly new. From desirable vari- ations of this sort new races may be developed, each succeeding generation tending to give greater fixity. In general, wide crosses or hybrids are more successful with plants than with animals, because the mutual adjustment ARTIFICIAL SELECTION 89 traits become more important in the more highly specialized organisms. Among animals, related species often cannot be crossed at all; the germ cells refuse to intermingle. Sometimes there is a very imperfect mingling and the resultant animal is divided within itself and does not live long. An example of this is seen in Dr. Moenkhaus's cross of the silverside (Menidia) with the killifish (Fundulus). The unmixed chromosomes of the germ-cell nucleus are seen unblended, through several segmen- tations of the egg. In the case of the mule, the cross of the horse with the ass, the hybridization is readily effected, but the resultant offspring is sterile. Presumably the hereditary difference in the repro- FIG. 55. Wild boar contrasted with modern domestic pig. (After Romanes.) ductive organs in the two parental strains is too great to allow the normal development of generative organs in the progeny. In general, crosses between closely related species are fertile, the degree of fertility being less as the parent species are more widely differentiated. Among animals, any great difference 90 EVOLUTION AND ANIMAL LIFE between the parent stocks renders hybridization impossible. But among plants, when hybrids are actually formed, fertility rather than sterility may be taken as the rule. This is the case with Mr. Luther Burbank's Primus berry, a cross between the Siberian raspberry (Rubus cratcegifolius) and the Calif ornian dew- berry or blackberry (Rubus ursinus). In this form the fruit excels in size and abundance either parent, and the hybrid breeds true from the seed, and ripens before either parent begins to bloom. It was fixed in the first generation, being in this re- gard a rare exception to the general rule of the aberration of hybrids. In this and in other respects the Primus, known to be an intentional cross of t\vo species, behaves as though it were a distinct species. In like fashion, the Logan berry, the product of an accidental cross at Santa Cruz, in California, of the European raspberry with the native dewberry, behaves also like a distinct species, and is also much superior in productive- ness to either parent. The fine art of the horticulturist is seen in the selection and fixing of the variations produced by crossing and hybridization. While most of the forms thus obtained are worthless, a few will show 7 decided advances. Often as much progress may be made in a single successful cross or hybridization as in a dozen or even a hundred generations of pure selection. By selection alone, however, important results may be obtained, with time and patience. Given a variation in a de- sired direction there is perhaps no actual limit bounding the possibilities of selection unless arising through external or me- chanical conditions. Thus selection for speed of horses is limited by the strength of the material of which a horse's leg is com- posed. The increase in the number of petals may be limited by the space on which petals can stand, and the number of leaflets in a leaf by the length of the rhachis. Still there are known cases in which a positive limit has been reached in at- tempting to modify organisms by selection alone. Accidental crossing within a species may form a useful basis for selection. Thus from the seeds in a single potato ball of the Early Rose variety, crossed by insects with an unknown parent, Mr. Luther Burbank reared potatoes of many different sorts: red potatoes, white potatoes, elongate potatoes, potatoes rela- tively smooth and potatoes all eyes and "eyebrows." Among all these, one form, long, white, smooth, and mealy, seemed far ARTIFICIAL SELECTION 91 superior to the others. From the subdivision of the tubers of this seedling arose the Burbank potato, the most valuable variety in its economic relations now cultivated in America. But with the choice of this form for preservation, selection ceased, as all plants of the Burbank potato in cultivation are FIG. 56. Heads of timothy, showing improvement by selection. (After Hays.) subdivisions of a single original plant. New forms would come from further selection of the Burbank potato seed. As illustrations of the more complex art of hybridization and selection, we give in the following paragraphs a brief account of the work of Luther Burbank, the most ingenious and successful of all recent experimenters in plant breeding. 92 EVOLUTION AND ANIMAL LIFE Burbank has originated and introduced a remarkable series of plums and prunes. No less than twenty varieties are included in his list of offerings, and some of them, notably the Gold, FIG. 57. Four types of plumcot: colors, red and yellow of various shades. (Photo- graph by Burbank; about one-half diameter.) Wickson, Apple, October Purple, Chalco, American, and Climax plums and the Splendor and Sugar prunes, are among the best known and most successful kinds now grown. In addition, he is now perfecting a stoneless plum, and has created the inter- FIG. 58. Seedlings from one hybrid plum. (After photograph by Burbank.) esting plumcot by hybridizing the Japanese plum and the apricot. The plumcot, however, has not yet become a fixed variety and may never be, as it tends to revert to the plum. ARTIFICIAL SELECTION 93 The stoneless and seedless plum is being produced by selection from the crossing of the descendants of a single fruit in a small wild plum with only part of a stone with the French prune; the percentage of stoneless fruits is gradually increasing with succeeding generations. The sugar prune, which promises to supplant the French prune in California, is a selected product of a second or third generation variety of the Petit d'Agen, a very variable French prune. The Bartlett plum, cross of the bitter Chinese simoni and the Delaware, a Burbank hybrid, has a fragrance and flavor extraordinarily like that of the Bartlett pear. The Climax is a cross of the simoni and the Japanese triflora. The Chinese simoni produces almost no pollen, only a few grains of it ever having been obtained, but these few grains have en- abled Burbank to revolu- tionize the whole plum shipping industry. Most of Bin-bank's plums and prunes are the result of multiple crossings, in which __J the Japanese Satsuma has FlG 59 ._ The larser plum is the direct seed . played an important part. ling of the smaller, produced by crossing Hundreds of thousands of the tri f Iiata ( Ja P an > P Ium and the little . maritima (Atlantic Coast) plum. (After seedlings have been grown photograph by Burbank.) and carefully worked over in the twenty years' experimenting with plums, and single trees have been made to carry as many as 600 varying seed- ling grafts. Burbank has originated and introduced the Van Deman, Santa Rosa, Alpha, Pineapple "No. 80," the flowering Dazzle, and other quinces; the Opulent peach, cross bred from the Muir and Wager; the Winterstein apple, a seedling variety of the Gravenstein; and has made interesting, although not profitable, crosses of the peach and nectarine, peach and almond, and plum and almond. Next in extent, probably, to his work with plums is his long and successful experimentation with berries. This work has extended through twenty-five years of constant attention, has involved the use of forty different species of Rubus, and has 94 EVOLUTION AND ANIMAL LIFE resulted in the origination and introduction of a score of new commercial varieties, mostly obtained through various hybridi- zations of dewberries, blackberries, and raspberries. * fc FIG. 60. Seedlings of one kind of hybrid plum: colors almost black, deep crimson, light crimson, scarlet, deep yellow, and shades of orange and yellow, green striped, spotted and speckled; long and short stems; sweet, sour, bitter, good, bad, and indifferent, firm and soft; flesh, yellow, white, pink, red, crimson, striped, and .shaded; stones of various shapes and sizes, large, small, oval, round, of different colors, some clingstones, some freestones; foliage varying as much as the rest, and growth from short and stalky and dwarf to rampant exuberance. (.Photograph by Bui-bank; about one-quarter diameter.) ARTIFICIAL SELECTION 95 Among these may especially be mentioned besides the Primus already spoken of, the Iceberg, a cross-bred white blackberry derived from a hybridization of the Crystal White (pistillate parent) with the Lawton (staminate parent), with FIG. 61. Seedlings of the Japanese quince, Pyrus Juponica: colors, orange yellow, or almost white, with crimson dots and splashes. (From photograph by Burbank.) 96 EVOLUTION AND ANIMAL LIFE beautiful snowy-white berries so nearly transparent that the small seeds may be seen in them; the Japanese Golden May- berry, a cross of the Japanese R. palmatus (with small, tasteless, dingy yellow, worthless berries) and the Cuthbert, the hybrid growing into treelike bushes, six to eight feet high, and bearing- great, sweet, golden, semitranslucent berries which ripen before strawberries; the Paradox, an oval, light-red berry, obtained in the fourth generation from a cross of Ciystal White Blackberry and Shaffer's Colossal Raspberry. While most of the plants from this cross are partly or w r holly barren, this particular out- come is an unusually prolific fruit producer. An interesting feature of Mr. Burbank's brief account, in his FIG. 62. Three walnuts: at left Japanese walnut, at right English walnut, and in middle a hybrid of these two. (From photograph by Burbank.) "New Creations' 7 catalogue of 1894, of the berry experimenta- tion is a reproduction of a photograph showing "a sample pile of brush 12 feet wide, 14 feet high, and 22 feet long, containing 65,000 two- and three-year old seedling berry bushes (40,000 Blackberry X Raspberry hybrids and 25,000 Shaffer X Gregg hybrids), all dug up with their crop of ripening berries.' 3 The photograph is introduced to give the reader some idea of the w r ork necessary to produce a satisfactory new race of berries. "Of the 40,000 Blackberry-Raspberry hybrids of this kind 1 Paradox ' is the only one now in existence. From the other 25,000 hybrids two dozen bushes were reserved for further trial." ARTIFICIAL SELECTION 97 Leaving Burbank's other fruit and berry creations un- noticed, we may refer to his curious cross-bred walnut results (Fig. 63), the most astonishing of which is a hybrid between FIG. 63. At left, leaf of English walnut, Jnglans regia; at right California black walnut, Juglans californica; and in the middle a leaf of the hybrid Paradox, first generation. (From photograph by Burbank.) Juglans Californica (staminate parent) and J. nigra (pistillate parent), which grows with an amazing vigor and rapidity, the trees increasing in size at least twice as fast as the combined growth of both parents, and the clean-cut, glossy, bright green 98 EVOLUTION AND ANIMAL LIFE leaves, from two to three feet long, having a sweet odor like that of apples. This hybrid produces no nuts, but curiously enough the result, of the reverse hybridization (i. e., pollen from nigra on FIG. 64. Hybrid seedling cactuses, Opuntia, after six months growth, showing num- erous varieties. (From photograph by Burbank.) f pistils of Calif ornica) produces in abundance large nuts of a quality superior to that possessed by either parent. Of new vegetables Burbank has introduced besides the Bur- bank and several other new potatoes, new tomatoes, squashes, asparagus, etc. Perhaps the most interesting of his experiments in this field is his attempt, apparently destined to be successful, to produce a spineless and spiculeless and unusually nutritious cactus (the spicules are the minute spines, much more danger- ous and harder to get rid of than the conspicuous long thornlike spines) edible for stock, and indeed for man. This work is chiefly one of pure selection, for the cross-bred forms seem to tend strongly to revert to the ancestral spiny condition. Among the many new flower varieties originated by Bur- bank may be mentioned the Peachblow, Burbank, Coquito, and Santa Rosa roses, the Splendor, Fragrance (a fragrant form), and Dwarf Snowflake callas, the enormous Shasta and Alaska ARTIFICIAL SELECTION 99 daisies, the Ostrich plume, Waverly, Snowdrift, and Double clematises, the Hybrid Wax Myrtle, the extraordinary Nico- tunia, a hybrid between a large, flowering Nicotiaria and a Petunia, several hybrid Nicotianas, a dozen new gladioli and ampelopses, several amaryllids, various dahlias, the Fire poppy (Fig. 65), (a brilliant, flame-colored variety obtained from a cross of two white forms), striped and carnelian poppies, and a blue Shirley (obtained by selection from the Crimson field poppy of Europe), the Silver Line poppy (obtained by selection from an individual of Papaver umbrosum, showing a streak of silver FIG. Go. At left, leaf and flower of the pale yellow poppy, Papaver pilusum; at right leaf and flower of the snow white poppy, Papaver somniferum; and in the middle, leaf and fire-crimson flower of the first generation hybrid of these two. (From photograph by Burbank.) inside) with silver interior and crimson exterior, and a Crimson California poppy (Eschscholtzia) , obtained by selection from the familiar golden form. Perhaps his most extensive experimenting with flowers has 100 EVOLUTION AND ANIMAL LIFE been done in the hybridizing of lilies, a field in which many plant breeders have found great difficulties. Using over half a hun- dred varieties as basis of his work Burbank has produced a mar- velous variety of new forms (Fig. 66) . " Can my thoughts be imagined," he says, in his " New Creations " of 1893, "after so many years of patient care and labor [he had been working over sixteen years], as, walking among them [his new lilies] on a dewy morning, I look upon these new forms of beauty, on which FIG. 66. An improved seedling lily with two petals. by Burbank.) (From photograph other eyes have never gazed? Here a plant six feet high with yellow flowers, beside it one only six inches high with dark red flowers, and further on one of pale straw, or snowy white, or with curious dots and shadings: some deliciously fragrant, ARTIFICIAL SELECTION 101 others faintly so; some with upright, others with nodding flowers; some with dark green, woolly leaves in whorls, or with polished light green, lancelike, scattered leaves." FIG. 67. An extraordinary apple, one-half being bright red and sour, and the other half greenish yellow and sweet; note in photograph the sharp line of demarkation between the different halves. (From photograph by Burbank.) So far no special reference has been made to the more strictly scientific aspects of Burbank's work. Burbank has been primarily intent on the production of new and improved fruits, flowers, vegetables, and trees for the immediate benefit of mankind. But where biological experimentation is being carried on so extensively it is obvious that there must be a large accumulation of data of much scientific value in its rela- tion to the great problems of heredity, variation, and species- forming. Burbank's experimental gardens may be looked on, from the point of view of the biologist and evolutionist, as a great laboratory in which, at present, masses of valuable data are, for lack of time and means, being let go unrecorded. Of Burbank's own particular scientific beliefs touching the "grand problems' of heredity we have space to record but two: first, he is a thorough believer in the inheritance of ac- quired characters, thus differing strongly from the Weismann school of evolutionists; second, he believes in the constant 8 102 EVOLUTION AND ANIMAL LIFE mutability of species, and the strong individuality of each plant organism, holding that the apparent fixity of characteristics is a phenomenon wholly dependent for its degree of reality on the FIG. 08. Seedlings of the Williams early apple, showing all the colors ever found in apples. (From photograph by Burbank.) length of time this characteristic has been ontogenetically re- peated in the phylogeny of the race. In like fashion to this working with plants, breeds of animals have been established by crossing and selection with a ARTIFICIAL SELECTION 103 view to the preservation of the best traits of both. In estab- lishing the stock farm at Palo Alto, Leland Stanford had the FIG. 69. Improvement in geranium: at left, the original wild form, and at right the latest improved form. (From photograph by Burbank.) conception of strengthening the trotting horse by a cross with the larger running horse or thoroughbred. The result was the formation of a peculiar type of horse, large, strong, supple, FIG. 70. Sports found among crossed amaryllids, the size and form markedly changed; the flowers are three inches in diameter. (From photograph by Burbank.) and intelligent, very clean of limb and sleek of coat. This group of horses held for some vears the world's records for 104 EVOLUTION AND ANIMAL LIFE speed in their various classes and ages, and the experiment was in the highest degree successful. In one sense such at- tempts are not experiments. The skillful breeder knows that out of the many combinations possible in crossing, some few will fall in line with his plans. He has only to preserve these, and to clinch them by in-and-in or segregated breeding to bring about a result he may have deemed possible or desir- able. It is possible, by intentional selection, to turn a nori- essential or race character into a selective or adaptive one. The Hampshire sheep have black ears, but by persistent se- lection the ears could probably be made white. Probably also the horns of the Dorsets could be bred on Hampshires by making use of possible occasional reversions to the horned stock. This result could be attained very rapidly by a cross- ing with Dorset stock, but this triumph of the breeder's art has rarely any homologue in the wild state or in the condition of unconscious selection. When selection ceases, the adaptive characters are likely to decline or disappear. Under cessation of selection, called by Weismann panmixia, no premium is placed on traits of excel- lence, from the human standpoint, such as long wool, plump- ness or symmetry of form; and only the purely vegetative ad- vantages of the individual count. But while the traits of excellence disappear, the race traits or nonadaptive characters persist unchanged. A herd of neglected Hampshire sheep is still a herd of Hampshires. The black face, ears, and legs remain black, with no tendency to fade. When the worst individuals are selected for breeding, we have the reversal of selection. A flock of Hampshire culls, feeble, loose-jointed, scant-wooled, unsymmetrical, could be used in breeding, and the adaptive characters usually sought for could be bred out of them. But they would still be Hamp- shires, for the hereditary characters which had persisted with- out the aid of selection would persist after selection ceases or even if it is reversed. When these same characters are made the object of selection, they are subject to the same laws as ordinary adaptive characters. What is true of a breed of sheep a product of geographical isolation with segregative breeding is true in a general way of any wild species of animals or plants. Its adaptive characters are due to natural selection. These change more rapidly than ARTIFICIAL SELECTION 105 the nonadaptive characters, and respond more readily to the conditions of panmixia or of reversal of selection. In matters of breeding we must distinguish between animals actually best and those potentially best. An animal is at its actual best when in prime condition, at the prime of its life. Another of far finer heredity, of far stronger ancestry, may be at any given time actually the inferior of the first. It may be too old, too young, in too poor condition to represent its own best status. It is generally recognized that, for all breeding purposes, the animal potentially best is superior to one which, otherwise inferior, may be actually best at the time. The tendency of heredity is to repeat the traits of the ideal individuals, which the parents ought to have been. More exactly, the tendency of heredity is to produce individuals which, under like conditions of food and environment, would develop as the parents have developed. But it is also recognized that the actual physical condition of the parent affects the offspring. A sick mother is likely to bear an enfeebled child. Immature or declining sires do not beget offspring as strong as those begotten by them when they are in perfect strength and health. In this matter, apparently, we have to deal with two different elements, as Weismann and others have pointed out. The first is true heredity, the quality of the germ cell, which is not affected by the condition of the parent. Weak or strong, the offspring is of the same kind or type as the parentage. The second element has been called Transmission. Its relations are with vegetative development. The embryo is ill nourished by the sick mother, and it enters on life with lowered vigor. The momentum, if we may use such a figure of speech, is reduced from the first, and the lost vitality may never be regained. The defects of the male parent are perhaps of less moment, but whatever their nature their results would be of the same kind. They would not enter into the heredity of the offspring, but they might play a large part in retarding its development. In the category of transmission, not of heredity, would belong the theme of Ibsen's " Ghosts" (Gjengangere) , the development of softening of the brain in the son of a debauchee, the alleged cause being that the father's nervous system was vermoulu (worm-eaten), if we are to accept the ghastly drama as an exposition of possible facts. 106 EVOLUTION AND ANIMAL LIFE The role played by the phenomena of transmission as distin- guished from that of heredity has never been clearly ascertained. Many eminent writers ascribe to it a large importance. It is a central element in Mr. Casper RedfiekTs theory of heredity, and he brings together a considerable array of facts and statis- tics to justify his conclusions. But the value of statistics in such matters is easily exaggerated, because of the difficulty in ascertaining the real causes behind the phenomena we try to record. It is fair to say as a broad proposition that, as a sound mind requires a sound body, soundness both of mind and body are factors in giving to offspring the best possible start in life. The heredity unchanged, there is still a great value in vigor of early development. The relation of these matters to the theory of organic evo- lution is mainly here: artificial selection as a process is of the same general character as natural selection; both represent a form of isolation or segregation, which prevents indiscriminate mating, and which holds certain groups of individuals as the agents of reproduction of the species within a given time or in a special area. Artificial selection intensifies useful or adaptive characters, using these words in a broad sense. At the same time, it per- petuates a series of characters, in no wise useful, and in no fashion adaptive. The,3e characters remain unchanged for long periods, and hence have more value in race distinction or in classification than the strictly adaptive characters have. A Southdown sheep is plump and fat, on the whole perhaps more so than any other type of sheep. Nevertheless, it is not by its plumpness that we know a Southdown. It is rather by the character of its wool, the color of its face and feet, the form of its head. So it is with breeds and races generally. They are formed primarily by isolation in breeding, the separation of a few from the many by geographical or similar causes, by the perpetuation of the traits of these few (the "survival of the existing"), all this being modified by the new range of natural and artificial selection and the new reactions under the varying conditions of a new environment. It interests us to know that a similar process takes place in nature. Geographical and topographical barriers are crossed in migration. These isolate a portion of a species under new conditions, with new reactions to the environment, and a ARTIFICIAL SELECTION 107 new range of natural selection. Adaptive characters change rapidly, and in ways more or less parallel, with similar altera- tions in related species. Characters nonadaptive, often slight in appearance and bearing no relation to the life of the animal, become slowly but surely fixed as characters of the species. As two closely allied breeds of animals are never found in the same region unless purposely restrained from free interbreeding, so two closely related species never develop in the same breeding area. As the nearest relative of some given breed of domestic animals is found in a given region nearly related geographically, so is the nearest relative to any given wild species found, in most cases, not far away. It is to be looked for on the other side of some geographic, topographic, or climatic barrier. In other words, the interrelation of variation, heredity, geographic isolation and environmental features generally seems to be the same in the formation of domestic races as in that of the formation of natural species. The principal new element intro- duced in the art of selective breeding is that of purposeful crossing, the removal of the barriers which separate well- differentiated forms, for the purpose of beginning a new series to be selected toward a predetermined end. It has been recently repeatedly stated that most races of domesticated animals or plants find their origin in a mutation or saltation of some sort. In our judgment, there is not suffi- cient evidence to prove this view. There are few cases of either races or species known to have originated in this way. That such is in fact the general law of race or species origin, we see little reason to believe. One of the few well-known illustrations of race-forming through saltation is that of the Ancon sheep. In 1791, in Massachusetts, a ram was born with unusually short legs. As this character was useful, preventing the sheep from leaping over stone walls, the owner of this sheep used the ram for breeding purposes, and succeeded in isolating a short-legged strain of sheep known as the Ancon sheep. So far as known to us, this type of sheep differed in this character alone from the common sheep of Connecticut. With the later advent of the more heavy-wooled, and therefore more profitable, Merino, the Ancon sheep disappeared. A recent similar case of race origin from a prepotent sport is that of the polled Here- fords arising in Kansas from a hornless Hereford bull. CHAPTER VII VARIOUS THEORIES OF SPECIES-FORMING AND DESCENT CONTROL The four factors named, variation, inheritance, selection, and sepa- ration, must work together in order to form different species. It is impossible to think that one of these should work by itself or that one could be left aside. ORTMANN. As mentioned in the introductory chapter on the factors of evolution (Chapter IV) , and as referred to several times in the chapter on natural selection, the factor of the segregation or isolation of groups of individuals must be taken into account in any discussion of species-forming causes. This factor has long been recognized by biologists, that phase of it, and undoubtedly the most important of its several phases, called geographic or topographic isolation or segregation being very clearly stated and its importance emphasized by Moritz Wagner in 1868. Alfred Russel Wallace gave much attention, in his years of active investigation, to the general subject of geographical distribution, and was a pioneer in calling the attention of natu- ralists to the great significance, in the light of the evolution theory, of the facts of the geographical distribution of both animals and plants. To-day, especially among American biologists, the factor of topographic segregation is recognized as one of the most important of species-molding influences. Indeed it seems self-evident to many naturalists that natural selection is impotent as an actual cause of species-forming with- out some effective sort of isolation factor to assist it. Because of the importance in the eyes of present-day naturalists of the geographic isolation factor we have given (Chapter VIII), a brief special discussion of this factor. In addition, in Chapter XIV, will be found a discussion of the more general subject of geographical distribution. 108 VARIOUS THEORIES OF SPECIES-FORMING 109 But it is conceivable that isolation may be effected in other ways than by actual segregation or geographic separation of individuals. Anything that could lead to exclusive or dis- criminate breeding among certain individuals of a species would result in the isolation of these individuals from the rest of the species as effectively as their actual separation from others by a geographic or topographic barrier. Now there are various influences or conditions that might conceivably bring about such a state of affairs, and some of these have been actually observed to exist. It is of interest to note that this kind of isolation differs, in a rather important way, from purely geo- graphic isolation in that the latter is almost sure to be wholly indiscriminate as regards the individuals comprised in an isolated group, while the former, which has been called physiological isolation, will be discriminate. That is, there will be a struc- tural or physiological peculiarity common to all the " isolated ' individuals, it being by virtue of this common peculiarity (something not common to other individuals of the same species) that the isolation actually exists. Romanes has been the chief champion of the physiological isolation factor. And we may advantageously refer directly to his writings for a specific statement of different forms or phases of this kind of isolation. In "Darwin and After Darwin," III, p. 7 et seq., he writes: "Now the forms of discriminate isolation, or homogamy, are very numerous. When, for example, any section of a species adopts somewhat different habits of life, or occupies a somewhat different station in the economy of nature, homogamy arises within that section. There are forms of homogamy on which Darwin has laid great stress, as we shall presently find. Again, when for these or any other reasons a section of a species becomes in any small degree modified as to form or color, if the species happens to be one where any psychological pref- erence in pairing can be exercised as is very generally the case among the higher animals exclusive breeding is apt to ensue as a result of such preference; for there is abundant evidence to show that, both in birds and mammals, sexual selection is usually opposed to the intercrossing of dissimilar varieties. Once more, in the case of plants, intercrossing of dissimilar varieties may be prevented by any slight difference in their seasons of flowering, of topographical stations, or even, in the case of flowers which depend on insects for their ferti- 110 EVOLUTION AND ANIMAL LIFE lization, by differences in the instincts and preferences of their visitors. "But, without at present going into detail with regard to these different forms of discriminate isolation, there are still two others, both of which are of much greater importance than any that I have hitherto named. Indeed, these two forms are of such immeasurable importance that were it not for their virtually ubiquitous operation, the process of organic evolution could never have begun, nor, having begun, continued. "The first of these two forms is sexual incompatibility either partial or absolute between different taxonomic groups. If all hares and rabbits, for example, were as fertile with one another as they are within their own respective species, there can be no doubt that sooner or later, and on common areas, the two types would fuse into one. And similarly, if the bar of sterility could be thrown down as between all the species of a genus, or all the genera of a family, not otherwise pre- r< ntcd from intercrosxiny, in time all such species, or all such genera, would become blended into a single type. As a matter of fact, com- plete fertility, both of first crosses and of their resulting hybrids, is rare, even as between species of the same genus; while as between genera of the same family complete fertility does not appear ever to occur, and, of course, the same applies to all the higher taxonomic divisions. On the other hand, some degree of infertility is not unusual as between different varieties of the same species; and, wherever this is the case, it must clearly aid the further differentiation of those varieties. It will be my endeavor to show that in this latter connection sexual incompatibility must be held to have taken an immensely important part in the differentiation of varieties into species. But meanwhile we have only to observe that wherever such incompatibility is concerned, it is to be regarded as an isolating agency of the very first importance. And as it is of a character purely physiological, I have assigned to it the name Physiological Isolation; while for the particular case where this general principle is concerned in the origination of specific types, I have reserved the name Physiological Selection." If the factors of variation, heredity, natural selection, and isolation are, in the minds of most naturalists, the chief factors in species-forming and descent control, and a combination of these factors is, in the belief of these same naturalists the so- called selectionists or Neo-Darwinians a sufficient causal ex- planation of organic evolution, there are many other natural- VARIOUS THEORIES OF SPECIES-FORMING 111 ists who have no such high esteem of the value of natural selection. These believe, variously, that (a) to the selection factors other auxiliary or helping ones are to be added, or (6) that various other factors are equally potent in species-forming, or (c) that these other factors are the more important ones, or finally (d) that the selection factors are of no importance at all, that is, have no reality. Before Darwin, the French naturalist Lamarck had clearly enunciated an explaining theory of species transformation, and there are to-day many naturalists who believe that the Lamarckian explanation, or its fundamental assumption, is true, or, at least, that it is based on the more important and effective factors in evolution. These natural- ists have been called Neo-Lamarckians. Some of these have formulated theories of their own based on Lamarckian funda- mentals, but developed in directions more or less obviously away from characteristic Lamarckism. Still other fundamental causal factors than the Darwinian ones of selection and the Lamarckian ones of accumulated effect of use, disuse, and functional stimulation are assumed in certain other theories of species change and general evolution. Nageli, a botanist and natural philosopher, believed in a special inherent vitalistic principle or force in living matter which tends to pro- duce progressive differentiation and evolution. Von Kolliker, Korschinsky, and de Vries believe that species-forming occurs by definite sudden small (or larger) fixed changes or mutations, so that for them a mutational or discontinuous variation is the fundamental causal factor in species transformation. Numerous paleontologists believe that variation follows determinate lines in its occurrence, so that evolution is orthogenetic, with its lines primarily fixed by determinate variation. We may then examine briefly some of the more important special theories or groups of theories put forward by biologists either as auxiliary and subordinate to the more generally known Darwinian theory, or as alternative with or substitutes for this theory. First to be mentioned should be the transmutation theory of Lamarck. In its simplest expression it is, that every individ- ual organism is, throughout its lifetime, reacting to environ- mental stimuli and conditions in such ways as to change its structure and its habits in greater or less degree from the structure and habits of its parents and ancestors, this change 112 EVOLUTION AND ANIMAL LIFE coming about specifically from the varying effects of use or disuse of parts, and the functional stimulation of other parts in response to such extrinsic conditions as light, contact, tem- perature, pressure, color, etc., etc. The changes effected will, in the nature of things, be essentially adaptive. Now, these adaptive changes, these variations, or new characters acquired during the lifetime of the individual will be, in Lamarck's belief, inherited, if not in full, at least in partial degree, by the offspring. These in turn submitted to similar or to different environ- mental influences will continue the changes either cumulatively or diversely. By this steady direct change and adaptation to environment the species is ever modifying and transforming. Evolution marches, and marches adaptively and advanta- geously. But modern naturalists find a most unfortunate impediment to this simple, direct, and sufficient explanation of species- forming and evolution in the apparent untruth of the assumption that the characters acquired by an individual in its lifetime are transmitted by inheritance to its young. This question, fun- damental to the Lamarckian theory, of the inheritance or non- inheritance of acquired characters has long been one of the most hotly debated points in evolution biology. As we have devoted a number of pages to its particular discussion in our later chap- ter on heredity (Chapter X), we need not anticipate that discussion here. It is sufficient to say that as far as scientific proof, that is, evidence from actual observation and experiment, goes, those naturalists led by Weismann, who deny this inheri- tance, have at present distinctly the better of the argument. The orthogenetic evolution theories of various authors, based upon the assumed occurrence of variations in determinate lines or directions (a restricted and determinate variation as compared with the nearly infinite, fortuitous, and indeterminate variation assumed in the selection theory) , are of several types. The mention of two will reveal pretty well the more important characters of all. Not a few biologists have always believed in the existence of a sort of mystic, special vitalistic force or prin- ciple by virtue of which determination and general progress of evolution is chiefly fixed. Such a capacity, inherent in living matter, seems to include at once possibility of specific adapta- tion and the possibility of progressive or truly evolutionary change. Not all evolution is in a single direct line, to be VARIOUS THEORIES OF SPECIES-FORMING 113 sure ; ascent is not up a single ladder or along a single genealogi- cal branch, but these branches are few (as indeed we actually know them to be, however the restriction may be brought about) and the evolution is always progressive, that is, toward what wo, from an anthropocentric point of view, are constrained to call higher or more ideal life stages and conditions. Other naturalists also seeming to see this course of determin- ate or orthogenetic evolution, but not inclined to surrender their disbelief in vitalism, in forces over and beyond the familiar ones of the physicochemical world, have tried to adduce a definite causomechanical explanation of orthogenesis. The best and most comprehensible types of this explanation are those essentially Lamarckian in principle, in which the direct in- fluence on living matter of environmental conditions, the direct reactions of the life stuff to stimuli and influences from the world outside, are the causal factors in such an explanation. But while every naturalist will grant that such factors do change and control in considerable degree the life of the individual, most see no mechanism or means of extending this control directly to the species. The stumbling block of heredity, the means and mode of inheritance, as we so far know them, are directly in the way of any general acceptance of such a theory of evolution under the direct control of such "primary factors of life. ;; Ontogenetic species, that is, conditions of structure and habit common to many individuals of one kind, the conditions due to sameness of intrinsic and extrinsic factors in development, constitute a category of organisms which at any given time and place seem very real, and are for the moment truly real. But their environment is remaining fairly constant. We speak easily of the flux of Nature: her everchangingness. And in the large we are speaking only of the truth. But during our brief period of observation of the few generations of this or that kind of animal or plant that come under our eyes and microscopes, the nature environing these generations may be nearly uniform. What are the changes in the desert in a score or a hundred or a thousand of years? What changes in life conditions on the barren storm-swept peaks of the mountain ranges? What in the waters of that brackish bay or sweet-water lake apart from the paths of man? Ontogenetic species have a seeming of reality, but so far as our present knowledge goes it is only a 114 EVOLUTION AND ANIMAL LIFE seeming: reality vanishes with the death of the individual: their young can perpetuate their specific peculiarities only if the environmental conditions of their development are identical with those which attended the growing up of their parents. Variations in this environment will determine variations in them, and their father's kind wdll exist no more. The authors of this book believe that more characters of species than are commonly thought are of this shifty, ephemeral character; that not a few so-called true species are only onto- genetic species held for a number of generations true to a type simply because the environment, the extrinsic factors in the development of all the individuals in these successive genera- tions, are the same. But how these individual characteristics and changes can be put into the heredity of the race we do not understand. "There is no fixity in species other than that due to the long-repeated ontogenetic reiteration of this or that characteristic," says Luther Burbank. And he speaks from the conviction forced on him through thirty years of the closest sort of observation and personal experience of the life of plants. And yet, however strongly our own minds respond to a desire to believe this it w r ould be so clarifying the obstinate "no mechanism '' objection stands boldly up to check our sympa- thetic reasoning. Finally we should refer to the theories of heterogenesis or species-forming by mutations or saltations, which have been proposed at various times as a substitute for the theory of species-forming by the gradual transformation through selec- tion. During the discussion in the first few years after the appearance of Darwin's "Origin of Species," the German zoolo- gist von Kolliker expressed the belief that the change from species to species would probably be found to be more sudden and more distinctive than Darwin's theory permitted one to assume. Later, the Russian botanist Korschinsky, on a basis of general observation and some not very extensive personal experimentation, definitely formulated a theory of species- forming by heterogenesis which he placed strongly in contrast with Darwin's theory of gradual transformation by selection, which later theory he claimed should be wholly given up. But not until the publication of de Vries's work, Die Mutations- theorie, in which are recorded the results of close personal observation and experimentation for twenty years on race and VARIOUS THEORIES OF SPECIES-FORMING 115 species-forming in plants has the theory of species-forming by mutations, or sudden fixed changes (lesser or greater) had any considerable adoption or even general attention. At the present moment, probably because of a strong re- action against the too blind acceptance and general over- emphasis of the selection doctrines, and because, too, of the unusually extensive character of de Vries's experimentation and observation, and his trenchant criticism of the weak places in the other theories, with the generally weighty character of his work and reputation, because of all this the theory of species- forming by mutations has at the present moment a fairly large body of adherents among reputable biologists. And yet the actual evidence of tested observation on which the theory rests is curiously meager. One hastens to admit, however, that similar evidence for the theory of direct species-forming by selection is also meager. While apparently no one has ever seen a case of species-making by the natural selection of slight fluctuating variations, de Vries seems to be almost the only one who has observed actual cases of species-making by hetero- genesis, and he has seen very few. And in the nature of things, the opportunities for this kind of evidence, that is, that of actual observation, ought to be much larger in the case of hetero- genesis than in that of general transformation by the selection of slight variations. An account of the exact character of the de Vriesian mutations is included in our later chapter on variation and mutation. Our readers should realize, that however much they may see of this theory in present-day popular scientific literature, and however strongly the case may be put in favor of the mutation theory of species origin, this theory is not accepted by the great body of biologists as entitled to replace the Darwinian theory. We may close this chapter with a reference to a pregnant sentence of the American paleontologist, Osborn, in a lecture entitled "The Unknown Factors of Evolution": "The general conclusion we reach from a survey of the whole field is that for Buffon's and Lamarck's factors we have no theory of heredity, while the original Darwinian factor, or Neo-Darwinism, offers an inadequate explanation of evolution. If acquired varia- tions are transmitted, there must be, therefore, some unknown principle in heredity; if they are not transmitted, there must be some unknown factor in evolution." Our present plight seems 116 EVOLUTION AND ANIMAL LIFE to be exactly this: we cannot explain to any general satisfac- tion species-forming and evolution without the help of some Lamarckian or Eimerian factor; and on the other hand, we cannot assume the actuality of any such factor in the light of our present knowledge of heredity. The discovery of the " unknown factors of evolution" should be the chief goal of all present-day biologic investigation. CHAPTER VIII GEOGRAPHIC ISOLATION AND SPECIES- FORMING "For me, it is the chorology of organisms, the study of all the important phenomena embraced in the geography of animals and plants, which is the surest guide to the knowledge of the real phases in- the process of the formation of species." -MoRiTZ WAGNER. A FLOOD of light may be thrown on the general problem of the origin of species by the study of certain evidence as to the l actual origin of species with which we may be familiar, or of which the actual history or the actual ramifications may in some degree be traced. In such cases, one of the first questions naturally asked is this: Where did the species come from? Migration forms a large part of the history of any species or group of forms. The fauna of any given region is made up of the various species of animals living naturally within its borders. The flora of a region is made up of the plants which grow naturally within its limits. Of all these, animals and plants, the inhabitants of most regions are apparently largely migrants from some other region. Some have entered the region in question before acquiring their present specific characters; others come after having done so. Which of these conditions apply to any given case can sometimes be ascertained by the comparison of the individuals along the supposed route of migration. Thus, Dr. C. Hart Merriam has undertaken to show the actual origin of nine species of Californian chipmunks (Eutamias) by an elaborate study of their distribution, adaptations, and transformations. He finds them closely related to one another, 1 A paper published in "Science," 1906, by the senior author, under the title "The Actual Origin of Species," has been freely quoted from in this chapter. 9 117 118 EVOLUTION AND ANIMAL LIFE FIG. 71. Some chipmunks of California, showing distinct species produced through isolation. (From nature, by William Sackston Atkinson.) GEOGRAPHIC ISOLATION AND SPECIES-FORMING 119 but not derived from one another by direct lines of descent. A closer study indicates that some of them " came from closely related forms in remote geographic areas, others from antece- dent forms now extinct, and not more than three or four from species still inhabiting the region." The nature of any fauna bears an immediate relation to the barriers, geographic, climatic, topographic, or bionomic, which may form its boundaries. By bionomic barriers we mean any condition of any sort which may check free inter- breeding, or which may tend to cause divergence within a species. A thickly peopled level area may be in this sense a barrier, because it prevents the animals on the one side of the area from interbreeding with those on the opposite side. If the two extremes have diverged to become different species, the individuals in the middle area, whose presence in a sense constitutes the bionomic barrier, are usually variously inter- mediate in the characters and habits which they possess. Whenever the individuals of a species move evenly over an area, its members freely interbreeding, the character of the species remains substantially uniform. Whenever freedom of movement and consequent freedom of interbreeding is checked, the character of the species is rapidly altered. It is changed even though external conditions seem to be absolutely identical on both sides of the barrier, and if there is no visible distinction in the original stock on the two sides. Presumably, there are subtle differences in the environment, producing changes in the process of selection and adaptation. Doubtless, there are differences equally subtle produced by the processes of varia- tion and their repetition by inheritance. The pregnant phrase of Dr. Cones applies in these cases: 'Migration holds species true: localization lets them slip." In other words, free interbreeding swamps incipient lines of variation, and this in almost every case. On the other hand, a barrier or check of any sort brings a certain group of individuals together. These are subjected to a selection different from that which obtains with the species at large, and under these conditions new forms are developed. This takes place rapidly when the conditions of life are greatly changed, so that a new set of demands is made on the species, and those individuals not meeting it are at once destroyed. The process is a slow one, for the most part, when the barrier in question interrupts the 120 EVOLUTION AND ANIMAL LIFE flow of life without materially changing its conditions. But this is practically a universal rule: A barrier which prevents the intermingling of members of a species will with time alter the relative characters of the groups of individuals thus separated. These groups of individuals are incipient species, and each may become in time an entirely distinct species if the barrier is really insurmountable. In the great water basin of the Mississippi, many families of fish occur and very many spe- cies are diffused throughout almost the whole area, occurring in all suitable waters. Once admitted to the water basin, each one ranges widely and each tributary brook has many species. In the streams of California, mostly small and isolated, the number of genera or families is much smaller. Each species, unless running to the sea, has a narrow range, and closely re- lated species are not found in the same river. The fact last mentioned has a very broad application and may be raised to the dignity of a general law of distribution. Given any species in any region, the nearest related species is not likely to be found in the same region nor in a remote region, but in a neighboring district separated from the first by a barrier of some sort, or at least by a belt of country, the breadth of which gives the effect of a barrier. Always the species nearest alike in structure are not found together nor yet far apart, and always a check to interbreeding lies between. Where two closely allied forms are not found to intergrade, they are called distinct species. If we find actual intergradation, the occurrence of specimens intermediate in structure, the term subspecies is commonly used for each of the recognizable groups thus connected. Widelv distributed across the United States and from / southern Canada to Arizona, we have the yellow warbler, Dcndroica ccstiva. This bird is chieflv yellow, olive on the / / back with chestnut streaks on the sides, the tail feathers colored like the body, and without the white spot on the outer feathers shown in most of the other wood warblers composing the genus Dendroica. The yellow warbler throughout its range is very uniform in size and color. Its nearest relative differs in having a shade less olive on the back and the brown streaks on the sides narrower. This form is found in the Sonoran region, and, as along the Rio Grande it intergrades with the first, it is called GEOGRAPHIC ISOLATION AND SPECIES-FORMING 121 a subspecies, Dendroica ccstiva sonorana. Further south, in central Mexico, this form runs larger in size and is recorded as Dendroica (estiva dugesi. Northward, through to Alaska, we have an ally of the parent bird, but smaller and still more greenish. This is Dendroica cestiva rubiginosa. In the West Indies, the golden warblers migrate not from north to south, but from the shore to the mountains, and, possibly in consequence of the less demand of flight, the wing is shorter and more rounded, while the tail is longer. As these forms do not clearly intergrade with those of the mainland, and, for the most part, not with each other, they are held to represent a number of distinct species, although doubtless derived from the parent stock of Dendroica cestiva. Some of these West Indian forms are relatively large, the wing more than five inches long, and the longest known of these, the type of the species for this reason, found in Jamaica, is called Dendro- ica petechia. On the island of Grand Cayman is a similar bird, a little smaller, Dendroica auricapilla. Of a deeper yellow than petechia, and equally large, is the golden warbler of the Lesser Antilles ranging from island to island, from Porto Rico to Antigua. This form, first known from St. Bartholomew, is Dendroica petechia bartholemica. A smaller bird, a little different in color, takes its place in the Bahamas. This is Dendroica petechia flaviceps. In Cuba, the golden warbler is darker and more olive, with other minor differences from the form called bartholemica, of which it may be the parent. This is Dendroica petechia gund- lachi. A similar bird, but with the crown distinctly chestnut, is Dendroica petechia aureola, the golden warbler of the Gala- pagos and Cocos Islands, off the coast of Ecuador and Peru. Scattered over other islands are smaller golden warblers with the wing less than five inches long, and with the crown tawny red, as in aureola. These are known collectively as Dendroica ruficapilla, the type being from Guadeloupe and Dominica. More heavily streaked, with the crown darker in color, is the golden warbler of Cozumel, Dendroica ruficapilla flavivertex, and with very similar but with darker crown is Dendroica ruficapilla flavida, of the island of St. Andrews. Always, the nearest form lies across the barrier, and among these island forms the chief barrier is the sea. With a darker chestnut crown is Dendroica ruficapilla rufopileata, of the island of 122 EVOLUTION AND ANIMAL LIFE Curasao, and still darker bay is the crown of Dendroica ruficapilla capitalist the golden warbler of the Barbadoes. Still other golden warblers exist, with the chin and throat chestnut as well as the crown. One of these, olive green on the back, is Dendroica rufigula, of Martinique. The others are more yellow. One of these, with the sides heavily streaked, inhabits the isthmus region, Dendroica erythacoides, called a distinct species, because no intergradations have been made out. Another, more faintly streaked, replaces it on the Atlantic coast from Yucatan to Costa Rica, Dendroica bryanti, while the Pacific coast, from Sinaloa to Costa Rica, has another form, with still fainter markings, Dendroica bryanti castaniceps. An extreme of this form with the throat and breast tawny, but not the crown, is found in Jamaica again and is known as Dendioica eoa. In this case, which is one typical of most groups of small birds, the relation of the species to the barriers of geography is so plain as to admit of no doubt or question. Given the facts of individual fluctuation and of heredity, it is manifest that while natural selection may produce and enforce adaptation to conditions of life, the traits which dis- tinguish these species bear little relation to utility. The individuals which, separated from the main flock, people an island, give their actual traits to their actual descendants, and the traits enforced by natural selection differ from island to island. If external conditions were alike in all the islands the progress of evolution would perhaps run parallel in all of them, and the only differences which would persist would be derived from differences in the parent stock. As some difference in environment exists, there is a corresponding difference in the species as a result of adaptation. If great differences in con- ditions exist, the change in the species may be greater, more rapidly accomplished, and the characters observed will bear a closer relation to the principle of utility. Doubtless, wide fluctuations or mutations in every species are more common than we suppose. With free access to the mass of the species, these are lost through interbreeding. Isolate them, as in a garden, or an enclosure or on an island, and these may be continued and intensified to form new species or races. Any breeder or any horticulturist will illustrate this. It is not claimed that species are occasionally associated with physical barriers, which determine their range, and which GEOGRAPHIC ISOLATION AND SPECIES-FORMING 123 have been factors in their formation. We claim that such conditions are virtually universal among species as they exist in nature. When the geographical relations of the origin of a species cannot be shown it is usually because the species has not been critically studied, from absence of material or from absence of interest on the part of naturalists. In a few cases, a species ranges widely over the earth, showing little change in varying conditions and little susceptibility to the effects of isolation. In other cases, there is some possibility that saltations, or suddenly appearing characters, may give rise to a new species within the territory already occupied by the parent form. But these cases are so rare that in ornithology, mammalogy, herpetology, conchology, and entomology, they are treated as negligible quantities. One of the most successful workers in this field is Rev. John T. Gulick, whose studies of the localization of species and sub- species of land snails in Oahu Island (Hawaii) have become classical. According to Mr. Gulick, the land snails of the wooded portion of Oahu have become split up into about 175 species of land shells represented by 700 or SCO varieties. He frequently finds a genus represented in several successive valleys by allied species, sometimes feeding on the same and similar plants. In every case, the valleys that are nearest to each other furnish the most nearly allied forms, and a full set of the varieties of each species presents a minute gradation between the more divergent types found in the more widely separated localities. Similar conditions are recorded among the land snails in Cuba and in other regions. In fact, on a smaller scale, the development of species of land and river mollu.sks has everywhere progressed on similar lines with that of birds and fishes. To recognize isolation as practically a necessary condition in the subdivision of species need not necessarily eliminate or belittle any other factor. Isolation is a condition, not a force. Of itself it can do nothing. Species change or diverge with space and with time: with space, be- cause geographical extension divides the stock and brings new conditions to part of it; with time, because time brings always new events and changes in all environment. The beginning of each species must rest with its variability of individuals. One of the most remarkable cases of group evolution is 124 EVOLUTION AND ANIMAL LIFE that of the song birds of Hawaii which constitute the family of Drepanidse. In this family are about forty species of birds, all much alike as to general structure, but diverging amazingly from each other in the form of the bill, with, also, striking differences in the color of the plumage. In almost all other families of birds the form of the bill is very uniform within the group. It is correlated with the feeding habits of the bird, and these in most groups of wide range become nearly uniform within the limits of the family. With a great range of com- petition, each type of bird is forced to adapt itself to the special line of life for which it is best fitted. But with many diverging possibilities and no competition, except among themselves, the conditions are changed, and we find Drepanidse in Hawaii fitted to almost every kind of life for which a song bird in the tropics may possibly become adapted. (Plate II.) In spite of the large differences to be noted there can be little doubt, as Dr. Hans Gadow, Mr. Henry W. Henshaw and others have shown, of the common origin of the Drepanidse. A strong peculiar goatlike odor exhaled in life by all of them affords one piece of evidence pointing in tlvs direction. There is, moreover, not much doubt that the whole group is descended from some stock belonging to the family of honey creepers, Ccerebidse, of the forests of Central America. Each of the Hawaiian Islands has its species of Drepanine birds, some olive green in color, some yellow, some black, some scarlet, and some variegated with black, white, and golden. The females in most cases, like the young, are olive green. On each island, most of the species are confined to a small district, to a single kind of thicket or a single species of tree, each species being especially fitted to these localized surroundings. With the destruction of the forests some of these species are already rare or extinct. With high specialization of the bill they lose their power of adaptation. In each of the several recognized genera there are numerous species, mostly thus specialized and local- ized, relatively few species being widely distributed throughout the islands. Most primitive of all, least specialized and most like the honey-creeper ancestry, is the olive green Oreomystis bairdi of the most ancient island, Kauai. This bird has a small straight bill, not unlike that of the slender-billed sparrows. It is said to be the most energetic and ubiquitous of the group, feeding PLATE II. 1, Chloridops kona Wilson, Hawaii; 2, Pseudonestor xanthophrys Rothschild 3, Hcmiqnathus procerus Cabanis, Kauai. (From specimens.) GEOGRAPHIC ISOLATION AND SPECIES-FORMING 125 on insects on the trunks of trees. If we assume that Oreo- mystis, or some other of the genera with short and slender bills, represents the original type of Drepanidse, we have two lines of divergence, both of them in directions of adaptation to peculiar methods of feeding. Next to Oreomystis. on the one hand, we have Loxops and Himatione, with the bill pointed, a little longer than in Oreo- wijstis, and slightly curved downward. The species, red or golden, of these two genera are distributed over the islands, each on its own mountain or in its own particular forest. Vestiaria, another genus, remarkable for its beautiful scarlet plumage, has the bill very much longer and strongly curved downward. Vestiaria coccinea, the iiwi of the islands, lives among the crimson flowers of the ohia tree (M etrosidcros) and the giant lobelia, where it feeds chiefly on honey, which is said to drop from its bill when shot. According to Mr. S. B. Wilson, the scarlet sickle-shaped flowers of a tall climbing plant (Strongylodon lucidus) found in these forests " mimic in a most perfect manner both in color and in shape the bill of the iiwi '' so that the plant is called nukuiiwi (bill of the iwii). The next genus, Drepanis, has the sickle bill still further prolonged, forming a segment of a circle, and covering nearly fifty degrees. Drepanis pacifica, one of the species, has the bill forming about one fourth of the total length. The species of this genus, black and golden in color, were very limited in range, and are now nearly or quite extinct. Still another group with sickle bills, Hemignathus, diverges from Vestiaria in having only the upper mandible very long and decurved, the lower one being straight and stiff. The numerous species are mostly golden yellow in color. The group contains long- billed forms like Hemignathus procerus of Kauai, and short- billed forms like Heterorhynchus olivaceus of Hawaii. In the short-billed forms the two mandibles are quite unlike: the upper very slender, much curved and about one fourth the length of the rest of the body, the lower mandible half as long and thick and stiff. These birds feed chieflv on insects in the dead limbs \) of the koa trees in the mountain forests. Some or all of them use the lower mandible for tapping the trees, after the fashion of woodpeckers, while with the long and flexible upper one they reach into cavities for insects or insect larvae or suck the honey of flowers. 126 EVOLUTION AND ANIMAL LIFE Mr. S. B. Wilson remarks: "Nature has shown great sym- metry in regard to the species of this genus (Hemignathus including H eterorhynchus) to be found in the Sandwich Archi- pelago, three of the main islands having each a long-billed and a short-billed form." This, of course, is most natural. Both long-billed forms (Hemignathus') and short-billed forms (H eterorhynchus) have spread from the island where they were originally developed to the other islands, each changing as it is isolated from the main body of the species and subjected to natural selection under new conditions. With the genus H eterorhynchus, the forms with slender bills reach their culmina- tion. Going back to the original stock, to which Oreomystis bairdi is perhaps the nearest living ally, we note first a divergence in another direction. In Rhodacanthis , the bill is stout like that of the large finch, not longer than the rest of the head, and curved downward a little at the tip. The species of this genus feed largely on the bean of the acacia and other similar trees, varying this with caterpillars and other insects. The stout bill serves to crush the seeds. In C.hloridops, the bill is still heavier, very much like that of the grosbeak. Chloridops kona is, according to Mr. Robert Perkins, a dull, sluggish, solitary bird and very silent; its whole existence may be summed up in the words "to eat." Its food consists of the fruit of the aaka (bastard sandal tree), and as this is very minute, its whole time seems to be taken up in cracking the extremely hard shells of the fruit, for which its extraordinarily powerful bill and heavy head are well adapted. "The incessant cracking of the fruits, when one of these birds is feeding, the noise of which can be heard for a considerable distance, renders the bird much easier to get than it otherwise would be. Its beak is always very dirty with a brown substance adhering to it which must be derived from the sandal nuts." In Psittacirostra and Pscudonestor the bill suggests that of a parrot rather than that of a grosbeak. The mandibles are still very heavy, but the lower one, as in Heterorhynchus, is short and straight, while the much longer upper one is hooked over it. Pseudonestor feeds on the larva* of wood-boring beetles (Clytanus) found in the koa trees (Acacia falcata), while the PLATE III. 1, Oreomystis bairdii Stejneger, Kauai; 2, Heterorhynchus oliva- ceus La Fresnaye, Hawaii. 3, Drcpanis funerea Xewton, Molokai. (From specimens.) GEOGRAPHIC ISOLATION AND SPECIES-FORMING 127 closely related Psittacirostra eats only fruits, that of the ieie (Freycinetia arbor ea) and the red mulberry (Morns sapyrifera) being especially chosen. In all these genera, there is prac- tically one species to each island, except that in some cases the species has not spread from the mountain or island in which we may suppose it to have been originally developed. There are a few other song birds in the Hawaiian Islands, not related to the Drepanidae. These are derived from the islands of Polynesia and have deviated from the original types in a degree corresponding to their isolation. In the case of the Drepanida?, it seems necessary to conclude that natural selection is responsible for the physiological adaptations characteristic of the different genera. Such changes may be relatively rapid, and for the same reason they count for little from the stand- point of phylogeny. On the other hand, the nonuseful traits, the petty traits of form and coloration which distinguish a species in Oahu from its homologue in Kauai or Hawaii, are results of isolation. These results may be analyzed as in part differences in selection with different competition, different food and different conditions, and in part to hereditary differ- ence due to the personal eccentricities in the parent stock from which the newer species was derived. In these as in all similar cases we may confidently affirm: the adaptive characters a species may present are due to natural selection or are developed in connection with the demands of competition. The characters nonadaptive which chiefly distinguish species do not result from natural selection, but are connected with some form of geographical isolation and the segregation of individuals resulting from it. The origin of races and breeds of domestic animals is in general of much the same nature. In traveling over Eng- land one is struck by the fact that each county has its own breed of sheep, each of these having its type of excellence in mutton, wool, hardiness, or fertility, but the breeds distin- guished by characters having no utility either to sheep or to man. The breeds are formed primarily by isolation. The traits of the first individuals in each region are intensified by the inbreeding resulting from segregation. Natural selection preserves the hardiest, the most docile, and the most fertile: artificial selection those which yield the most wool, the best mutton and the like. The breed once established, artificial 128 EVOLUTION AND ANIMAL LIFE selection also tends to intensify and to preserve its nonadaptive characteristic marks. The more pride the breeders take in their stock, the more certain is the preservation of the breed's useless peculiarities. Very few of the characters which usually distinguish a breed of domestic animals have the slightest phys- iological value to the species. Each of them would disappear in a few generations of crossing, and in each race prized by the breeder the actual virtues exist wholly independently of these race marks. Analogous to the race peculiarities of domestic animals are the minor traits among the men of different regions. Cer- tain gradual changes in speech are due to adaptation, the fitness of the word for its purpose, analogous to natural selection. The nonadaptive matters of dialect find their origin in the exigencies of isolation, while languages in general are ex- plainable by the combined facts of migration, isolation, and the adaptation of words for the direct uses of speech. In the animal kingdom generally we may say therefore: Whenever a barrier is to some extent traversable, the forms separated by it are likely to cross from one side to the other, thus producing intergradations, or forms more or less inter- mediate between the one and the other. For every subspecies, where the nature of the variation has been carefully studied, there is always a geographical basis. This basis is defined by the presence of some sort of physical barrier. It is ex- tremely rare to find two subspecies inhabiting or breeding in exactly the same region. When such appears to be the case, there is really some difference in habit or in habitat: the one form lives on the hills, the other in the valleys; the one feeds on one plant, the other on another; the one lives in deep water, the other along the shore. There can be no possible doubt that subspecies are nascent species, and that the accident of inter- gradation in the one case and not in the other implies no real difference in origins. For a final example, we may compare the species of Ameri- can orioles constituting the genus Icterus. We may omit from consideration the various subspecies, set off by the mountain chains, and the usual assemblage of insular forms, one in each of the West Indies, and confine our attention to the leading species as represented in the United States. (See frontispiece.) GEOGRAPHIC ISOLATION AND SPECIES-FORMING 129 The orchard oriole, Icterus spurius, has the head, back, and tail all black, the lower parts chestnut, and the body relatively small, as shown by the average measurements of different parts. In the hooded oriole, Icterus cucullatus, the head is all golden orange except the throat, which is black, the tail is black, and the wings are black and white. This species, with its subspecies, ranges through southern California and Arizona, and over much of Mexico. Our other orioles have the tail black and orange. In the common Baltimore oriole, Icterus galbula } of the east, the head is all black and the under parts orange. In the equally common Bullock oriole, Icterus bullocki, of the California region, the head is yellow on each side, the belly rather yellow than orange. The females of all the species are plain olivaceous, the color and proportions of parts varying with the different species, while in the males of each of the many species black, white, yellow, orange, and chestnut are variously and tastefully arranged. Each species again has a song of its own, and each its own way of weaving its hanging nest. That which interests us now is that not one of these varied traits is clearly related to any principle of utility. Adaptation is evident enough, but each species is as well fitted for its life as any other, and no transposition or change of the distinctive specific characters or any set of them would in any conceivable degree reduce this adaptation. No one can say that any one of the actual distinctive characters or any combination of them enables their possessors to survive in larger numbers than would otherwise be the case. One or two of these traits, as objects of sexual selection or as recognition marks, have a hypothetical value, but their utility in these regards is slight or uncertain. Naturalists now look with doubt on sexual selection as a factor in the evolution of ornamental structures, and the psychological reality of recognition marks is yet un- proved, though not impossible. It may be noted in passing, that the prevalent dull yellowish and olivaceous hues of the female orioles of all species seem to be clearly of the nature of protective coloration. It has been shown statistically that certain specific charac- ters among insects have no relation to the process of selection. Among honey bees the variation in venation of the wings and in the number and character of the wing hooks is just as 130 EVOLUTION AND ANBIAL LIFE great among the bees which first come from their cells as in a series of individuals long exposed to the struggle for existence. Among ladybird beetles of a certain species (Hippodamia convergens), eighty-four different easily describable " aberra- tions'' or variations in the number and arrangement of the black spots on the wing covers have been traced. These variations are again just as numerous in individuals exposed to the struggle for life as in those just escaped from the pupal state. In these characters, there is, therefore, no rigorous choice due to natural selection. Such specific characters, without individual utility, may be classed as indifferent, so far as natural selection is concerned, and the great mass of specific characters actually used in systematic classification are thus indifferent. And what is true in the case of the orioles and the ladv- fc/ birds is true as a broad proposition of the related species which constitute any one of the genera of animals or plants. All that survive are sufficiently fitted to live, each individual, and therefore, each species, matched to its surroundings as the dough is to the pan, or the river to its bed, but all adaptation lying ap- parently within a range of the greatest variety in nonessentials. Adaptation is presumably the work of natural selection; the division of forms into species is the result of existence under new and diverse conditions. CHAPTER IX VARIATION AND MUTATION It becomes imperative that we should carry out the most exact research possible by means of experiment and also wean ourselves of the convenient, but, as it seems to me, highly pernicious habit of theo- retical explanations from general propositions. Otherwise there is great danger that the bright expectation which Darwin has opened out to us by his theory may be baffled the prospect of gradually bringing even organic Being within reach of that method of inquiry which seeks to discern mechanical efficient causes. SEMPER. THUS far in our discussion of evolution factors and theories we have taken for granted the actuality of the two fundamental factors, variation and heredity. No one disputes their reality; nor does anyone deny their fundamental and indispensable character in relation to the origin of species and the evolution of organisms. All the theories to explain evolution build on these two basic factors or vital conditions. The subjects of doubt or denial are such postulated factors as selection, muta- tion, orthogenetic progress, etc.; variation and heredity never. But the character, the influence, the regularity or irregu- larity of variations, their behavior in heredity, whether trans- missible or not, whether acquired or congenital, whether deter- minate or indeterminate, etc. these are the problems that the factor variation or variability presents to biologists. Heredity, too, has its problems. These we shall take up in another chapter. That variations exist is too obvious to everyone to need any discussion. Any litter of kittens or puppies, of mice or pigs, shows us the differences in pattern, shape, and physiology of in- dividuals born at one time and of the same parents. In wild nature the variations among brothers and sisters are no less real than among these domesticated animals. 131 132 EVOLUTION AND ANIMAL LIFE Collect a few thousand individuals, at one time in one place, of a single species of insect, as a spotted ladybird beetle; then go over these carefully, looking for variation in some single characteristic, as the color pattern. What do you find? Let us FIG. 72. Diagram showing variation in elytral pattern of the convergent ladybird, Hippodamia convergens : 1, Mode; 2-9, variations in size of spots; 10-17, variations by coalescence of spots; 18-40, variations by reduction in number of spots. (After Kellogg and Bell.) VARIATION AND MUTATION 133 answer by calling attention to Figs. 72, 73, and 74 and what these variations signify. Note also Fig. 75, showing the FIG. 73. Diagram showing variations in elytral pattern of convergent ladybird, Hippodamia convergens: 1-5, Variations by different reduction in number of spots in the two elytra; 6-9, variations by conditions of spots. (After Kellogg and Bell.) variation in elvtral blotching to be found in a series of Individ- *J C.5 uals of the California flower beetle, Diabrotica soror; see also Fig. 76, showing the vari- ations in the black and yellow color pattern of the abdomen of the common yellow jacket (Vespa sp.); and Fig. 77 showing the variation in the pattern of the prothorax in a series of 178 individuals of a common California!! flower bug, all these individuals collected at one time by sweeping a net over a few rods of alfalfa and Baccharis on the campus of Stanford University. FIG. 74. Diagram showing variations in prothoracic pattern of the convergent ladybird, Hippodamia convergens. (After Kellogg and Bell.) These are all color and pattern variations; but in- sects show variations in structural parts as well. Fig. 78 shows a common red-legged locust and one of its hind tibiae enlarged 10 134 EVOLUTION AND ANIMAL LIFE FIG. 75. Diagram showing variations in elytral pattern of the California flower beetle, Diabrotica soror. (After Kellogg and Bell.) to show the spines. In eighty-nine individuals of this species of locust collected at Ithaca, N. Y., the number of spines in the outer rov, T of the right tibiae varies from nine to fifteen, in the inner row from eleven to sixteen. One not given to the systematic study of insects may think spines on the hind legs very trivial structures in- deed; but the entomologist, using exactlv such character- O *j istics as the number of these structures as a means in help- ing him to distinguish and define his species, knows how considerable this variation really is. The dog-days cicada (Fig. 79) also has spines on its hind tibiae, but only a few, usually, indeed, two. But in any series of individuals of this insect some individuals will be found with but a single spine, some with three, and a few with four even, although the very great majority will have two. For example, FIG. 76. Diagram showing variation in pattern in the yellow jacket, Vespa ger- manica. (After Kellogg and Bell.) VARIATION AND MUTATION 135 in a series of 98 male individuals collected at Indianapolis, In- diana, at one time, 12 individuals had one spine in the outer row of the right tibiae, 83 had two spines, 2 had three spines, and one had four spines. In the outer row of the left tibiae of the same individuals, there were three spines in 6 individuals, two in 75, and one in 17. In the inner rows of tibial spines in these same FIG. 77. Diagram showing variation in pattern of the prothorax of a flower bug c (After Kellogg and Bell.) individuals there were in the right tibiae, five spines in 5, four spines in 40, three spines in 43, two spines in 9, and one spine in 1 individual: in the left tibiae, five spines in 2 individuals, four spines in 48, three spines in 39, and two spines in 8. In the paper from which we have taken these illustrations of the actuality of variation, studied and statistically tabulated, are given the data showing the actual extent and frequency of variations in various characters, such as color patterns of head, thorax, and abdomen, character of antennal segments, number of tibial spines, character of elytra! striation, character of vena- 136 EVOLUTION AND ANIMAL LIFE tion, number of wing hooks, etc., in two dozen different insect species. Long ago Dr. J. A. Allen, of the American Museum of Natural History, gave similar data of the actual variation in FIG. 78. Red-legged locust, Melanoplus FIG. 79. The seventeen-year locust, Cicada femur-rubrum, and hind tibia, showing septendecim, and its hind tibia, showing inner and outer rows of spines. (After inner and outer spines. (After Kellogg Kellogg and Bell.) and Bell.) various familiar American bird species, his data referring chiefly to variations in dimensions; as length of whole body, length of tail, of wing, of bill, of tarsus and claw, etc. CARDINALIS V/ftG/WANUS 58 specimens, Florida. Tail. Length of Bird Winy. V.Vo >< FIG. 80. Diagram showing variation in length of tail, body, and wing in fifty-eight specimens of the card'^al, Cardinalis (formerly called virginianus), from Florida. (After Allen.) And anyone with means of collecting considerable series of individuals of single species can, if he but give the time and study to it, reveal similar variations in almost any part or characteristic of any species or kind of plant or animal. " What VARIATION AND MUTATION 137 parts vary?" some one asks. All parts vary, but some more than others. Darwin, in Chapter V of his "Origin of Species," postulated certain so-called laws of variability, which attempt to answer this question, "What parts vary? J; These so-called "laws' which to-day would hardly be dignified with the name of law, are summed up by Darwin at the end of this chapter as follows: VARIATION OF ICTERUS BALTIMORE.20.* Tail. m m "Our ignorance of the laws of variation is profound. Not in one case out of a hundred can we pretend to assign any reason why this or that part has varied. But whenever we have the means of instituting a comparison, the same laws appear to have acted in pro- ducing the lesser differences between varieties of the same species, and the greater differ- Wing. Tarsus. Middle Toe. f Hind Toe. f. Bill, Length. Bill, Width. ences between species of the same genus. Changed condi- tions generally induce mere fluctuating variability, but sometimes they cause direct and definite effects; and these may become strongly marked in the course of time, though we have not sufficient evidence on this head. Habit in pro- ducing constitutional peculiarities, and use in strengthening, and disuse in weakening and diminishing organs, appear in many cases to have been potent in their effects. Homologous parts tend to vary in the same manner, and homologous parts tend to cohere. Modifications in hard parts and in external parts sometimes affect softer and internal parts. When one part is largely developed, perhaps it tends to draw nourishment from the adjoining parts; and every part of the structure which can be saved without detriment will be saved. FIG. 81. Diagram showing variation in di- mensions in twenty male specimens of the Baltimore oriole, Icterus galbula (formerly called bdltimore). (After Allen.) 138 EVOLUTION AND ANIMAL LIFE Changes of structure at an early age may affect parts subsequently developed ; and many cases of correlated variation, the nature of which we are unable to understand, undoubtedly occur. Multiple parts are variable in number and in structure, perhaps arising from such parts not having been closely specialized for any particular function, so that their modifications have not been closely checked by natural selection. It follows, probably from this same cause, that organic beings low in the scale are more variable than those standing higher in the scale, and which have their whole organization more specialized. Rudimentary organs, from being useless, are not regulated by natural selection, and hence are variable. Specific characters that is, the characters which have come to differ since the several species of the same genus branched off from a common parent are more variable than generic characters, or those which have long been inherited, and have not differed within this same period. In these remarks we have referred to special parts or organs being still variable, because they have recently varied and thus come to differ; but we have also seen . . . that the same prin- ciple applies to the whole individual; for in a district where many species of a genus are found that is, where there has been much former variation and differentiation, or where the manufactory of new spe- cific forms has been actively at work in that district and among these species we now find, on an average, most varieties. Secondary sexual characters are highly variable, and such characters differ much in the species of the same group. Variability in the same parts of the organization has generally been taken advantage of in giving secondary sexual differences to the two sexes of the same species, and specific differences to the several species of the same genus. Any part or organ developed to an extraordinary size or in an extraordinary manner, in comparison with the same part or organ in the allied species, must have gone through an extraordinary amount of modification since the genus arose; and thus we can understand why it should often still be variable in a much higher degree than other parts; for variation is a long-con- tinued and slow process, and natural selection will in such cases not as yet have had time to overcome the tendency to further variability and to reversion to a less modified state. But when a species with an ex- traordinarily developed organ has become the parent of many modified descendants which in our view must be a very slow process, requiring a long lapse of time in this case, natural selection has succeeded in giving a fixed character to the organ, in however extraordinary a manner it may have been developed. Species inheriting nearly the same constitution from a common parent, and exposed to similar VARIATION AND MUTATION 139 Neck Body Lacerta oceltata hind Legs Neck , , .Body Lacerta viridis M Hind Legs .Tail .Body Lacerta agilis ^ _ H indLegs Tail Neck Body Lacerta muralls ^^^ Hind Legs *mmmmmm*TaH I Neck Lacerta velox ** Hind Legs M Tail M Neck Lacerta deserti M Hind Leg, FIG. 82. Diagram showing variations in dimensions of lizards. (After Wallace.) influences, naturally tend to present analogous variations, or these same species may occasionally revert to some of the characters of their ancient progenitors. Although new and important modifications may not arise from reversion and analogous variation, such modifications will add to the beautiful and harmonious diversity of nature. 140 EVOLUTION AND ANIMAL LIFE Num. of Variates 500 50 Classes...O I "Whatever the cause may be of each slight difference between the offspring and their parents and a cause for each must exist we have reason to believe that it is the steady accumulation of beneficial differ- ences which has given rise to all the more important modifications of structure in relation to the habits of each species." Modern investigation of variation, which includes at least two phases of study that have been developed since Darwin's time, namely the statistical and quantitative, and the experimental study of variation, has been able to add much information about the mode and the character of variations, and has effected a sort of classifi- cation of them which helps at once to express and to clarify and organize our knowledge of variability. But it has added as yet no great funda- mental generalizations really worthy to be called laws. One generalization there is, perhaps, of application and value far reaching enough to be called law (although it ap- plies to only a single category of variation, but a large one), and that is the law formulated in 1846 (ten years before Dar- win's "Origin of Species"), by the Belgian anthropologist, Quetelet, on the basis of the examination of the height and chest measurements of soldiers. As it applies only to what are variously called fluctuating, individual, continuous, or Dar- winian variations, we may note before stating the law the cur- rent mode of classifying the variations which occur in plants and animals. Variations may be either congenital or acquired: that is, may be such as are apparently determined in the organism at conception, or such as are imposed on it during its development 61 8 9 10 FIG. 83. -- Diagram showing curves of distribution of frequency of variation in glands of swine. (After Davenport.) VARIATION AND MUTATION 141 by the influence of extrinsic factors. Or variations may be di- vided into determinate and indeterminate ; that is, those (if there really are such) which are apparently controlled by some, to us unknown, influences and are by these influences confined to cer- tain definite lines or directions of change; and, on the other hand, those which are apparently wholly accidental, or rather which may represent any conceivably possible line or kind of change. Finally, variations may be distinguished as to their general character as discontinuous and continuous ; that is, variations oc- curring irregularly, mostly large and comparatively rarely, and small, abundant variations occurring in graded series. Among the former are to be ranked the occasional sports and monsters familiar to all breeders; while in the latter, Darwin believed him- self to have at hand the necessary ever-present materials to serve natural selection as a basis for species transformation. Hence the slight but abundant and ever-present fluctuating continuous variations are often called "Darwinian variations." Now the law of Quetelet applies solely to the Darwinian variations. The law is, that these variations occur according to the law of probabilities (or law of error): that is, that the slightest variations away from the modal or average type will be the most abundant, and that the number of varying individ- uals will be progressively less the farther away from the modal type the variations of these individuals are. That is, if the vari- ations in some characteristic of a species be determined for, say, 10,000 individuals of the species, and tabulated, and a curve erected to express graphically the facts of this variability, this curve will practically coincide with that one which would simi- larly express the variation, if the variation actually occurred according to the mathematical law of the frequency of error; this theoretical curve being obtained by the formula deduced originally by Gauss at the beginning of the last century. Fig. 83 shows graphically how certain studied cases of continuous variation reveal the condition expressed by Quetelet ; s law. As compared with discontinuous and sport variation, con- tinuous variation is by great odds the more common. Bateson, an English student of variations, has attempted to show that discontinuous variations are more common than is generally believed, and has filled a large volume with accounts and illus- trations of such alleged variations. But it has been proved that many of these are cases of teratogenic regeneration, or ab- 142 EVOLUTION AND ANIMAL LIFE normal restoration of injured parts. Others, too, are of a char- acter which, to many people, will not seem to be discontinuous at all, but continuous. For example, differences in number of antennal or tarsal segments in insects are called by Bateson cases of discontinuous variation if the differences are only by one segment. But as the differences cannot well be less than a whole segment, variations in number of segments, if represented by all the successive numbers between the lowest and highest number of segments observed, may fairly be called continuous: that is, strictly gradatory. It may be of interest to note, for the purposes of explaining by concrete examples the various phases or categories of varia- tion already named, some specific examples exemplifying each category. The following are taken from a paper on variation in insects, which records a number of statistical studies of varia- bility made by the junior author of this book and Mrs. Bell- Smith. To distinguish absolutely between acquired variation and congenital (or blastogenic) variation is a matter which can be done in but comparatively few cases. Whether a variation be congenital or whether it be acquired during the development or life-time of the individual showing it, this variation cannot be recognized until after a considerable part of the development has been undergone; if it is a variation in an adult structure or function, all of the development must have been completed. The variation is apparent only after it is unfolded: only after the part it appears in has reached its definite stage of completed growth and development. Now, who is to say whether this variation was or was not */ imposed on the individual showing it, during this long develop- ment and immature life as a result of some external influence brought to bear on the varying part during the development? We know that such extrinsic influences do modify parts and functions during individual development, and so we must be very careful when we claim that this or that variation is con- genital and not acquired. Yet, how all-important it is to make the distinction is apparent when we recall the fact that most biologists are agreed that acquired characters (variations) can- not be inherited, so that new species can be built up only on the basis of congenital characteristics. In the case of insect variations, a criterion for distinguishing VARIATION AND MUTATION 143 between the congenital and acquired condition is at hand, thanks to the unusual character of the development of certain specialized insects, namely, all those that undergo a complex metamorphosis. "Without by any means exhausting the subject of the postembry- onic development of insects, entomologists have become sufficiently well acquainted with the phenomena attending this development to be able to confirm absolutely (in essential characters) Weismann's dis- coveries in the larva of the i imaginal discs ' as the independent embry- onic centers from which develop the wings, legs, antennae, and some other parts of the winged adults (imagines) of insects with complete metamorphosis. That is to say, in all the insects which hatch from the egg in a larval condition markedly different from the definitive condi- tion of the species in its fully developed, mature stage, many of the adult organs, as the external parts of the head, and the legs and wings, are produced not by a gradual development, growth, and transforma- tion of the corresponding larval parts, but by a special development in late larval life and during the pupal stage, the final structures being formed from small groups of previously undifferentiated subembryonic cells. These cells are derived in the case of the external parts just named chiefly from invaginations of the larval cellular skin layer. In the larva (maggot) of a house fly, for example, there are no functional legs or wings : there are no external signs (buds, pads) of these organs at any time in the larval stage. "In the larval life there can be no possible molding influence on these future adult organs of the nature of a direct response or reaction to the immediate environment. We might assume such an influence possible if the wings and legs were slowly transforming external struc- tures subject to attempts at or actual functional use in flight or crawling during the larval life. At pupation, the wings and legs suddenly appear as external parts, but still equally functionless, and now wholly concealed and protected by the opaque chitinized wall of the puparium. With the final issue of the adult, the wings and legs appear for the first time in functional condition, and with the simple need of unfolding, expanding, and drying the outer wall, an operation requiring but few moments, they appear at this time in their definitive fully developed condition. The wings have the arrangement of veins and number of spines and fringing hairs; the legs have the armature of spines and spurs and the number of segments which they retain unchanged through the short or longer adult life. The wings and legs of the adult of all 144 EVOLUTION AND ANIMAL LIFE insects with complete metamorphosis and the insects of this category include all the beetles (Coleoptera), two-winged flies (Diptera), moths and butterflies (Lepidoptera), ants, bees, wasps, gall flies and ich- neumons (Hymenoptera), and some other orders are exposed during their development, to just one type of extrinsic influences, namely, those of nutrition, temperature, humidity, etc. These influences affect the whole body and metabolism of the body-developing insect. But they have no direct relation to specific parts. "An important special environing condition of life, and one that certainly works direct and obvious influence on the body wall of certain animals, is what may be called the chromatic condition of the environ- ment. Color and pattern adapted to the needs of protection or ag- gression are phenomena familiar throughout the animal series. Most of such color and pattern conditions, catalogued under the head of protective resemblance, mimicry, warning colors, etc., are fixed con- ditions as far as the individual is concerned, presumably brought about by the age-long action of natural selection. "Not a few animals display the capabilities of achieving marked adaptive changes, i. e., acquired variations, during their immature life (postembryonic development). But it is obvicus that insects of com- plete metamorphosis, which possess in adult stage a color scheme and pattern wholly different from that of the larva or pupa and one which is not apparent until it appears in fixed definitive condition on the emergence (and drying) of the imago from the pupal cuticle, cannot be conceived to show, in their color pattern, variations due to individual adaptive changes. That is, variations in this color pattern among the individuals of a species are not acquired, but are strictly congenital, except in so far as they are produced by the general influences of nutrition, temperature, etc., working without reference to the external chromatic conditions of the environment. "Even such all-pervading influences as nutrition, temperature, humidity, and light maybe, and in many cases obviously are, so nearly practically identical for all the members of one brood, or even for all the individuals of the species, that they can have little or no influence in causing variations. For conspicuous example, the case of the honey bee may be noted. Here, all the larvae live side by side under identical conditions (those of the hive) of temperature, humidity, and light, and the distribution of exactly similar food to them in similar quantity is probably as nearly exactly uniform as could be guaranteed under our most careful artificial experimental conditions. The pupte are, more- over, under identical conditions of temperature, moisture, and light, so VARIATION AND MUTATION 145 that when the adults issue, the variations to be found in any of their parts may with complete confidence be ascribed to prenatal influences, to intrinsic causes. They are purely blastogenic. Similarly, the con- ditions of life of the developing individuals of all the other social insects, the termites, ants, and social wasps, are practically identical. "The variations, therefore, in the color pattern of Diabrotica (Fig. 75), Hippodamia (Figs. 72, 73 and 74), and Vespa (Fig. 76) (insects of complete metamorphosis with all adult external structures never exposed to outside conditions until in definite unchangeable condition), are congenital variations. Of the same nature are also the structural variations in the character of the venation and the number of wing hooks in the honey bee (see Fig. 94). But the variations in the pat- tern of the prothorax of the flower bug (Fig. 77), and in the number of spines on the tibiae of the red-legged locust (Fig. 78), and the cicada (Fig. 79), may be in part acquired. In these latter cases the insects, not having a complete metamorphosis, have during their immature life these color and structural characters in formative condition, and to some extent in use. They are therefore exposed to the continuous influence of their environment." It might be thought that we could determine whether varia- tions are congenital or acquired in cases in which we are thor- oughly acquainted with the character of the environment or ex- trinsic influences which have surrounded the individuals during their development. In experimental cases we can control this environment and make it identical for all of a given lot of individuals, or measurably varying for different lots. Then by comparison we can determine what characteristics still vary among those individuals exposed to identical environment- these variations should be congenital and what new kinds of variations appear in those individuals exposed to different en- vironments these should be acquired variations. This has been done experimentally for silkworm moths. By varying the food supply, etc., marked variations have been produced in the size of larvae and moths, weight of silken cocoons, duration of larval stages (instars). These variations are manifestly acquired, and wherever in nature simple variations in dimen- sions are found among individuals of a species, this is due un- doubtedly, to greater or lesser extent, to differing conditions of nutrition. But we know well that a practically identical food supply given to domesticated animals or human beings can 146 EVOLUTION AND ANIMAL LIFE never make all the individuals of a single brood or family of the same size. Part of the dimensional variation is due, therefore, to congenital causes. In a beehive, the condition of temper- ature, humidity, and food supply are practically identical for all the developing bees, and yet bees born of eggs laid by a single queen, reared at the same time in the same hive, vary largely in such easily determinable and important matters as venation of the wings, number of hooks used in holding the two wings of one side together, color pattern, etc. Undoubtedly, these variations are strictly congenital, hence inheritable, and therefore of a character to serve as a basis for species change. With regard to examples of continuous and discontinuous variations, we take the FIG. 84. Head of deer following from the paper on "Variation in with one prong of horns markedly dif- ferent from the other. (After Bateson.) continuous variations we reter to those variations mentioned above, variously called fluctuating, individual, etc., which are present in any series of indi- viduals of a species, and which cluster about the modal or most abundantly represented forms of the species, as would be expected from the law of error (law of probabilities) dis- cussed above. "Morgan, in 'Evolution and Adaptation/ ob- jects to the use of ' continuous ' as a descriptive name for these variations, on the ground that the word suggests persistence or continuity through successive generations. It seems to us, however, that the name is an apt one, if ' continuous ' be taken to mean that the occurring variations in any (sufficiently large) set of individuals form a continuous series, the extremes being connected or immediately merging into each other by a series of small gradatory steps. By 'discontinuous' variation, we would mean, in contrast to continuous, such considerable and radical changes as have been variously called single variations, sports, saltations, mutations, etc.; that is, variations which are not members of graded series and do not group themselves in orderly manner about the modal FIG. 85. Turtle with two heads. (After Bateson.) VARIATION AND MUTATION 147 FIG. 86. Child with six toes on each foot. (After Bateson.) species form according to the law of error. Although often not large, they are yet rarely so minute as those differences which distinguish the adjacent members in any series of individuals arranged on a basis of continuous or fluctuating variation. Mutations, according to the usage of de Vries, discontinuous variations may or may not be. Thus, all mutations might be called discontinuous variations, although not all discontinuous variations are necessarily de Vriesian muta- tions, that is, certain to breed true under varying conditions of environment. "Asa matter of fact, not all continuous variation follows the law of error : the curve or polygon of frequency is not infrequently an unsymmetrical one : ' skewness ' prevails ; that is, the highest part of the curve may be nearer one end, or the curve may even be bimodal. But neverthe- less the 'continuity' of the variations is unmistakable. In a suffi- ciently large series the extremes of the range are perfectly connected with the mode or modes and hence with each other by gradatory steps very small in size. Whatever the largeness of the difference between the extremes, any two adjacent' members of the series are hardly distin- guishable. This gradual kind of variation, in- sensible, but yet effective (as regards widely sepa- rated members of the series), is most typically illustrated in cases of what Bateson calls ' sub- stantive' variation, that is, where the varying characteristic is one of pattern, of length, width, or bulk, of the curving of a vein or leg or spine. Excellent examples of this continuous substantive variation are presented by the abdominal and face patterns of Vespa (see Fig. 76), and the elytral pattern of Dia- brotica (see Fig. 75). "According to Bateson, variations in number of antennal and tarsal segments, number of spines, hairs, or other processes, and other FIG. 87. Eyestalks of a decapod dissected out: on the right an antenna has regenerated out in place of an amputated eye; opt., optic nerve. (After Herbst.) 148 EVOLUTION AND ANIMAL LIFE such numerical or, as called by him, meristic variations, must be looked on as different in kind from the substantive variations those capable of perfect merging from one condition to another in other words, practically incapable of quantitative measurements. These meristic variations are called discontinuous by Bateson. Typical examples are the variation in the number of the costal wing hooks in bees and ants, the number of tibial spines in the locust and cicada, the number of metathoracic tactile hairs in biting bird lice, etc. But when one stops to consider the fact that in all these cases variation could hardly occur FIG. 88. Variations in pattern of wings of Peronea cristana. (After Clark.) by any steps less than those of one hook or one spine or one hair, that a half hook or half antennal segment is inconceivable, serious doubts as to the validity of Bateson 's classification of variations as continuous and discontinuous will certainly result. The doubt is strengthened by the difficulty of a clean classification presented by such cases as that of Hippodamia convergens (Figs. 72, 73 and 74). Here we have a substantive variation in pattern, appearing, however, in such a way as to demand numerical, i. e., meristic, expression. One specimen has nine elytral spots, another ten, another eleven, and so on; the whole range is indeed from naught to eighteen, with every number between represented, each by various combinations of spots. " But it is conceivable, and indeed is really the case among our specimens, that these spots might be either of normal size, or of any lesser size down to the limits of visibility. Some of the spots are of the diameter of pin points, some of the pin shaft, and some of pin heads. There is perfect gradation and continuity in this variation. VARIATION AND MUTATION 149 And even in such cases as variations in spines and hairs, this gradation might exist: and indeed it does. Although in our consideration of the variation in the number of the tibial spines of the locust and cicada and in the number of the tactile hairs of the bird lice, we have referred to these variations only numerically, i. e., meristically, as a matter of fact there are obvious differences in the length, i. e., size, of the spines and hairs, so that it would be wholly fair to break down the unit differ- ences and speak of differences by one quarter, one third, and two thirds of a spine. For the tibial spines of the locust, we have actually re- corded the conditions in the form of frac- tions. But in the case of a hook or an antennal or a tarsal segment it is a unit or nothing. ' To our mind, the distinction between substantive and meristic variation is not at all equivalent to a distinction between continuous and discontinuous variation. It is a distinction between two categories of variation only in that one category in- cludes such conditions as permit more readily of extremely slight, nearly insensi- ble, practically immeasurable differences, as those of pattern or shape or extent, while the other category includes partic- ularly conditions in which any variation must of necessity be fairly obvious, and usually capable of numerical expression. ' But we believe, nevertheless, that variations really discontinuous occur among insects. For example, the occurrence of interpolated, wholly new, and complete cells (determined by the presence of new cross veins or branches of longitudinal veins) in the fore and hind wings of drone honey bees (Figs. 93 to 96) and the occurrence of curious malformations of venation among drone bees must be looked on as sports or truly discontinuous variations. The regular occurrence of a four-segmented foot, perfectly complete, functional in those numer- ous specimens of cockroach (Fig. 89), in which natural regeneration has taken place, may be looked on as an example of discontinuous variation. Although no difference in tarsal segments less than that of one is conceivable, it is quite conceivable that the foot with one fewer than the normal number might be in such condition that it would be obviously a five-segmented foot with one segment dropped out: in 11 FIG. 89. Cockroach, showing varying number of tarsal seg- ments in legs. (After Kellogg and Bell.) 150 EVOLUTION AND ANIMAL LIFE other words, that when compared with a normal five-segmented foot it would appear to be a modification of such a foot with some one segment wanting. But that condition is not at all what appears after the cock- roach regenerates a foot. The new foot is only very little, if any, shorter than the normal five-segmented foot (see Fig. 89) : one cannot say that it is precisely this or that segment which is lost. It is a new kind of foot, apparently just as capable, as 'fit/ as useful as the five- segmented kind. We have regularly occurring, in these cases of re- generation, the development of an entirely changed organ, similar as a whole to the old one, but different from it in all its parts; this differ- ence not being one of incompleteness, or serial addition or subtraction, but the difference of newness. It is the regenerative mutation of an organ ! " In five years of experimental rearing of the silkworms for the sake of studying phenomena of heredity and variation, the junior author has been able to record numerous cases of discon- tinuous or sport variation such as the absence of the usually well-developed caudal horn of the larva, melanism in larvae, double cocooning or absence of cocoon in the pupal condition, congenital monstrous loss of a whole wing in the adult, striking aberration of the wing pattern in the adult, etc. But the great mass of variation ever present and readily observable among the scores of thousands of silkworm individuals reared and carefully scrutinized has been of the continuous (fluctuating or Darwinian) type. A special type or kind of discontinuous variation, that exemplified by the so-called de Vriesian mutations, is discussed at the end of this chapter. The matter of determinate variation is discussed as follows in the same paper: "The theory of determinate variation is based on the hypothesis that fluctuating variations are not in all cases, nor necessarily in any case, purely fortuitous and scattering, but that because of some in- trinsic or extrinsic influence they tend to occur along definite or determinate lines. The need for the theory rests on the claimed inad- equacy of slight fortuitous variation in offering selection a sufficient 'handle' for action. The greatest logical difficulty with the theory is that none of the influences which are known is adequate to cause such an effect as that of producing persistent determinate variations. In the case of any developing individual, determinate variation can be VARIATION AND MUTATION 151 attained by controlling the environment (kind and quantity of food, degree of temperature, humidity, and light, etc.), but if such variations (modifications) acquired during development are not inherited, there will be no advance, generation after generation, along any line. There will be no cumulative effect of such determinate variation. The con- stant repetition of a certain environment on generation after genera- tion of a certain species would of course produce a constant repetition of certain individual modifications (orthoplasy), but we do not know as yet of any actual effect on the species of such persistent ontogenic variations. "The need, however, for some such factor in species-forming as de- terminate variation is obvious and strongly felt. There are certainly few selectionists left who honestly believe that the minute fluctuating variations in pattern, in size, in curve of a vein, in length of a hair, etc., have that life-and-death value which is the sole sort of value that an ' advantageous variation ' must have to be a serviceable handle for the action of natural selection. As a matter of fact, no systematist will have escaped having had it distinctly impressed on him that he recog- nizes differences in the pattern of ladybird beetles, in the number of fin rays in fishes, in the branching of a vein in flies' wings, that no enemy, no agent of natural selection, can recognize, at least to the extent of pronouncing sentence of death (or not pronouncing it) on its basis. And further, no biologist really satisfies himself with the worn statement: 'We must not presume to judge the value of these triv- ial, these microscopic differences, for we do not know all the complex interrelation and interaction of the organism and its environment/ We do not ; but we do know for many cases that such differences are not actually of life-and-death selective value, and reason compels us to believe to a moral certainty that in other cases these fortuitous trivi- alities have similar lack of life-and-death importance. " Directly touching this point are our data of the variation of series of honey bees collected from free-flying individuals after exposure as adults to the rigors of outdoor life, as compared with the variation in the series of bees, adults, but collected just when issuing from the cells before being exposed as adults in any way to the external dangers of living. Series of both drones and workers representing both exposed and unexposed individuals were studied. The results of this examina- tion are, that the variation among the exposed individuals is no less than that among the unexposed individuals. This means that these various, mostly slight, blastogenic, variations (although in such im- portant organs as the wings), which occur among bees at the time of 152 EVOLUTION AND ANIMAL LIFE their issuance as active, winged creatures, are not of sufficient advan- tage or disadvantage to the individuals to lead to a weeding out by death or saving of such varying individuals by immediate selective action. Whatever the rigor and danger of the outdoor bee life, these variations seem to be insufficient to cut any figure in the persistence or nonpersistence of any individual in the face of this rigor. "A case which really seems to illustrate deter- minate variation is that of the variation of the flower beetle, Diabrotica soror (Fig. 75). Among a thousand in- dividuals collected on the University campus in 1895, a certain condition of variation in the elytral pattern exists, as represented graphically by Fig. 90. In 1901 and 1902, other thousands collected from the 'same place and examined to determine the condition of the variation in this pattern, show a dis- tinctly different status, as il- lustrated in Figs. 91 and 92. (To be sure that a series of a thousand individuals really reveals the conditions of this pattern variation, repeated series of 1,000 individuals each were examined and found practically identical.) The difference in the varia- tion status between the 1895 lot and the 1901-2 lots consists in the dominance in 1901-2 of one of the two modal conditions found to exist in the species, which in 1895 was not the dominant one. There has been a marked change in seven years, not in the pattern itself but in the prevalence or dominance of one type of pattern. Has the change been brought about by natural selection? Or is it the result of a determinate variation caused by we know not what intrinsic or extrinsic factors ? >6( 2( 8( 14( ?00 _ = == a - m Mi mm m 6( 1 1 ) 1 1 2( i 8( Ll- A( = Classes ':\ 'ariates ; 38: K|:: * 55 ..!.. ..!_. ..). . 64 qu !* 203 MISCEL 200totAl905 FIG. 90. Frequency polygon of variation of elytral pattern in 905 specimens of the Cali- fornia flower beetle, Diabrotica soror, collected at Stanford University, 1895. (After Kellogg and Bell.) VARIATION AND MUTATION 153 "When one straightens up after a careful microscopic examina- tion of the pattern of Diabrotica to determine its variation, one is sure that no other enemy of these flower beetles can be conceived to use such discrimination as ours. Does the fly catcher swoop- ing from its station on fence post or tree branch determine which of two heavily flying Dia- broticas shall be its prey on the basis of 'two middle spots on left elytron partial- ly fused' in one and 'these two spots not touching' in the other? To our minds the change in variation status, the dominance of one mode to-day which was the subordinate mode in 1895, is not due to the action of selection. We do not indeed hesitate to believe in those 'unknown factors of evolu- tion' which may produce, among other results, that condition of affairs best named 'determinate varia- tion.' This variation is not necessarily to be conceived of as purposeful or even advantageous ; if by its cumulation it becomes a disadvantage of life - and - death value, natural selec- 36 . i i 1 ' 32 Tl M 28 24 20 16 | 1 1 A U 1 1 12 M P" H J d i 1 Classes * * * _ . . _ -I- MlSCEl I04 : tot a!905 *3I3 32 60 396 FIG. 91. Frequency polygon of variation of elytral pattern in 905 specimens of the Cali- fornia flower beetle, Diabrotica soror, col- lected at Stanford University, October, 1901. (After Kellogg and Bell.) tion, which is after all a logical necessity and un- doubtedly an actual actively regulative factor in species control, will take care of it." In the light of the foregoing discussion of the categories and characters of variations, it is obvious that a well-grounded knowledge of variability and variations, a knowledge based on careful extensive statistical and experimental studies, is essen- tial as a basis for any effective investigation of the factors and 154 EVOLUTION AND ANIMAL LIFE processes involved in species-forming, that is, evolution. The methods and phenomena of evolution are intimately linked with indeed throughout are based upon the methods and phenomena of variation. What causes variation is a contrib- utory cause in evolution, and one of the funda- mental and all-important causes. Concerning the causes of variation, at least of those of congenital varia- tion, we are almost wholly in the dark. Only such influences as can affect the actual germ cells are presumably potent to ef- fect congenital variation. Such influences are not proved to the satisfac- tion of many biologists to be numerous. In the fusion of the germ cells of two individuals, the phenomenon called by him amphimixis, Weis- mann finds the most ef- fective cause of variation. Now the wider apart the two parents are in struc- tural and functional char- acteristics, the greater is the variation in their off- spring likely to be. Hence hybridization, or the mating of unlike parents, even to the degree of race and species un- likeness, is a great resource of the breeder who would have in his hands large variation. But if the parents are too unlike, their mating, even if possible, proves sterile. Usually parents must be of the same species, although experiment has shown that considerable extraspecific hybridization is possible. Among cultivated plants and animals the artificially selected races differ very much, but these races are mostly easily hybridiz- teO \l( ) W ) i L ;oo I : ) 2 \ 1 \ 3 LL I lasses foriate! .. _ _ _ MISCPL 09 tot *19Q5 i3l * 40 55 3881 FIG. 92. Frequency polygon of variation of elytral pattern in 905 specimens of the Cali- fornia flower beetle, Diabrotica soror, col- lected at Stanford University, October, 1902. (After Kellogg and Bell.) VARIATION AND MUTATION 155 able. The nature and results of fertilization and amphimixis are treated in Chapter XIII. But parthenogenetically produced individuals (that is, young born from unfertilized eggs as the honey-bee drones, certain whole generations of various gall flies, saw flies, aphids, etc., etc., regularly are) also vary. In the case of male bees, male ants, female aphids, etc., etc., the individuals differ quite as much as do individuals of the same species of bisexual parentage. Comparing the variation in drone bees (par- thenogenetically produced) as com- pared with that of the workers (from fertilized eggs), we find that this is true. The organs examined for variation in these series of bees were the wings, organs used by both drones and workers, and having no immedi- ate relation either structurally or physiologically to the differ- entiation of those two castes or kinds of individuals of the honey-bee species. The workers are "incomplete" only in that most of them are infertile: in no other structural or physiological feature of their makeup are they less " complete ' : than the drones. They are indeed distinctly the more specialized of FIG. 93. Fore and hind wings of honeybee (drone), showing normal venation. (After Kel- logg and Bell.) FIG. 94. Part of costal margin of hind wing of honeybee, much magnified to show hooks. (After Kellogg and Bell.) the two, and according to one of the early Darwinian canons of variation might be expected to differ more than the drones. But the drones are males and, according to another commonly accepted belief, this is the explanation for a larger variation on their part, if such larger variation occurs. As a matter of fact, it does. The drones, in all the many series studied, show mark- edly more variation in the venation of the wings than do the workers, while they show quite as much variation as the work- ers in the number of the hooks which hold the two wings together in flight. (See Figs. 93 to 96.) Both these characters, i. e., wing venation and wing hooks, are not so-called "male char- 156 EVOLUTION AND ANIMAL LIFE acters": they are not to be compared with those secondary sexual characters such as ornamental or aggressive spines, horns, patterns, etc., which are the characteristics that give males their special reputation for ultravariation. FIG. 95. Fore wings of honeybee (drone) , showing variations in venation. (After Kellogg and Bell.) Finally, with regard to the causal influence in variation-pro- ducing of the "primary factors of evolution/' such as temper- ature, light, humidity, pressure, and extrinsic physicochemical conditions generally, summed up commonly in the phrase climate and environment, we have one all-important considera- tion to keep constantly in mind. However po- tent and obvious the ef- fects of these influences are on the individual, we have no proof as yet of a nature to com- pel the general accept- ance of biologists, that such effects can be car- ried directly over to the race or species. Onlv ten vears after */ J Darwin published the "Origin of Species," von Kolliker, the great German zoologist, in criticising the assumptions on which species-forming by natural selection was based in the Darwinian theory, proposed an alternative theory of heterogenesis or species-forming by leaps (saltations or mutations). These saltations need not of necessity to be large, but must be changes definite and fixed. Later, Korschinsky, a Russian botanist, outlined in some de- FIG. 96. Hind wings of honeybee (drone), show- ing variations in venation. Note the interpola- tion of the cells. (After Kellogg and Bell.) VARIATION AND MUTATION 157 tail and with greater emphasis such a theory of species-form- ing by mutations; and finally in 1901 Hugo de Vries, the famous botanist of Amsterdam, published in extenso the details of many years of observation and experiment on the subject of mutations, and reformulated definitively a theory of species- forming by mutational or saltational variation, the now famil- iar mutation theory. The following paragraphs from Morgan (" Evolution and Adaptation/' pp. 294-297, 1903) give a concise statement of the actual details of the mutations in the evening primrose ob- served by de Vries : "We may now proceed to examine the evidence from which de Vries has been led to the general conclusions given in the preceding pages. De Vries, found at Hilversam, near Amsterdam, a locality where a number of plants of the evening primrose, (Enothera lamarck- iana, grow in large numbers. This plant is an American form that has been imported into Europe. It often escapes from cultivation, as is the case at Hilversam, where for ten years it had been growing wild. Its rapid increase in numbers in the course of a few years may be one of the causes that have led to the appearance of a mutation period. The escaped plants showed fluctuating variations in nearly all of their organs. They also had produced a number of abnormal forms. Some of the plants came to maturity in one year, others in two, or in rare cases in three, years. " A year after the first finding of these plants de Vries observed two well-characterized forms, which he at once recognized as new elementary species. One of these was 0. brevistylis, which occurred only as female plants. The other new species was a smooth-leafed form with a more beautiful foliage than 0. lamarckiana. This is 0. Icevifolia. It was found that both of these new forms bred true from self-fertilized seeds. At first only a few specimens were found, each form in a particular part of the field, which looks as though each might have come from the seeds of a single plant. "These two new forms, as well as the common 0. lamarckiai/a, were collected, and from these plants there have arisen the three groups or families of elementary species that de Vries has studied. In his garden other new forms also arose from those that had been brought under cultivation. The largest group, and the most impor- tant one, is that from the original 0. lamarckiana form. The accom- panying table shows the mutations that arose between 1SS7 and 1899 158 EVOLUTION AND ANIMAL LIFE from these plants. The seeds were selected in each case from self- fertilized plants of the lamarckiana form, so that the new plants ap- pearing in each horizontal line are the descendants in each generation of lamarckiana parents. It will be observed that the species, 0. ob- longata, appeared again and again in considerable numbers, and the same is true for several of the other forms also. Only the two species, 0. gigas and 0. scintillans, appeared very rarely. "(ENOTHERA LAMARCKIANA Elementary Species GENERATION. 8 Gener ) VIII. 1899 annual ) 7 Gener ) VII. 1898 annual ) 6 Gener ) VI. 1897 annual ) 5 Gener V. 1896 annual 4 Gener ) IV. 1895 annual ) 3 Gener ) III. 1890-91 biennial ) 2 Gener ) II. 1888-89 biennial ) 1 Gener ) I. 1886-87 biennial ) AIK^IO Oblon- Ruhri- Lamarck- Nan- T Scin- Gigas. Albida. gata nerv|s jana _ Lata. 5 1 1,700 21 1 1 11 9 V 3,000 y 29 3 V 1,800 9 5 V 25 135 15 176 20 8,000 49 1 12 6 8 14,000 60 73 1 1 10,000 3 3 15,000 5 5 V 9 "Thus de Vries had, in his seven generations, about fifty thousand plants, and about eight hundred of these were mutations. When the flowers of the new forms were artificially fertilized with pollen from VARIATION AND MUTATION 159 the flowers on the same plant, or of the same kind of plant, they gave rise to forms like themselves, thus showing that they are true elemen- tary species. 1 It is also a point of some interest to observe that all these forms differed from each other in a large number of particulars. "Only one form, 0. scintillans, that appeared eight times, is not constant as are the other species. When self-fertilized, its seeds pro- FIG. 97. At left, section of chestnut, Castanea vesca, showing unusual variations; at right, a branch of Mercurialis annua, which presents several variations. (After de Vries.) duce always three other forms, 0. scintillans, 0. oblongata, and 0. lamarckiana. It differs in this respect from all the other elementary species, which mutate not more than once in ten thousand individuals. From the seeds of one of the new forms, 0. Icevifolia, collected in the field, plants were reared, some of which were 0. lamarckiana, and others 0. Icevi/olia. They were allowed to grow together, and their descendants gave rise to the same forms found in the lamarckiana 1 0. lata is always female, and cannot, therefore, be self-fertilized. When crossed with O. lamarckiana there is produced fifteen to twenty per cent of pure lata individuals. 160 EVOLUTION AND ANIMAL LIFE family, described above, namely, 0. lata, cUiptica, nanneUa, mibri- nervis, and also two new species, 0. spatulata and leptocarpa. "In the lata family, only female flowers are produced, and, therefore, in order to obtain seeds they were fertilized with pollen from other species. Here also ap- peared some of the new species, already mentioned, namely, al- bida, nannella, lata, oblongata, ru- brinervis, and also two new species, elliptica and subovata. "De Vries also watched the field from which the original forms were obtained, and found there many of the new species that ap- peared under cultivation. These were found, however, only as weak young plants that rarely flowered. Five of the new forms were seen either in the Hilversam field, or else raised from seeds that had been collected there. These facts show that the new species are not due to cultivation, and that they arise year after year from the seeds of the parent form, 0. lamarckiana." FIG. 98. Stamens of a hybrid willow, Salix auritax purpurea, showing dif- ferent degrees of varying. FIG. 99. A branch of a Japanese tree, Cryptomeria japonica, showing an atavistic variation. (After de Vries.) As to this we may observe: It has long been known that individual variations of an extreme degree sometimes occur, VARIATION AND MUTATION 161 and that these may be to a degree persistent in heredity. Of such nature was the Ancon sheep, the Mauchamp sheep, the iceberg blackberry, and numerous other races or forms known in the domestication of animals, or the cultivation of plants. The generally normal structure of such individuals distinguishes FIG. 100. A branch of the green Georgine, in which the inflorescence leaves and some of one branch (the right-hand one) are green like the rest of the plant, while the other varieties are red and in normal condition. (After de Vries.) them from monstrosities, which are usually freaks of develop- ment rather than of heredity. The name " saltation, ;; or in recent years "mutation/ 7 has been applied to extreme fluctuation, the immediate cause of which is unknown. The experiments of de Vries on the salta- tions of the descendants of the evening primrose (called (Eno- thera lamarckiana) have drawn general attention again to the possibility that saltation has had a large part in the process of 1 The word mutation was first used not for saltations but for the slow fluctuation in successive geological periods. 162 EVOLUTION AND ANIMAL LIFE formation of species. As to this it may be said that the possible variation within each species is much greater than the range of the individuals which actually survive. The condition of domestication favors the development of extreme variation, be- cause such individuals may be preserved from interbreeding with the mass, and they may survive even if their characters are unfavorable to competition in the struggle for existence. Among plants it is noticed that new soil and new conditions seem to favor large variation in the progeny, although the traits thus produced are not usually hereditary. Cases more or less analogous to those noted by Dr. de Vries are not rare in horti- culture. The cross breeding of variant forms favors the ap- pearance of new forms. Among actual species in a state of nature, there are very few which seem likely to have arisen by a sudden leap or mutation. The past and the future of de Vries' evening primroses are yet to be shown. The species called by de Vries (Enothera lamarckiana is not at present known in its wild state anywhere in North America, the parent region of all the species of evening primroses or (Enothera; so that we have as yet no reason to assume that the various mutants of the evening primrose are really comparable to the wild species of the same group now existing in America. While saltation remains as one of the probable sources of specific difference, the actual role of this process in nature is yet to be proved. CHAPTER X HEREDITY "Vom Vater hab' ich die Statur, Des Lebens ernstes Fiihren ; Vom Miitterchen die Frohnatur Und Lust zu fabuliren. Urahnherr war der Schonsten hold, Das spukt so hin und wieder. Urahnfrau liebte Schmuck und Gold, Das zuckt wohl durch die Glieder. Sind nun die Elemente nicht An dem Complex zu trennen ; Was ist denn an dem ganzen Wicht Original zu nennen?" GOETHE, * " Zahme Xenien," vi. HEREDITY is the rule of persistence among organisms. The existence of such a law, or "ascertained sequence of events," is a matter of common observation. "Like produces like, 1 "Stature from father and the mood Stern views of life compelling; From mother, I take the joyous heart And the love of story-telling. "Great-grandsire's passion was the fair. What if I still reveal it? Great-grandam's, pomp and gold and show, And in my bones I feel it. "Of all the various elements That make up this complexity, What is there left when all is done, To call originality?" BAYARD TAYLOR'S translation in part. 163 ' ' 164 EVOLUTION AND ANIMAL LIFE "Blood will tell," "Blood is thicker than water," these proverbs in all languages indicate the general fact that each organism is likely to resemble its parents, and that the basis of fundamental resemblance among organisms is found in kinship by blood. It is equally a matter of common observation that the law of hered- ity is inseparable from a law of variation. No one organism is quite an exact copy of another. The prevention of such a con- dition is one of the effects of the process of double parentage. Except in certain exceptional forms in which parthenogenesis or hermaphroditism appear, each complex organism springs from two organisms of the same species: the one male, the other female. The resultant organism partakes of the qualities of each of these in some degree, and through these to a degree also it partakes of qualities of the parents or ancestors of each. The phrase, "Kinship by blood," used in connection with all studies of heredity, is a survival of an ancient theory that the physical basis of heredity is found in the actual blood. "Blood is quite a peculiar juice," as was observed by Mephistopheles, but its peculiarities are not concerned with heredity. The func- tion of blood is concerned with the nourishment of tissues and the removal of their waste. The actual vehicle of transfer of hereditary qualities, the physical basis of heredity, is found in structures within the protoplasm of the germ cell. The germ cells, male or female, are alike in all characters essential to this discussion. On the average, the potency of the male and the female cell is exactly the same, there being nowhere constant advantage of one sex over the other. Each cell, male or female, is one of the vital units, or body cells, set apart for the special purpose of reproduction. It is not essentially differ- ent from other cells in structure or in origin, but in its poten- tialities. Its function is that of repeating the original organism, ' with the precision of a work of art." Heredity is shown in the persistence of type, in the existence of broad homologies among living forms, in the possibility of natural systems of classification in any group, in the retention of vestigial organs, in the early development and subsequent obliteration of outworn structures once useful to individuals of the race or type. In a general way, the individual inherits from both parents the common structure of organisms of the species to which it belongs. The special peculiarities of the individual organism HEREDITY 165 are also inherited, but in much less certainty of degree. These traits belonging to a member of a single generation have a smaller "inheritance fund" on which to draw. In each gener- ation some of these individual qualities are latent or "reces- sive," others are potent or "dominant.' 3 The recessive or an- cestral characters reappear with a certain regularity. They may form a sort of mosaic, by mixing with other dominant traits, or they may make a more or less perfect blend. Resem- blance to some remote ancestor occurs at times, being known as atavism. Each ancestor has some claim in the formation of the new individual, and behind the grandfather and grandmother dead hands from older graves call in their direction. The past will never let go, though with each generation there is a deeper crust over it. These old claims grow less with time, because with each new generation there are twice as many of these com- petitors. Moreover past generations can affect the heredity of the individual only through the agency of his immediate parents. Out of these elements Mr. Galton frames the idea of a " mid-parent/' a sort of center of gravity of heredity, though, as Dr. Brooks has observed, it is doubtful if this mid-parent is more than a logical abstraction. The bluer the blood in any species, that is, the more closely alike the ancestors are, the more certain will be the personal resemblance among the de- scendants. But characters actually latent are very real in heredity. Dr. Brooks says: "When a son of a beardless boy grows up and acquires a beard, we may say that he has inherited his grandfather's beard, but this is only a figure of speech, and he actually inherits the beard his father might have acquired, had he lived, nor would the case of a child descended from a series of ten or a hundred beardless boys be different." It is, moreover, certainly true that a beard can be as well inherited from the mother who has none as from the father. The inheritance is that of the beard the mother might have developed had she been a man. And, in general, in matters of heredity, the child is not derived from the parents as they actually are, but from the parents as they might have been. The traits transmitted in heredity are chosen from the whole line of parental possibilities. And with the process of conception, 12 166 EVOLUTION AND ANIMAL LIFE the union of the two parental germ cells, "the gate of gifts is closed. " No trait or quality can ever be acquired of which at least the elements are not involved in the original inheritance. "What is transmitted to the infant/' observes Dr. Archdall Reid, "is not the modification [of the parent], but only the power of acquiring it under similar circumstances. The power to acquire fit modifications in response to appropriate stimula- tion is that which especially differentiates high animal organ- isms from low animal organisms." Atavism or reversion is the process of "throwing back/' by which in some degree an individual resembles a distant ancestor. Under the name of "atavism/' according to Yves Delage, are included three very different things : (a) The transmission in one family of individual characters, which, latent for several generations, suddenly reappear. This is family atavism, and its nature is readily recognized. (6) The reappearance, more or less regularly in a race, of characters of an allied race, from which the first race may have been derived. This is race atavism. Of this nature are the zebra stripes sometimes seen in mules. (c) The appearance of characters abnormal for the race in which they appear, but which are normal in other races sup- posed to be ancestral. This is atavism of teratology. An illus- tration is the occasional appearance in the modern horse of rudi- ments of additional toes, with partly developed hoofs. "Everything is possible in heredity/' observes Delage. "One may always find examples of election, of blending (of mosaic), of combination, of resemblance direct, and of resem- blance reversed. To give to these groupings the name of laws would be an abuse of language, since not one of these rules is exclusively true. In reality there is no law of resemblance be- tween a child and its parents. All is possible, from a difference so great that there is not a trait in common, to an almost perfect identity with one or the other parent, with every intermediate degree of blending of characters and combination of resem- blances." The name "telegony ' is given to the supposed influence of the first male on the future offspring of the female. This theory of telegonv rests mainlv on a case of a mare which was first im- <_? / *. pregnated by a quagga, and whose subsequent colts from males of her own species had quagga-like markings. The supposed HEREDITY 167 facts on which the theory is based are inadequate or unproved, and it is probable that the phenomena called telegony have no real existence. Equally uncertain are the phenomena known as "prenatal influences/ 3 In the process of evolution, the development of the female has brought her to be more and more the protector and helper of the young. She gives to her progeny not only her share of its heredity, but she becomes more and more a factor in its development. In the mammalia the little egg is retained long in the body and fed, not with food yolk, but with the mother's blood. The parent thus becomes an immediate and most important part of the environment of the young. In man, by the growth of the family the parental environment becomes a lifelong influence. The father as well as the mother becomes a part of it. It has long been a matter of common belief that among mammals a special additional formative influence is exerted by the mother in the period between conception and birth. The patriarch Jacob is recorded as having made a thrifty use of this influence in relation to the herds of his father-in-law, Laban. This belief is part of the folklore of almost every race of intelli- gent men. In the translations of Carmen Silva, that gentle woman whom kind nature made a poet and cruel fortune a queen, we find these words of a Roumanian peasant woman: "My little child is lying in the grass, His face is covered with the blades of grass. While I did bear the child, I ever watched The reaper work, that it might love the harvests; And when the boy was born, the meadow said, 'This is my child.'" In the current literature of hysterical ethics we find all sorts of exhortations to mothers to do this and not to do that, to cherish this and avoid that on account of its supposed effect on the coming progeny. Long lists of cases have been reported illustrating the law of prenatal influence. Most of these records serve only to induce scepticism. Many of these are mere co- incidences, some are unverifiable, others grossly impossible. There is an evident desire to make a case rather than to tell the truth. The whole matter is much in need of serious study, and 168 EVOLUTION AND ANIMAL LIFE the entire record of alleged facts must be set aside to make a fair beginning. There are also many phenomena of transmitted qualities that cannot be charged to heredity. Just as a sound mind de- mands a sound body, so does a sound child demand a sound mother. Bad nutrition before as well as after birth may neu- tralize the most vigorous inheritance within the germ cell. A child well conceived may yet be stunted in development. Even the father may transmit weakness in development as a handicap to hereditary strength. The many physical vicissitudes between conception and birth may determine the rate of early growth or the impetus of early development. In a sense, the im- pulse of life comes from such sources outside the germ cell and outside heredity. All powers may be affected by it. Per- fect development demands the highest nutrition, an ideal never reached. In such fashion the child may bear the in- cubus of Ibsen's "Ghosts," for which it had no personal re- sponsibility. " Spent passions and vanished sins " may impair germ cells, male or female, as they injure the organs that produce them. In a thoughtful article on problems of heredity (The Horse- man, April 17, 1906), Mr. C. B. Whitford maintains that better results in the trotting horse come from breeding from untrained horses of good blood than from horses which have been elabo- rately trained to the highest speed on the racecourse. "Trotting horses that are overbred show the effects of their inten- sified breeding in a variety of ways. But the usual difficulty is ex- treme nervousness and want of ability to stand training. Sometimes a horse of this kind will show great promise when he is first hitched to a sulky. He will show great flashes of speed and will have a smooth, easy action and the trotting instinct well pronounced." But he is overnervous, lacks constitutional strength and will not do well. " The trouble with a horse of this kind is that he has not inherited the necessary fuel with which to create energy. He is 'burnt out' by heredity. That which he needed to train on was so largely used up by his ancestry in their process of development that they had not enough to transmit to their progeny." 169 FIG. 101. Diagram showing arrangement of bones in the hand or foot of various animals : 1, man ; 2. gorilla ; 3, orang ; 4, dog ; 5, sea lion ; 6, dolphin; 7, bat; 8, mole; 9, Ornithorhynchus. (After Haeckel.) 170 EVOLUTION AND ANIMAL LIFE If this is true, it would appear that nervous overstrain of the parent is unfavorable to normal nerve development of the offspring. This would be apparently a case of transmission of parental conditions, as above indicated, and not one of true heredity. It may be conceived that, at the moment of impregnation, the resultant germ cell is sexless. It begins its development at once, and, in the higher animals, turns very soon toward the formation of those structures which distinguish the one sex or the other. Each individual ultimately becomes either male or female. Relatively few animals, and those among the lower 7 FIG. 102. Limb skeletons of extinct and living animals, showing the homologous bones: 1 , salamander; 2, frog; 3, turtle; 4, Aetosaurus; 5, Plesiosaurus; 6, Ichthyo- saurus; 7, Mososaurus; 8, duck. forms, are ever really hermaphrodite, or representative of both sexes at once. Among the invertebrate animals the numerical relations of the sexes are subject to great variation. Among vertebrates, in general, the sexes are practically equal in number, as is shown by count of large series of individuals. This is true whether the species be monogamous, polygamous, or promiscuous in its sex relations. It is therefore apparent that the sex tendencies in HEREDITY 171 the germ are held on a very fine balance. A very slight impulse the one way or the other determines the sex direction the em- bryo shall take. Although much investigation and very much speculation have been devoted to this problem, it is still ] un- solved. We are not able, in the vertebrate animals, nor in fact in animals generally, to determine the nature of the stimulus, or of any of the various impulses, if more than one exists, which to.. "to,. FIG. 103. Limb skeletons of various animals, showing homologous bones: 9, Orni- thorhynchus; 10, kangaroo; 11, Megatherium; 12, armadillo; 13, mole; 14, sea lion; 15, gorilla; 16, man. leads the individual germ cell to develop as male or female. It is also possible that each germ cell is really bisexual from the beginning. One sex or the other becomes dominant and the other recessive as the embryo develops. But in this event we are still in doubt as to the nature of the determining factor or 1 The latest studies of the problem are chiefly concerned with an attempt to determine whether or not there exists a chromosome sex determinant, and whether sex determination may not be brought under Mendel's law of heredity (see later paragraphs in this chapter) in a modified form. 172 EVOLUTION AND ANIMAL LIFE stimulus. Among ants and the social bees and wasps the males develop parthenogenetically from unfertilized cells, the fertilized cells yielding either females or workers which are sterile females. But this specialized mode of development is peculiar to particu- lar groups. For a few lower species it has been ascertained that variation in nutrition may be a factor in sex determination. Favorable nutrition seems to increase the number of females. Most higher plants are hermaphrodite, the central leaves (car- pels) in the bud which becomes the flower, yielding ovules or fe- FIG. 104. Limb skeletons of various animals, showing homologies of the bones; at left, mole; next, giraffe; next, bat; next, porpoise. male germ cells. The next whorl (stamens) yields male germ cells or pollen. The outer whorls (corolla, calyx) serve as pro- tective organs only, and are without sex. The bonds of union among organisms which stand at the basis of all classification are known as "homologies " (Figs. 101- 104). A homology is a real likeness, as distinguished from one, merely superficial or apparent. To superficial likeness we give the name of analogy. Homology means fundamental iden- tity of structure, as distinguished from incidental similarity of form or function. Thus, the arm of a man is homologous with the foreleg of a dog, because in either we can trace deep-seated resemblance or homologies with the other. In each detail of each bone, muscle, vein, or nerve of the one we can trace the HEREDITY 173 a corresponding details of the other. But in comparing the arm of man with the "limb" of a tree, the arm of a starfish, or the foreleg of a grasshopper, we find no correspondence in details. In a natural classification, or one founded on fact, organisms showing the closest homologies are placed together. An arti- ficial classification is one based on analogies. Such a classifica- tion might place together a cricket, a frog, and a kangaroo, because they all jump, or a bird, a bat, and a butterfly, because they all fly, even though the wings are very dif- ferently made (Fig. 105) in each case. The very existence of such terms as animals and plants, insects and mollusks imply relation- ships, and relationships in different degrees. Classification is the process of reducing our knowledge of these grades of likeness and unlikeness to a system. By bringing together those which are funda- mentally alike, and separating those which are unlike, we find that these traits are the tmtcome of long-continued influences. Classification is defined as "the rational lawful disposition of observed facts." It rests on the results of the operations of natural laws, or forces which bring about inevitable results. For it is a matter of common observation that the closest homologies are shown by those animals which have sprung from a common stock. The fact of blood relationship shows itself always in homology. So far as we know, homology is never produced in any other way, therefore the actual presence of homologies among animals or plants implies, as we shall see in a later chapter, their common descent from stock possessing these same characters. In our primitive use of the trunk of the tree to imply unity in life, we can see that this trunk represents FIG. 105. Diagram of wings, showing homol- ogy and analogy: a, wing of fly; b, wing of bird; c, wing of bat. 174 EVOLUTION AND ANIMAL LIFE homology, and that it is the representation of the current of heredity. The resemblances arise from common origin, the FIG. 106. Ears of various anthropoid apes and of man, showing human vestigial characters: 1, hairy human ear; 2, Barbary ape; 3, chimpanzee; 4 and 5, human ears; 6, ear of human foetus; 7, orang-outang. variations from the demand of differing external conditions. It may be said that the inside of an animal tells what it is, the out- side where it has been. In the internal structure, ancestral _ _^^ traits are perpetuated with little change through geologic ages. The external characters affected by every feature of the surroundings may be rapidly altered through response to demands of environ- ment and through the destruction of in- dividuals whose life fails of adjustment. It is in the persistence of heredity that we find the explanation of vestigial organs. An organ well developed in one group of animals or plants may in some other be reduced to an imperfect organ or rudiment so incomplete as to serve no purpose whatever. Such rudimentary or functionless structures may be found in the body of any of the higher animals and in most or all of the higher plants. As a rule such structures are more fully FIG. 107. Head of a five- months human embryo showing embryonic hair- covering. (After Ecker.) HEREDITY 175 FIG. 108. Andrian Jeftichjew, the Russian dog man, showing extraor- dinary covering of hair on the face. (After Wiedersheim.) developed in the embryo than in the adult, becoming atrophied with age. Familiar examples are the appendix vermiformis and the unused muscles of the ears in man, the atrophied lung, pelvis, and limbs of the snake, the air bladder of the fish, the "thumb' (or rather index fin- ger), of the bird, the splint bone of the horse, and the like. The anatomist Wiedersheim has recorded 180 vestigial or- gans in man. These structures occur in all the systems of organs, integument, skeleton, muscles, nervous system, sense organs, digestive, respiratory, circulatory, and urino-genital systems. Most of these rem- nants of structures are to be found completely developed in other vertebrate groups. Eleven of them are characteristic as functional organs of fishes only, four of amphibians and reptiles. The fact that structures are vestigial is shown often by cases of atavistic de- velopment. Within the brain of man, near the optic lobes, is a little spheroid structure scarcely larger than a pea, known as the "pineal gland' or conarium. It has no evident function, and Descartes once sug- gested that it might be the seat of the soul. It is larger in the embryo and still larger in the brains of some of the lower verte- brates. Recent investiga- tions have shown that it deve loped in 176 EVOLUTION AND ANIMAL LIFE certain lizards, notably in a very primitive New Zealand lizard of the genus Sphenodon (Hatteria) (Fig. 109), andfcthat, in these lizards, the pineal body ends in a more or less perfect eye-like structure placed between the true eyes in the center of the forehead. A trace of this eye is shown in the limbless lizard called slow worm (Anguis), of Europe, and in several American species. In the horned toad (Phrynosoma) (Fig. 110) its place ^^-^^^&m^^^- v. , .c-^.-"'--' -' :-<: k ' i^rt- .l 2 ?^ - -&'-. . -..t^lfefie^yr FIG. 110. Head of lizard or horned toad, Phrynosom blainvillei, showing translucent pearly skin covering the pineal eye. (From specimen.) is covered by a translucent pearly scale. These lizards have in fact three eyes, and the pineal body is the nervous gang- lion from which the third eve arises. The natural conclusion V from this that all vertebrates originally had three eyes, is prob- ably a too-hasty one. Perhaps the pineal body was an organ of sense, which developed into an eye in the lizards and their ancestors only, not in any of the Amphibians or fishes, and not in any mammals or birds, although these are descended from reptilian stock. Whatever the origin or primitive function of the pineal ganglion, its existence in man as a vestigial organ is due to the persistence of heredity. HEREDITY 177 In the living species of horse, E quits, there is but a single toe, with its basal -bones. On each side of the base bone of this toe is a small bone known as a splint bone. The splint bones are apparently useless to the horse, but in extinct species of horse these bones are developed as digits, bearing small hoofs. Occa- sionally even now colts are born in which these splint bones bear rudimentary hoofs. In the museum of Stanford University is the leg of a high-bred colt from Milpitas, California, bear- ing a small hoof on each of the two splint bones. The remains (Fig. Ill) of over thirty different ancient horse-like animals have been found in the rocks of the Tertiary era. The Eohippus, the earliest of these horselike animals, found in the oldest Tertiary rocks, was little larger than a fox, and its forefeet had four hoofed toes, with the rudiment of a fifth, while the hind feet had three hoofed toes. In the later rocks is found the Orohippus, also small, but with the rudi- mentary fifth toe of the fore- foot gone. Still later appeared the Mesohippus and Miohip- pus, horses about the size of sheep, with three hoofed toes only, on both forefeet and hind feet, but with the rudiment of the fourth toe in the forefeet, of the same size in Mesohippus, smaller in Pliohippus. Also, the middle toe and hoof of the three toes in each foot was distinctly larger than the others in both Mesohippus and Mio- hippus. Next came the Protohippus, a horse about the size of a donkey, with three toes, but with the two side toes on each foot reduced in size, and probably no longer of use in walking. The middle toe and hoof carried all the weight. Still later in the Tertiary era lived the Pliohippus, an "almost complete horse." The side toes of Pliohippus are reduced to mere rudi- FIG. 111. Foot changes in evolution of the horse: a, Equus, Quaternary (re- cent) ; b, Pliohippus, Pliocene; c, Pro- tohippus, Lower Pliocene; d, Miohip- pus, Miocene; e, Mesohippus, Lower Miocene; /, Orohippus, Eocene. (After FIG. 254 of "Animal Studies.") 178 EVOLUTION AND ANIMAL LIFE ments or splints. This animal differs from the present horse somewhat in skull, shape of hoof, length of teeth, and other minor details. Lastly came the present horse, Equus, with the splint bones or concealed rudiments of the side toes' very small and the hoof of the middle toe rounder. In spite of the great difference between the one-toed foot of the living horse and the dog's five-toed foot there was once a kind of horse which had a five-toed foot, and there is after all a close relationship between the foot of the horse and the foot of the dog. jpl FIG. 112. Homology of digits of four odd-toed mammals, showing gradual reduction in number and consolidation of bones above. (After Romanes.) In man there is deA T eloped at the proximal end of the caBcum or blind sac of the large intestine a small structure as shown in Fig. 113. This appendage has no function, and it is subject to inflammation or suppuration, known as appendicitis. In the embryo the appendix vermiformis is notably larger than in the adult man; and in the lower animals, as in the dog or the kangaroo (see Fig. 113), it may be recognizable as a prolon- gation of the ca3cum, scarcely less in diameter than the intestine itself. The appendix vermiformis is therefore a vestige of a long creciim which had its part in the process of digestion. In the embryo of all chordate animals, without exception, respiratory or gill slits are developed, homologous with those seen in the embryo of the fish. The presence of these slits or their vestiges is one of the most important secondary distinctive characters of the great group of Chordata, which includes the vertebrates. The human embryo is, in this regard, at certain HEREDITY 179 stages essentially similar to the embryo of the fish. But in the course of development the gill slits in man and the higher ver- tebrates disappear. Their position is, however, indicated by the course of certain blood vessels. These follow the lines blocked out in the embryo when they led to the gill slits, al- though no other trace of these slits persists in the adult, and this direction is not one which we could conceive as likely to have arisen except for the results of inheritance from the lower ver- tebrates. In the veins of the higher animals valves are present, so arranged as to prevent the flow of blood backward and espe- cially downward from the heart. In the lower animals, these valves are adjusted to the position on all fours. Their adjust- ment is the same in man, notwithstanding his erect posture. Apparently the adjustment of the valves was completed before the position on all fours gave way to the erect posture. FIG. 113. At left, appendix vermiformis of kangaroo; at right appendix vermiformis of human embryo. (After Wiedersheim.) In the embryo of man there exists a regular tail, supported by eight distinct bones, like the tail of any other mammal. In the process of development, these bones are reduced in number and are joined, forming the coccyx or rudimentary tail. In various species of fishes, lizards, salamanders, crayfishes, and other animals living in caves or buried in the ground, the eyes are atrophied. Numerous cases (Fig. 114) of this sort have been studied by Dr. Carl H. Eigenmann. He finds in general that the young cave fish have normally developed eyes, but that 180 EVOCu: TION AND ANIMAL LIFE with growth atrophy sets in affecting different species differ- ently, in some cases the muscles, in others the lenses, but in all cases reducing the size of the organ to a functionless structure more or less covered by the skin. In all cases, the ancestry of these blind species can be traced to forms with well-developed eyes inhabiting the same region. Among the species examined are the blind fish of Mammoth Cave (Amblyopsis spelceus), the cave blind fish of Kentucky and Indiana (Typlichthys subterra- neus) , descended from the Dismal Swamp fish (Chologaster cornutus), the Missouri blind fish (Troglichthys rosw), the blind fishes of the caves of Cuba (Lucifuga sub- terranea, and Stygicola dentata) and the blind goby of Point Loma (Typhlogobius calif orni- ensis] . In Dr. Eigenmann's opinion, the retention of eyes in these species is due to the influence of heredity, the vesti- gial structures being each and all necessary to life in the light. Their degeneration he ascribes to the inheritance of the individual effects of disease, a matter we discuss in another chapter. Hundreds of cases of vestigial organs in plants have been recorded, among which we may mention the barren stamen in Pcntstemon which completes the number of five usual in the group of Scrophulariacese to which Pentstemon belongs. Other illustrations are the rudimentary leaves, with rudimentary stomata, found on the joints of species of cactus (Opuntia), etc.; the cilia found on the spermatozoa of cycads, which would en- able these structures to move freely in the water, although they are not deposited in the water, and these cilia are never actually used. By the theory of special creation it was supposed that these FIG. 114. Fishes showing stages in loss of eyes and color: A, Dismal Swamp fish, Cliologaster cornutus, ancestor of the blind fish; B, Agassiz's cave fish, Chologaster agassizi; C, cave blind fish, T.yphlichthys subterraneus. HEREDITY 181 rudiments were created in accordance with the tendency in creative processes to adhere to an ideal type. But it can- not be too clearly understood that tendencies in biology exist only as functions of particular organs. The tendency to adhere to a type is a part of heredity, the function of the germ cell. In the light of our knowledge of organic evolution it is clear that the presence of vestigial organs is simply a fact of heredity. They are organs once useful, but which through changed conditions of life have become needless. It is a recognized fact that useless organs tend to dwindle away, but the cause of this phenomenon is not so clear. It may be due in part to (a) panmixia or cessation of selection, the organ being no longer held to a high grade of efficiency, to (6) reversal of selection, the advantage lying with those individuals in which the organ is no longer functional or (c) the inheritance of the results of functional disuse. The latter offers an explana- tion which at first sight appears adequate, and its reality has been stoutly maintained by various writers of the Neo-Lamarck- iaa school. In their views, changes in the individuals unques- tionably due to individual or ontogenetic disuse are carried over to the species as phylogenetic disuse. Against this view is opposed its inconsistence with current theories of heredity, and also the positive fact that there is as yet no proof of the in- heritance of acquired characters. When we say that, through heredity, the offspring inherits the characters of the parent, we are speaking only a large and general truth. The details of this inheritance reveal in what regards this general statement must be modified. We have already noted the inevitable occurrence of at least small varia- tions in all body parts in all individuals. In addition to this ex- ception to identical inheritance, certain characters of the parent may not, as just mentioned, appear at all in the offspring. And this may be due to any one of several causes. First, certain parental characters are apparently really not heritable, namely, those new characters which have been ac- quired by the parent during its lifetime as the result of mutila- tion, disease, special use or disuse of parts, any change of parts due to direct reaction to a functional stimulus or to an environ- mental stimulus or cause, such as a bleaching due to lack of light, a thickening of the skin in certain places due to con- 13 182 EVpfrTION AND ANIMAL LIFE tact, etc. At least, there is not recorded any satisfactory proof of the inheritance of these acquired characters, and there is definite proof that many of them are not inherited. And most biologists, as helpful in many ways to a clearing up of the problem of adaptation and species-forming as the actu- ality of such inheritance would be, believe themselves un- able to accept this fact, in the light of our present knowl- edge. (This matter of the inheritance of acquired characters is discussed in Chapter XI. The assumption of this inheritance is a fundamental part of the Lamarckian explanation of evo- lution.) Second, certain characters peculiar to sex are inherited only according to sex and not by all the young. These characters include not only the differing reproductive organs themselves, but those many, various, and often most remarkably developed so-called secondary sexual characters, such as the tufts and plumes and brilliant plumage of male birds, the antlers of male deer, the specialized antennae, skeletal processes, and color pat- t cms of many male insects, and the reduced wings of many female insects, etc., etc. Even in cases of parthenogenetic reproduc- tion (i.e., reproduction in which the male takes no part), sex and the sex characters of the offspring have no direct relation to the sex and sex characters of the mother. The queen honey bee produces, in fact, exclusively drones (male bees) when she lays unfertilized eggs, while on the contrary the parthenogenetic offspring of the Aphids (plant lice) are all females for several generations, and then in a single generation both males and females. Finally, certain parental characters, even though blast ogenic, may not appear in the offspring, but be inherited by them in latent condition, to appear in their young or perhaps even in a later generation. It is obvious, too, that where a certain char- acter in the mother is represented in the father by one of oppo- site condition, as where the mother is very short, the father very tall, the mother a brunette, the father light-haired, a given child can inherit the character in only one condition. That is, in all cases of biparental reproduction, and they compose the majority of cases in both animal and plant kingdoms, the inherited char- acters cannot be all those possessed by both parents, but must be either those of one or the other, or a mosaic of them, or a blend or fusion of them. And this introduces us to that phase HEREDITY 183 of the study of the results of heredity which to-day is being most investigated, the determination of the "laws" of inheritance of characteristics. The similarity or dissimilarity of the two mating parents is a matter of much importance in regard to the results of inherit- ance. To produce a fertile mating the two parents have at least to be nearly allied. We are accustomed to take this for FIG. 115. Romulus, the striped colt of a horse mother and zebra father. (After Ewart.) granted, but the actual degree of phyletic relationship necessary in fertile mating is a point of much biologic interest. In most cases both parents must belong to the same species or kind, but among animals and plants there have been noted exceptions to this rule, these exceptions constituting the facts of hybridi- zation. Hybridism is practically limited to mating of different species of the same genera. Only in a few recorded cases have organisms of different genera mated in nature with the produc- tion of offspring. In zoological gardens and menageries the 184 ETOLUTION AND ANIMAL LIFE race feeling of the confined animals seems to break down, and unusual cases of hybridism are occasionally noted. Also men- tion must be made of the artificial induction of the fertilization of sea-urchin eggs by the sperm cells of starfishes (animals not only of different genera but of different families) , and a few other similar exceptional cases accomplished by Loeb and other ex- perimenters. In many examples of hybridism the immediate offspring are unable to produce young and so no continuous series of generations results. In other fewer cases the off- spring of hybridization are fertile, and thus constitute the beginnings of a new race or variety of animal or plant. Many of our domesticated animal races and cultivated plant varie- ties have originated by hybridism often artificially induced by man. For the most part, however, both parents of any brood of young belong to the same species, and hence they are at least as like each other as the other members of the same species have to be. But this may still permit great superficial dissimi- larity: many attributes, such as size, color, texture, outline, etc., of the body parts, especially the external ones, may be quite different. For within any species there may be several subspecies or varieties, the individuals of all of which are capable of fertile mating with each other. And even where there is no distinctly recognizable subspecific distinctions there may yet be much superficial dissimilarity among the individ- uals composing a single species. So in all studies of the results of heredity, of the actual inheritance of parental characters, the degree of likeness or unlikeness of the parents must be taken into account. So that the " laws " of heredity, as formulated on a study not of its mechanism but of its results, refer to the character of the parental union, whether pure or crossed, and if crossed whether the parents are of different varieties of one species or of actually different species. As a matter of fact the crossing of parents with a few to many dissimilar characters has been the actual means of getting at some of the most important evidence as to the behavior of heredity that we have. For the very dissimilar- ity of the parental attributes makes it possible to trace in the progeny of succeeding generations the workings or results of heredity with reference to these particular characters. Galton's Law of Ancestral Inheritance may be stated in few HEREDITY 185 words, although for an understanding of the character of the evidence on which it is based, and for an appreciation of its whole significance some full account of it, preferably Galton 's own statement and discussion of it in his memoir entitled "The Average Contribution of Each Several Ancestor to the Total Heritage of the Offspring," published in 1897, should be read. From a study of the carefully kept pedigree book of the kennels of the Basset Hounds Club, with records extending through twenty-two years, and a study of inheritance in the British Peerage made possible by the complete genealogic records kept for these families, together with a consideration of va- rious other less detailed but at least helpful records of inher- itance, Galton formulated the statement that any organism of bisexual parentage derives one half its inherited qualities from its parents (one fourth from each parent), one fourth from its grandparents, one eighth from its great-grandparents, and so on. These successive fractions, whose numerators are one and w r hose denominators are the successive powers of two, added together equal one or the total inheritance of the organism: thus i + l + l + iV + T& + A: + .... =1. The English mathematician and natural philosopher, Karl Pearson, has made computations showing that Galton 's law thus simply expressed is only a close approximation to the actual inheritance relations, and that the fraction indicating the contribution of any given ancestor must be slightly modified by introducing into it another factor. In general, though, the Galtonian formula received a very general acceptance among biologists. And only recently, in the light of the discov- ery of Mendel's investigations and conclusions and their confir- mation in essential principle by the recent researches of various botanists and zoologists, has Galton 's law been looked on as altogether too simple and incomplete a formulation of the facts of inheritance. It is not yet quite certain whether Galton's formula is consonant with the Mendelian formula or not. But at best Galton's law only expresses a part of what may now with confidence be said to be known of the regular course of inher- itance. Before taking up the actual Mendelian results and conclu- sions, however, it is important for us to note the different modes or kinds of behavior of inheritance which characteristics may show in their transmission. Cuenot has made a rough but EVOLUTION AND ANIMAL LIFE suggestive classification of these inheritance categories as follows : In cross mat ings and by " cross mating," students of hered- ity do not necessarily mean mating between distinct species or even varieties, but mating between parents which disagree in the condition of one or more specifically referred to characteris- tics in cross mating between the parents A and B, if we con- sider a single pair of corresponding characters a and b which differ in the two parents, the young produced by the crossing may (1) all present the same parental character a without any trace of the character b, the character a being then termed dominant or prepotent or prevalent, the other recessive or latent; or, (2) the young may all agree in presenting a new char- acter differing from the parental characters a and 6, this new character apparently being a simple physical mixture or a real chemical combination or blending of a and b ; or (3) , the young may differ from one another in regard to the parental characters a and b, some showing the character a, some showing the char- acter b or (4) the young may differ among themselves in regard to the characters a and 6, some showing the character a, some the character 6, and some various characters intermediate be- tween a and fr; or (5) the young may show the characters a and b side by side in each individual in small separated parts, even in neighboring but distinct cells. These differences undoubt- edly depend partly on the nature of the characteristics them- selves, partly on the kind of organism, and partly on extrinsic influences. It is obvious also that for certain characteristics by no means all five of these ways are open. Many characters are so wholly antagonistic that no blend nor any mosaic of them can occur in a single individual, leaving only ways (1) and (3), viz., exclusive or alternative inheritance open to them. To these five general categories of the actual transmission of certain obvious parental characters may here be added for consideration those cases of the appearance in the young of a character or characters having no obvious relation to either a or 6, but sometimes explicable as reversions or reappearances of characters possessed by ancestors more or less remote and other times as obviously wholly new and heretofore never ex- istent characters which, if pronounced, are called "sports" or sudden or discontinuous variations. Also must be taken into account the possible appearance among the young, of a few to HEREDITY 187 many individuals showing many simultaneous, usually slight but real differences from the parents in various parts and func- tions. These are the differences called mutations by de Vries and his followers, and are the basis of the at present consid- erably accepted theory of species-forming by heterogenesis or sudden complete fixed modifications of organic types. In the light of the observations and experiments of de Vries, these mu- tations are of special importance in any consideration of hered- ity and variation. (See p. 157, Chapter IX, for a brief account of these mutations.) The Mendelian "laws" apply only, probably, to certain par- ticular categories of inheritance, or rather categories of char- acters. That is, so far as worked out, the Mendelian principles seem to have definite application only to cases of inheritance in which the characteristics under observation are mutually ex- clusive or alternative in character; categories (1) and (3) in our list in a preceding paragraph are the only ones under the rule of the Mendelian principles, and there are even some ex- ceptions in these categories. The various other kinds of inher- itance, called blended or combined (where the two characteristics fuse or blend to form a new condition) , and mosaic or par- ticulate (where both parental characteristics exist side by side in each individual among the young), apparently require for their explanation something besides the Mendelian principle. At some time between 1855 and 1865 Gregor Johann Mendel, an Augustinian monk in the small Austrian village of Brimn, carried on in the gardens of his cloister pedigree cultures of peas and some other plants from which he derived data which he read, together with his interpretation of their significance, before meetings of the Natural History Society of Briinn, and which in the same year of their reading (1865) were published under the title "Experiments in Plant-hybridization," in the Abhand- lungen (vol. iv), of the society. Mendel was the son of a peas- ant, and had been educated in Augustinian foundations and ordained a priest. For two or three years he studied physics and natural science in Vienna, and refers to himself in one of his papers as a student of Kollar. He became abbot of his cloister, and was for a time president of the Briinn Natural History Society. Such are the essential details of the education and work of the man whose name will undoubtedly live forever in the annals of biological science. EVOLUTION AND ANIMAL LIFE Mendel's principal data were derived from the crossing of varieties of peas (Pisum sativum} in which he found several pairs of well-marked contrasting characters. Bateson gives a clear and concise summary account of MendePs methods and results which we quote in the following paragraphs. For the purposes of his experiments Mendel selected seven pairs of characters as follows : 1. Shape of ripe seed, whether round; or angular and wrinkled. 2. Color of "endosperm" (cotyledons), whether some shade of yellow; or a more or less intense green. 3. Color of the seed skin, whether various shades of gray and gray-brown; or white. 4. Shape of seed pod, whether simply inflated; or deeply constricted between the seeds. 5. Color of unripe pod, whether a shade of green; or a bright yellow. 6. Nature of inflorescence, whether the flowers are arranged along the axis of the plant; or are terminal and form a kind of umbel. 7. Length of stem, whether about six or seven feet long, or about three fourths to one and one half feet. "Large numbers of crosses were made between peas differing in respect of one of each of these pairs of characters. It was found that in each case the offspring of the cross exhibited the character of one of the parents in almost undiminished intensity, and intermediates which could not be at once referred to one or other of the parental forms were not found. "In the case of each pair of characters there is thus one which in the first cross prevails to the exclusion of the other. This prevailing character Mendel calls the dominant character, the other being the recessive character. 1 "That the existence of such 'dominant' arid 'recessive' charac- ters is a frequent phenomenon in cross breeding, is well known to all who have attended to these subjects. " By letting the cross-breds fertilize themselves Mendel next raised another generation. In this generation were individuals which showed 1 "Note that by these novel terms the complications involved by the use of the expression 'prepotent' are avoided." HEREDITY 189 the dominant character, but also individuals which presented the recessive character. Such a fact also was known in a good many instances. But Mendel discovered that in this generation the numer- ical proportion of dominants to recessives is on an average of cases approximately constant, being in fact as three to one. With very con- siderable regularity these numbers were approached in the case of each of his pairs of characters. "There are thus in the first generation raised from the cross- breds seventy-five per cent dominants and twenty-five per cent re- cessives. "These plants were again self-fertilized, and the offspring of each plant separately sown. It hext appeared that the offspring of the recessive remained pure recessive, and in subsequent generations never produced the dominant again. " But when the seeds obtained by self-fertilizing the dominants were examined and sown it was found that the dominants were not all alike, but consisted of two classes: (1) those which gave rise to pure dom- inants, and (2) others which gave a mixed offspring, composed partly of recessives, partly of dominants. Here also it was found that the average numerical proportions were constant, those with pure domi- nant offspring being to those with mixed offspring as one to two. Here it is seen that the seventy-fi ve-per-cent dominants are not really of similar constitution, but consist of twenty-five which are pure dominants and fifty which are really cross-breds, though, like the cross-breds raised by crossing the two original varieties, they only exhibit the dominant character. "To resume, then, it was found that by self-fertilizing the original cross-breds the same proportion was always approached, namely: 25 dominants, 50 cross-breds, 25 recessives, or ID : 2DR : 1R. "Like the pure recessives, the pure dominants are thenceforth pure, and only give rise to dominants in all succeeding generations studied. "On the contrary the fifty cross-breds, as stated above, have mixed offspring. But these offspring, again, in their numerical pro- portions, follow the same law, namely, that there are three dominants to one recessive. The recessives are pure like those of the last genera- tion, but the dominants can, by further self-fertilization, and exam- ination or cultivation of the seeds produced, be again shown to be made up of pure dominants and cross-breds in the same proportion of one dominant to two cross-breds. JO EVOLUTION AND ANIMAL LIFE "The process of breaking up into the parent forms is thus con- tinued in each successive generation, the same numerical law being followed so far as has yet been observed. "Mendel made further experiments with Pisum sativum, crossing pairs of varieties which differed from each other in two characters, and the results, though necessarily much more complex, showed that the law exhibited in the simpler case of pairs differing in respect of one character operated here also. "In the case of the union of varieties AB and ab differing in two distinct pairs of characters, A and a, B and b, of which A and B are dominant, a and b recessive, Mendel found that in the first cross-bred generation there was only one class of offspring, really AaBb. "But by reason of the dominance of one character of each pair these first crosses were hardly if at all distinguishable from AB. "By letting the AaBb's fertilize themselves, only four classes of offspring seemed to be produced, namely: "AB showing both dominant characters. "Ab showing dominant A and recessive b. " aB showing recessive a and dominant B. " ab showing both recessive characters a and 6. i The numerical ratio in which these classes appeared was also regular and approached the ratio 9AB : 3.46 : 3aB : lab. "But on cultivating these plants and allowing them to fertilize themselves, it was found that the members of the Ratios 1 ab class produce only ab's. o j 1 aB class may produce either all aB's, ( 2 or both aB's and ab's. o j 1 Ab class may produce either all Ab's ( 2 or both Ab's and ab's. ' I AB class may produce either all AB's 2 or both AB's and Ab's, 2 or both AB's and aB's, ,4 or all four possible classes again, namely, AB's, Ab's, aB's, and ab's, and the average number of members of each class will approach the ratio 1 : 3 : 3 : 9 as indicated above. 'The details of these experiments and of others like them made with three pairs of differentiating characters are all set out in Mendel's memoir.' HEREDITY 191 Perhaps the most striking thing about Mendel's work is the singularly suggestive and luminous interpretation which he gave of just why the pea characteristics were transmitted exactly as they were; why, in general, the peculiar numerical ratio be- tween dominant and recessive should be, and why it should */ persist so uniformly. This interpretation or explanation is now well known in biology as the theory of the " purity of the germ cells," or, as Cuenot has called it, the theory of "gametes dis- joints," or "la disjonction des characteres dans les gametes des hybrides ' (the separation of characters in the germ cell of hybrids) , the Spaltungsgesetz of de Vries. This interpretation is simply that in the young of the first generation after a cross-mating, although because of dominance but one of the contrasting pair of parental characters will show itself in the body make-up, yet when these young form their germ cells the two parental characteristics will be represented, but only one in any one germ cell; that is, in the case of Mendel's peas that the pollen cells and ovule cells produced by the cross-bred young would carry each one of the alternative or mutually exclusive parental varietal characters. If this were the case and if, on an average, the pollen cells and ovule cells were evenly divided as to the two characteristics, then by miscellaneous or random mating (mating according to the law of probabilities) between these cells we should get in the de- veloped young just such conditions with regard to the con- trasting characteristics as Mendel actually did get in his peas. For twenty-five per cent of the pollen grains representing the dominant character would unite with twenty-five per cent of the ovule cells representing the dominant character, twenty-five per cent of the recessive pollen grains with twenty-five per cent of the recessive ovule cells, and the remaining fifty per cent of each kind with each other ; that is, of every four pollen grains and every four egg cells we should get by random pollination 1 pollen dominant X I ovule dominant; 1 pollen recessive X 1 ovule re- cessive; 1 pollen dominant X 1 ovule recessive; 1 pollen reces- sive X 1 ovule dominant. This condition would bring it about that the fully developed young would show the contrasting characteristics (remembering the dominance of one of the char- acteristics in those cases in which dominant and recessive are united), in this condition: 3D, 1R. Which is exactly what occurred in Mendel's peas, and has since been noted to occur 92 EVOLUTION AND ANIMAL LIFE in many other cases recorded by post-Mendelian observers and experimenters. These records are of both plants and animals, and are fast multiplying. Thus the so-called Mendelian laws of heredity refer to two phases of the problem of inheritance viz.: (1) how inherited characters are actually distributed, and (2) the fundamental cause, lying in the germ plasm, for this particular kind of dis- tribution. Like Gait on 's formula, Mendel's law expresses the regularity of heredity based on actual recorded statistics of inheritance ; but it also gives a satisfying fundamental reason for this regularity. Biologists, with few exceptions, see in the establishment of the Mendelian principles of heredity in biologic science the greatest advance toward a rational explanation of inheritance that has been made since the beginning of the scientific study of the problem. The extraordinary fact that Mendel's work lay practically unnoted for thirty-five years (actually the only reference to it in scientific "literature " in all that time seems to have been one by Focke in 1881 in Die Pflanzenmischlinge, p. 109), has been partly explained by Bateson as due to the driving interest felt through all that time by biologists generally in other phases of investigation; but it remains a curious commentary on the possibilities of the temporary obscurity that may be in store for even the best scientific work. The "discovery' of Mendel's work seems to have been made in 1900 by three investigators almost simultaneously, who also discovered independently the same important facts of the transmission behavior in inheritance of exclusive or alternative characteristics. These men are de Vries, Tschermak, and Correns, and their published papers not only verify Mendel's particular work on the peas, but confirm his principles or laws on the basis of much added experimenta- tion and observation on other plants. In the last five years zoologists, notably von Gnaita working with mice, Cuenot, Darbishire, Davenport, Bateson and Castle with mice, rabbits, guinea-pigs and chickens, McCracken with certain beetles, and Toyama. Mrs. Bell and Kellogg with silkworms, have shown that Mendelian principles obtain in animal as well as in plant inheritance. For the results of all of these investigations in large measure confirm our confidence in the Mendelian prin- ciples of dominance and recessivity and of the purity of germ cells. But also in nearly all of these studies the investigators HEREDITY 193 have found some inconsistencies and have caught glimpses of other principles which, when finally grasped, will undoubtedly considerably limit the application of Mendel's laws, but will, almost certainly, not detract from their importance, nor lessen in any degree the high place in science that belongs to the patient, persistent, clear-minded August inian monk of the cloister gardens of Briinn. One of the modifications of the Mendelian behavior of hybrids which has been shown to exist in certain cases, is that the young of the cross-mated parents may not all exhibit in the same degree the dominant characteristic, although in the subsequent generations the regular Mendelian three-to-one splitting up into dominant and recessive appearance may occur. The young of the first generation may include a very few individuals showing the recessive character, as de Vries found in mating two varieties of Papaver somniferum (ninety-seven per cent showed the dominant character, three per cent the re- cessive). Or the first generation may show a sort of pseudo- blend condition, approaching but not duplicating exactly the dominant characteristic, as occurs when Hyoscyamus pallidus is crossed with H. niger (de Vries, "Die Mutationstheorie," Bd. II., p. 162.) When silkworm moths of the race Shanghai, with white cocoons, are crossed with moths of the race Yellow Var. with rose-yellow cocoons, the hybrid offspring make straw-yellow cocoons of a tint just between the two parent tints. The color- ing matter of the grapevine Aramon has the chemical formula C46H3GO20, and the coloring matter of the race Teinturier has the formula C44H4oO2o* the hybrid offspring called Petit- Bouschet, of a crossing of these two, has coloring matter of the formula C45H 3 802o> exactly intermediate. Mendel himself got as the result of a crossing between two pea races, one one foot in height and the other six feet, hybrids measuring from six to seven and one half feet high. These are specific cases of blended inheritance and there are many others known. Also when the plant Mirabilis jalapa 9 , with red flowers, is crossed with a male variety with white flowers the hybrid offspring exhibit red flowers (maternal type), white flowers (paternal type), and flowers streaked with the two colors. So when corn with blue kernels is crossed with corn with white kernels, a hybrid is obtained exhibiting on a single ear blue 194 EVOLUTION AND ANIMAL LIFE kernels, white kernels, kernels of intermediate bluish-white tint and kernels streaked with blue and white. The streaked flowers and kernels of these two cases are due to mosaic inheritance. Or the apparent dominance of the contrasting charac- teristics may be proved to have something of real dominance about it, as Miss McCracken has so clearly shown in her studies of the inheritance of dichromatism in the little beetles Lina lapponica (Fig. 117) and Gastroidca dis- similis. Here the FIG. 116. At left an ear of field corn; at right at) ear of sweet corn; and in the middle a hybrid of these two, showing alternation of kernels resembling those of each different parent. (After de Vries.) first two or three generations behave in true Mendelian manner, but with successive generations the dominant character is plainly seen to be gradually extinguishing the recessive character in the cross- bred groups, so that in the seventh generation after the original cross-mating the Mende- lian ratio of 2 to 1 in the cross-bred group is changed to 28 to 1. There may occur also a breaking up or decom- position of the parental varietal characters, which may mean that the domi- nant and recessive char- acters are not simple ones but are complex- i. e., really the resultant of several combined characteristics; or it may mean that there exists a real instability in the parent type and that the stimulus or influence of the cross-mating is all FIG. 117. Lina lapponica, showing its two forms, one black and one spotted. (After McCracken.) HEREDITY 195 that is needed to break down this weak apparent stability of the type and allow its component characters (the elementary units of de Vries) to recombine into various new and differing types. This condition seems to be that which results in the extraordi- nary variation so commonly observed by plant and animal breeders as brought out by hybridization, and which' is con- stantly made use of by these breeders. Luther Burbank de- pends very largely on this initial abundant and eccentric varia- tion induced by wide hybridizations for " starters " for his work of producing "new creations. " So in accepting Mendel's laws of heredity we must bear clearly in mind that they by no means apply to all, or, at any rate, that our present knowledge of them does not include their application to all, cases and categories of inheritance. CHAPTER XI INHERITANCE OF ACQUIRED CHARACTERS Tout ce qui a ete acquis, trace ou change dans ^organisation des individus, pendant le cours de leur vie, est conserve par la generation et transmis aux nouveaux individus qui proviennent de ceux qui out eprouve ces changements. LAMARCK. Heaven forfend me from Lamarck nonsense of a tendency to pro- gression, adaptation from the slow willing of animals, etc. ; but the conclusions I am led to are not wholly different from his, though the means of change are wholly so. DARWIN to HOOKER, 1848. THE "fourth Law of Evolution," as expressed by Lamarck in his "Zoological Philosophy," reads as follows: "All that has been acquired, begun, or changed in the structure of individuals in their lifetime, is preserved in reproduction and transmitted to the new individuals which spring from those which have in- herited the change." This principle was used by Lamarck as one of the funda- mental elements in his theory of the transmutation of species. For nearly a hundred years it attracted little attention, being accepted as a part of the law of heredity by most persons, even by those most opposed to the essential part of Lamarck's theory, the derivation or transmutation of species. Among others, Darwin accepted it as one of the factors in evolution of forms. With Herbert Spencer it became one of the fundamental prin- ciples of the philosophy of Evolution. Mr. Spencer states the proposition in this way: "Change of function produces change of structure: it is a tenable hypothesis that changes of structure so produced are inherited." For the supposed inheritance of characters produced by the impact of environment or by resultant activities of the individual the term progressive heredity has been devised. The fact of the existence of pro- 196 INHERITANCE OF ACQUIRED CHARACTERS 197 gressive heredity, more or less taken for granted by writers of the last century, was flatly denied by Dr. August Weismann, who insisted that it was necessary that the theory of the inheritance of characters acquired in the lifetime of the indi- vidual should no longer be accepted without definite proof. In the theory of heredity through the development of the germ cell controlled by influences exerted by structures within the nucleus, Weismann found no room for the inheritance of characters not preestablished within this germ. External in- fluences in general cannot reach the germ cells, and throughout nature the germ cells are elaborately protected from the direct influence of external conditions. This attack upon an ancient theory roused its supporters to defend their faith and to search for evidence to support it. A temporary division of naturalists into two schools arose as a result of this discussion. Those who held with Lamarck and Spencer that characters gained in the life time of the individual, and not received from ancestors possessing them, became hered- itary, were known as Neo-Lamarckians. Those w r ho, with Weismann, denied the existence of this factor and from a neces- sity, real or fancied, laid special stress on the Darwinian principle of natural selection, assumed the title of Neo-Darwiriians. In their hands the Darwinian principle became the all-powerful factor in evolution, a theory of Allmacht which was soon questioned from other quarters and by those not considered as Neo-Lamarckians. Prominent among the leaders of the Neo- Lamarckians were Herbert Spencer, Haeckel, Niigeli, Cope, Eimer, Hyatt, Gadow, Dall, Packard, and others. Among the recognized Neo-Darwinians were Weismann, Wallace, Hux- ley, Gray, Brooks, Lankester, and others. After some years of controversy, mostly theoretical, the dis- cussion has been tacitly dropped by biologists generally. It is recognized that the sole crucial test is that of experiment, that experiment is not easy, inasmuch as it is very difficult to show that any given trait in heredity really belongs to the category of acquired characters, and that in no case has it been indu- bitably shown that any character not inborn has been inherited. Moreover the studies of the germ cell and the physical basis of heredity tend to show that the structures of the germ cell are more complex and that the processes of heredity are in a sense more mechanical than could have been supposed in the time of 14 198 EVOLUTION AND ANIMAL LIFE Lamarck or even that of Darwin or Spencer. The characters shown by any adult individual are all in a sense acquired char- acters, their development dependent largely on nutrition and on the influences of environment. The facts of heredity show that it is not the actual traits of the parents, but rather their poten- tialities which are inherited. Moreover, acquired characters are simply matters of degree of development. They represent in no case anything qualitatively new. Taking the modern theories of heredity, it is perhaps not conceivable that "all that is acquired, begun, or changed' 1 in the physical or mental life of the individual should produce a corresponding change in the germ cells, or in the cells from which these are thrown off. On the other hand, Dr. Weismann has admitted the possi- bility that one-celled animals and animals of simple structure in which the germ cell shares in the general relation of the body cells to the environment may be effected by developmental con- ditions. In other words, the inheritance of acquired characters may be a reality in the development of Protozoa, the simpler Metazoa, and the lower types of plants, but this condition does not obtain among the higher forms. In much of the discussion on this subject the term "ac- quired characters " is used with an uncertain or double meaning. The term should be limited to traits of the individual which were not inborn or blastogenic, and would not be exhibited in the natural or usual development of the individual. In general, such traits would arise either from the operation of use or disuse of parts, or other functional stimulation derived from the en- vironment. An illustration of an acquired character resulting from use and disuse would be the increased size of the arm in the black- smith, or the decreased leg muscles of the tailor. The training of a musician or of a mathematician would give increased power along the lines of the training. The neglect of the musical or of mathematical ability would lead to the relative mediocrity of this form of ability. Education in a general way increases mental capacity: neglect of education allow r s it to become relatively less. The supposed inheritance of results of civilization forms an important part of the philosophy of Herbert Spencer. Is civilization the inheritance of the power gained by past suc- cesses, or is it simply the acquisition of the machinery which past successes have produced? As to this Herbert Spencer INHERITANCE OF ACQUIRED CHARACTERS 199 remarks: "Considering the width and depth of the effects which the acceptance of one or the other of these hypotheses must have on our views of life, the question, Which of them is true? demands beyond all other questions whatever the atten- tion of scientific men/ 3 Other illustrations of the supposed effect of use and disuse are thus discussed by Dr. Edwin Grant Conklin: "In the first place, this whole line of argument starts with the assumption that the individual habits of an animal are inherited, and that these habits ultimately determine the structure, an assumption which really begs the whole question ; for, after all, the substratum of any habit must be some physical structure, and if modified habits are inherited it must be because some modified structure is inherited. I take an example which will serve as an illustration of a whole class: Jackson says that the elongated siphon of Mya, the long-necked clam, is due to its habit of burrowing in the mud, or to quote his words: ' It seems very evident that the long siphon of this genus was brought about by the effort to reach the surface, induced by the habit of deep burial.' It certainly would be pertinent to inquire where it got this habit, and how it happened to be transmitted. It is surely as diffi- cult to explain the acquisition and inheritance of habits, the basis of which we do not know, as it is to explain the acquisition and inheri- tance of structures which are tangible and visible. Such a method of procedure, in addition to begging the whole question, commits the further sin of reasoning from the relatively unknown to the relatively known. "This case is but a fair sample of a whole class, among which may be mentioned the following: The derivation of the long hind legs of jumping animals, the long forelegs of climbing animals, and the elon- gation of all the legs of running animals through the influence of an inherited habit. All such cases are open to the very serious objection mentioned above. '' Another whole class of arguments may be reduced to this propo- sition: Because necessary mechanical conditions are never violated by organisms, therefore modifications due to such conditions show the inheritance of acquired characters. Plainly, the alternative propo- sition is this : if acquired characters are not inherited, organisms ought to do impossible things. 'Many of the arguments advanced to prove the inheritance of characters acquired through use or disuse seem to me to prove entirely 200 EVOLUTION AND ANIMAL LIFE too much. For example, Professor Cope argues very ably that bones are lengthened by both stretch and impact, and that modifications thus produced are inherited. Even granting that this is true, how would it be possible for this process of lengthening to cease, since in active animals the stretch and impact must be continual? Professor Cope answers that the growth ceases when 'equilibrium' is reached. I confess that I cannot understand this explanation, since the assumed stimulus to growth must be continual. But, granting again that growth may stop when an animal's legs become long enough to 'sat- isfy its needs,' how on this principle are we to account for the shorten- ing of legs, as, for example, in the turnspit dog and the ancon sheep and numberless cases occurring in nature? If any one species was able, by taking thought of mechanical stresses and strains, to add one cubit unto its stature, how could the same stresses and strains be invoked to decrease its stature? ' These evidences are, I know, not the strongest ones which can be adduced in support of the Lamarckian factors. There are at present a relatively small number of such arguments which seem to be valid and the great force of which I fully admit. But the cases which I have cited are, I believe, fair samples of the majority of the evidences so far presented, and in the face of such 'evidence' it is not surprising that one who is himself a profound student of the subject and a con- vinced Lamarckian prays that the Lamarckian theory may be deliv- ered from its friends." * As to the inheritance of the effects of extrinsic forces on the individual, we find little in the way of direct evidence. In all the members of the large family of flounders and soles, the adult fish rests flat on the bottom and swims on its side, the cranium being twisted so that both eyes appear on the upper side. As a rule color cells are developed on the upper side only, the lower cells remaining largely uncolored or white. In the young of all species the head is symmetrical, both eyes being normally situated, and the fish swims vertically in the water. Little by little, as development goes on, the fish turns over to one side, and the eye of the lower side passes around or through the forehead to join its fellow on the upper side. On the upper side pigment cells develop, while on the lower side they remain 'H. F. Osborn, "Evolution and Heredity," Wood's Holl Biological Lectures, 1890. INHERITANCE OF ACQUIRED CHARACTERS 201 imperfect. However, in a flounder reared under conditions in which the light falls on the lower side, pigment cells are de- veloped also on that side. It has been claimed by certain writers, as Cunningham, that the twisting of the head in the flounder is due to the inheritance of an acquired character. A flat fish without air bladder, rest- ing on the sea bottom, naturally falls on one side. The eye thrust into the sand is naturally twisted around to the upper side, and this tendency begun in very young individuals becomes hereditary, while the lack of pigment on the under side is also transmitted by inheritance. But it is just as easy to claim that the first trait of adaptation is due to natural selection, and that the whiteness of the blind side is ontogenetic, due to the absence of light in the growth of the individual. In any case, no specific theory of the origin of the twist of the flounder's head can be regarded as proved. It is well known, as Dr. Conklin observes, that certain water snails "if reared in small vessels are smaller than when grown in large ones," and this case has been cited as showing the influence of environment in modifying species. There is good evidence, however, that this modification does not affect the germinal protoplasm, for these same gasteropods will grow larger if placed in larger vessels. It seems very probable that the diminished size of these animals is due to deficient food supply, but this has so little modified the somatic protoplasm that, although they may be fully developed as shown by sexual maturity, they at once increase in size as soon as more abundant food is provided, and this takes place by the active growth and division of all the cells of the body. In higher animals, once maturity has been reached, there is little chance for growth, apparently because many of the cells are so highly differen- tiated that they can no longer divide; consequently the growth is limited, and hence the size of the adult may depend in part upon the amount of nutriment furnished to the embryo. This limitation of growth is due to the high degree of differentiation of the somatic cells. But as the germ cells are not highly dif- ferentiated and are capable of division, it follows that they would not be permanently modified by starving. It may be, as Professor Brewer argues, that long-continued starving and consequent dwarfing of animals may leave its mark on the germinal plasm; but, as he also remarks, this influence must be 202 EVOLUTION AND ANIMAL LIFE very slight as compared with the cumulative effects of selection in breeding, and it is safe to assert that there is no such whole- sale and immediate modification of the germinal plasm due to nutrition as some people seem to suppose." As a matter of fact experiment has shown that the results of dwarfing due to lack of food are shown for three generations in silkworms (these subsequent broods of larvae being full fed but producing dwarfed moths). But with succeeding genera- tions the moths became larger and resumed their normal ap- pearance. Mutilations of any sort are not inherited. The tails of sheep have been cut off for countless generations. Yet each lamb is born with a tail. This law holds good for docked tails, docked ears, pierced ears, and the many mutilations to which domestic animals and men have been subject since the begin- ning of civilization. Influences of climate, of heat, of cold are not inherited so far as experiment shows, nor has it been made clear that any extrinsic influence exerted on the individual really modifies the forces of heredity. Even Lamarck admits this. He ob- / serves: "Circumstances change the forms of animals. But I must not be taken literally, for environment can effect no direct / j changes whatever upon the organization of animals." In Spencer's view, the phenomena of instinct are to be ex- plained as the inheritance of habits of the individual. The Neo-Darwimans see in the adaptations of instinct only the re- sults of natural selection acting upon the endless variations to which individual instincts are subject. In most cases the latter view seems the most probable. In some cases it hardly offers a plausible explanation. The young mocking bird shows an inborn dread of owls and cats, while it is relatively indifferent to the presence of dogs or chickens. It seems hardly reasonable to suppose that all mocking birds without this instinct of dread for these particular animals have been destroyed, while the others have survived. Still more deep seated is the dread of snakes possessed by all the monkey species known to us, as well as by their human allies. Most men and most monkeys have a different feeling in regard to snakes from that exhibited toward any other sort of animals. This feeling is inborn. It may be suppressed, but not often wholly conquered. To call it an inherited experience is easy, INHERITANCE OF ACQUIRED CHARACTERS 203 but in default of other evidence for the inheritance of experi- ences, the explanation is not satisfactory. But it is not easy to believe that in early times those without this instinct fell victims to venomous snakes through their own fearlessness. It is perhaps not necessary to take sides on this question. Any view we may adopt rests for confirmation mainly on the im- probability of what we conceive to be the opposite alternative. Conklin further observes that the "so-called facts [of progressive heredity] are merely probabilities of a higher or lower order, and to one man they seem more important than to another. No conviction based even upon a high degree of proba- bility can ever be reached in this way. There is here a deadlock of opinion, each challenging the other to produce indubitable proof. This can never be furnished by observation alone. Possibly even experiment may fail in it, but at least it is the only hope." We shall not assist science, says Osborn, "with any evolution factor grounded upon logic rather than upon inductive demonstration. A retrograde chapter in the history of sci- ence would open if we do so and should accept as established laws those which rest so largely upon negative reasoning." Meanwhile we may regard the theory of the inheritance of acquired characters as a piece of useful scaffolding which has served its purpose in the development of the facts of the deriva- tion of species. At present most of it- -perhaps all of it must be taken down, but it may be that from the same base will arise a better constructed theory which will again serve a purpose in the study of organic evolution. Similar conditions in life tend to develop or encourage analogous adaptations in groups of animals not homologous in structure nor closely related by lines of descent. In many cases these adaptations are so very similar and are so subtle in their parallelism as to deceive even the trained naturalist. In other cases, the convergence and its consequent analogies are less perfect, and the separate influence of like selection under like environment is easily traceable. Examples of this sort are seen in the density of the fur of all Arctic animals, whatever the group to which they belong. An- other illustration is found in the white winter dress of weasels, 204 EVOLUTION AND ANIMAL LIFE rabbits, owls, ptarmigans, and other birds and mammals, this color aiding alike in defense or attack as against the back- ground of snow. Similar convergence of characters is seen in the gray hues of almost all desert animals, in the thorny stems and scant thick foliage of almost all desert plants. In swift streams, fishes of various types (sculpins, darters, gobies, cat- fish, and minnows) protect themselves from the current by the reduction of the air bladder, by the instinct to lie flat on the bottom, and the instinct to make short quick darts from place to place in the swift waters. To this end also, certain fins are in each case especially increased in size and force. Convergence of characters is shown in the black colors, soft bodies, and luminous spots, characteristic of different groups of deep-sea fishes. It also appears in the development of eellike forms in groups of fishes which have no affinity with eels, and of snakelike forms among .lizards and salamanders, which have no real affinity with snakes. Like conditions of life bring about like structures. We may instance the occurrence of blind fishes of various groups in the different cave areas, these species being derived in all cases from fishes of neighboring regions having well-developed eyes. Thus the blind cave fish of Missouri (Trog- lichthys roscc), and those of Indiana and Kentucky (Amblyopsis spelccus, Typhlichthys subterraneus) , are separately derived from the once widespread type of the Dismal Swamp fish (Cholo- gaster cormdus). The blind fishes of Cuba (Stygicola, Lucifiiga} are derived from ancestors of a marine cusk (Brotula) now found in the Cuban seas. The blind catfish of Pennsylvania (Gronias nigrilabris) is modified from an existing species (Ameiurus pnllus) found in the same region. The blind salamanders of Austria and Texas are derived from former inhabitants of the same regions which possessed well-developed eyes. Parallelisms of this sort are found in every group of animals and plants. It is generally easy to distinguish analogous variations or results of convergence of characters, by the study of comparative anatomy. Resemblances induced by like selec- tion or by like conditions are usually superficial and do not affect those structures which do not come into direct contact with external conditions. But sometimes even deep-seated characters have been reached and affected by environmental influences. In this case a finer test is found in the study of embryonic development. In general, creatures actually closely INHERITANCE OF ACQUIRED CHARACTERS 205 related in descent have inherited common methods of develop- ment. Thus to embryology we have looked for the final test as to the real affinities of any given form. But even this test is sometimes delusive, for selection and environmental influences may affect embryonic development, as they may affect every other character or instinct. Certain writers carry this thought further, and find no real basis for discrimination between homologies and analogies. They would hold that the progressive inheritance of effects of similar environment might in time produce forms not imme- diately related into a condition of practical identity each with the other. Professor Hans Gadow observes : " When Gegenbaur had become the founder of modern comparative anatomy by putting it on the basis of evolution, it became gradually an axiom that homologies determined the degrees of affinity, and now in turn the position of an animal in the system is appealed to for de- termining whether a given organ is homologous or only analogous. It is a vicious circle. 'Only analogous' is the usual expression. In reality these cases of analogy, homoplasy, convergence have become of supreme interest in our science. Their solution implies the greatest of problems, and it is only the thoughtless orthodox fanatic who be- lieves that similarity in structure must mean relationship. To him two and two make four, no matter what the twos are composed of." But most naturalists believe that homology, which involves common descent, and analogy, which rests on similar experi- ences, are quite distinct elements, and that they can always be distinguished by recognized biological tests. No one can question the vast influence of extrinsic or en- vironmental influences on the history of a species of animal or plant. In the analysis of such influences we find a wide variety of opinions. According to some writers, these forces are dy- namic, shaping the development of the individual, and by heredity determining through the individual the future of the species. Dall uses these striking words: "The environment stands in relation to the individual as the ham- mer and anvil to the. blacksmith's hot iron. The organism suffers during its entire existence a continuous series of mechanical impacts, none the less real because invisible.' 206 EVOLUTION AND ANIMAL LIFE Others, not questioning the reality of the direct effects of environmental forces on the individual, find no evidence that these impacts are perpetuated in heredity. Besides direct effects of these outside influences, we have to consider an infinite variety of reactions, which these forces or impacts may set up in the organism. These again have been supposed to be hereditary, for the species changes under them in what seems to be very much the same fashion that the indi- vidual does. Use and disuse in the species bring about parallel results to those shown by use and disuse in the individual, and are by some therefore referred to the same cause. But again there is grave reason to question the fact of the inheritance of such reactions, and to doubt whether the effects of use and disuse in the species rest on the same set of causes as the results of use and disuse in the individual. There remains the supposition, adopted at least tentatively by a large proportion, probably the majority, of the naturalists of to-day, that the direct effects of environment, as well as the reactions or indirect effects on the individual, are not repeated in heredity, and that the selective influence of environmental causes is the measure of their influence in the transformation of species. The question of the nature of dynamic forces in evolution is one of the most recent and most interesting phases of the long- continued discussion of the inheritance of acquired characters. A vast range of variations are ontogenetic, or dependent on influences affecting directly the life of the individual. These ontogenetic variations are, strictly speaking, individual, ap- pearing as collective only when many individuals have been subjected to the same conditions. They may be divided into environmental variations and functional variations, two cate- gories which cannot always be clearly separated, as variations due to food conditions partake of the nature of both. More than thirty years ago, Dr. J. A. Allen demonstrated that climatic influences affect the averages in measurements and in color among birds. For example, in several species of birds, the total length is greater in specimens from the north, while the bills and toes are actually longer in southern specimens. That this condition is due to the influence of climate on develop- ment is apparently shown by the fact that numerous species are affected in the same way. It is noticed also that specimens INHERITANCE OF ACQUIRED CHARACTERS 207 B from the northeast and the northwest of the United States are darker in color than those from the interior, and again that red shades are more common in the arid southwest. Similar effects have been recently shown by a study of species of wasps. Modi- fications of this type may be produced at will by subjecting the larvae and pupae of certain in- sects to artificial heat and cold. The butterflies of the glacial regions and those developed in the ice chest have a pale coloration, and a warm environment deepens the pigment. The woodpeckers and other birds of the rainy forests, northwest and northeast, have always darker and more sooty plumage than those birds of the same type found in more sunny regions. A typical case is found in the various species of sticklebacks (Fig. 119) constituting the genera Gaster- osteus and Pygosteus of the Northern Temperate Zone. In both genera, the marine species are armed for the whole length of the body by a series of about tw r enty to thirty vertically oblong enameled bony plates. In brackish waters in Europe, America, and Asia alike, the stickle- backs in all the various species are only partially mailed, having vari- ously from three to fifteen bony plates, these smaller than in the marine forms and covering only the anterior part of the body. In these fishes also, the spines of the fins are less developed than in the marine forms. In strictly fresh waters, sticklebacks of various types are found entirely destitute of bony plates. These unarmed fishes have been regarded as distinct species and as distinct subspecies. At present they are usually simply regarded as variant :< forms," to which distinctive scientific names need not be applied. It has not been proved, but it is probably a fact, that the differ- FIG. 118. Specimen of Cera- tiurn, collected (A) out of the Guinea Coast stream and (B) out of the South Equatorial stream; note the marked dif- ference in development of the spines. (After Weismann and Chun.) 208 EVOLUTION AND ANIMAL LIFE '/; '\?4g ; -'-'-!' ' : ' .- FIG. 119. Specimens of the stickleback, Gasierostus cataphractus, collected in different kinds of water, and showing marked variations in the number of lateral bony plates and in the size of the dorsal fin rays; at the top, specimens collected in the salt water (note many lateral plates and large dorsal fin rays); next figure below, specimens collected in brackish water; next below, specimens collected in a river mouth; at the bottom, specimens collected in a river, with no lateral plates and small dorsal fin rays. INHERITANCE OF ACQUIRED CHARACTERS 209 ence is one due to the environment of the individual. Those in the sea find adequate salts from which to develop their coats of mail. Those in fresh water do not find this, while those in river mouths and other brackish situations develop armature in intermediate degrees. In the genus Eucalia, a stickleback confined to fresh waters of the Middle Western States, plates are never developed. The Loch Leven trout, Salmo levcnensis, is distinguished from the brook trout of England, Salmo eriox (/an'o), in its native waters by certain obvious characters. These disappear when the eggs are planted in brooks in England or in California, and the species develops as the common English brook trout. But it is conceivable that the obvious or ontogenetic traits of the Loch Leven trout are not the real or phylogenetic distinc- tions, and that the latter, more subtle, engendered through in- dividual variation, inheritance, selection, and isolation, really exist, although they have escaped the attention of ichthy- ologists. After the Loch Leven trout was planted in the Yosemite Park in 1896, it remained for nine years unnoticed. In 1905 individuals sent to Stanford University were, so far as could be seen, exactly like English brook trout. But it is conceivable that differences in food and w r ater have caused slight ontogenetic distinctions. It is certain that in isolation from all parent stocks they will in time develop larger differences w T hich, after many thousand generations, will be specific or subspecific. At present, these trout are quite unlike the native rainbow trout (Salmo irideus gilberti} of the Yosemite. The ontogenetic char- acters will perhaps approach those of the latter, but the phylo- genetic movement may be in quite another direction. Another ontogenetic species is the little char or trout (Sal- vdinus tudes Cope) from Unalaska. In Captain's Harbor, Una- laska, the Dolly Varden trout, Salvclinus malma, swarms in myriads, in fresh and salt water alike, reaching in the sea a weight of from six to twelve pounds. A little open brook, which drops into the harbor by an impassable waterfall, contains also an abundance of Dolly Varden trout, mature at six inches and weighing but a few ounces. This is Salvelinus tudes. In the harbor the trout are gray with lighter gray spots, and fins scarcely rosy. In the brook, the trout are steel blue, with crimson spots and orange fins, striped with white and black. In all visible 210 EVOLUTION AND ANIMAL LIFE phylogenetic characters, the two forms of trout are one species. We have reason to believe that fry from the bay would grow up as dwarfs in the brook, and that the fry from the brook would be gray giants if developed in the sea. But it is also supposable that in the complete isolation of the brook fishes, with free interbreeding, there would be some sort of phylogenetic bond. There may be a genuine subspecies, tudes, characterized not by small size, slender form, and bright colors, but by other traits, which no one has found because no one has looked deeply enough. In no group of vertebrates are the life characters more plastic than among the trout. The birds have traits far more definitely fixed. Yet differences in external conditions must produce cer- tain results. We should not venture to suggest that the dusky woodpeckers or chickadees of the rainy forests of the northeast and northwest are purely ontogenetic species or that they should be erased from the systematic lists. But it will be a great ad- vance in ornithology when we know what they really are and when we understand the real nature of the small-bodied, large- billed, southern races of other species of birds. It would be worth while to know if these are really ontogenetic purely, or if they are phylogenetic through "progressive heredity," the inheritance of acquired characters, such as are produced by the direct effects of climate or as the reaction from climatic influences. Or again may there be a real phylogenetic bond through geographical segregation, its evidences obscured by the more conspicuous traits induced by like experiences? Or are there other influences still more subtle involved in the formation of isohumic or isothermic subspecies? To sum up, there is no convincing evidence that the direct influence of environment is a factor in the separation of species, except as its results may be acted upon by natural selection. We have no proof to show that the environment of one genera- tion determines the heredity of the next and yet perhaps most naturalists feel that the effects of extrinsic influences work their way into the species, although a mechanism by which this might be accomplished is as yet unknown to us. CHAPTER XII GENERATION, SEX AND ONTOGENY "Unter jedem Grab liegt erne Weltgeschi elite " (German proverb). EACH animal, each plant, must have its individual beginning, its "creation/' and its individual development from this begin- ning to full grown, completely developed condition. For no organism is born fully developed. Even the simplest organ- isms, the one-celled kinds, whose " creation' is accomplished simply by the splitting in two of a previously existing individual of their kind, are not produced full-fledged. They have at least to increase from half size to full size, that is, to grow, and there are very few if any of them that do not have to effect changes in their body structure during this period of growth; that is, they have to undergo some development. The begin- ning, then, is always from a previously existing organism but how could it always have been? and between this beginning and the normal mature or full-fledged creature there has always to be some development. The beginning is called generation; the development, ontogeny. We are all so familiar with the fact that a kitten comes into the world only through being born as the offspring of parents of its kind, that we shall likely not appreciate at first the full significance of the statement that all life comes from life; that all organisms are produced by other organisms. Nor shall we at first appreciate the importance of the statement. This is a generalization of modern times. It has always been easy to see that cats and horses and chickens and the other animals we familiarly know give birth to young or new animals of their own kind; or, put conversely, that young or new cats and horses and chickens come into existence only as the offspring of parents of their kind. And in these latter days of microscopes and 211 212 EVOLUTION AND ANIMAL LIFE mechanical aids to observation it is also easy to see that the smaller animals, the microscopic organisms, come into ex- istence only as they are produced by the division of other similar animals, which we may call their parents. But in the days of the earlier naturalists the life of the microscopic organisms, and even that of many of the larger but unfamiliar animals, was shrouded in mvsterv. And what seem to us ridiculous / - beliefs were held regarding the origin of new individuals. The ancients believed that many animals were spontane- ously generated. The early naturalists thought that flies arose by spontaneous generation from the decaying matter of dead animals; from a dead horse come myriads of maggots which change into flesh flies. Frogs and many insects were thought to be generated spontaneously from mud. Eels were thought to arise from the slime rubbed from the skin of fishes. Aristotle, the Greek philosopher, who was the greatest of the ancient naturalists, expresses these beliefs in his books. It was not until the middle of the seventeenth century Aristotle lived three hundred and fifty years before the Christian era- that these beliefs were attacked and began to be given up. In the beginning of the seventeenth century, William Harvey, an English naturalist, declared that every animal comes from an egg, but he said that the egg might "proceed from parents or arise spontaneously or out of putrefaction." In the middle of the same century Redi proved that the maggots in decaying meat which produce the flesh flies develop from eggs laid on the meat by flies of the same kind. Other zoologists of this time were active in investigating the origin of new individuals. And all their discoveries tended to weaken the belief in the theory of spontaneous generation. Finally, the adherents of this theory were forced to restrict their belief in spontaneous generation to the case of a few kinds of animals, like parasites and the animalcules of stagnant water. It was maintained that parasites arose spontaneously from the matter of the living animal in which they lay. Many parasites have so complicated and extraordinary a life history that it was only after long and careful study that the truth regarding their origin was discovered. But in the case of every parasite whose life history is known, the young are offspring of parents, of other individuals of their kind. No case of spontaneous genera- tion among parasites is known. GENERATION, SEX AND ONTOGENY 213 The same is true of the animalcules of stagnant water. If some water in which there are apparently no living organisms, however minute, be allowed to stand for a few days, it will come to be swarming with microscopic plants and animals. Any organic liquid, as a broth or a vegetable infusion exposed for a short time, becomes foul through the presence of innumerable bacteria, infusoria, and other one-celled animals and plants, or rather through the changes produced by their life processes. But it has been certainly proved that these organisms are not spontaneously produced by the water or organic liquid. A few of them enter the water from the air, in which there are always greater or less numbers of spores of microscopic organisms. These spores (embryo organisms in the resting stage) germinate quickly when they fall into water or some organic liquid, and the rapid succession of generations soon gives rise to the hosts of bacteria and Protozoa which infest all standing water. If all the active organisms and inactive spores in a glass of water are killed by boiling the water, "sterilizing" it, as it is called, and this sterilized water or organic liquid be put into a sterilized glass, and this glass be so well closed that germs or spores can- not pass from the air without into the sterilized liquid, no living animals will ever appear in it. It is now known that flesh will not decay or liquids ferment except through the presence of living animals or plants. To sum up, we may say that we know of no instance of the spontaneous generation of organisms, and that all the animals whose life history we know are produced from other animals of the same kind. " Omne vivwn ex vivo/' "All life from life." The method of simple fission or splitting binary fission it is often called, because the division is always in two by which the body of the parent becomes divided into two equal parts -into halves is the simplest method of multiplication. This is the usual method of Amteba (Fig. 120) and of many other of the simplest animals. In this kind of reproduction it is hardly exact to speak of parent and children. The children, the new Amcebce, are simply the parent cut into halves. The parent persists; it does not produce offspring and die. Its whole body continues to live. The new Amccbce take in and assimilate food and add new matter to the original matter of the parent body; then each of them divides in two. The grand parent's body is now divided into four parts, one fourth of it forming one half 15 214 EVOLUTION AND ANIMAL LIFE of each of the bodies of the four grandchildren. The process of assimilation, growth, and subsequent division takes place again and again and again. Each time there is given to the new Amoeba an ever-lessening part of the actual body substance of the original ancestor. Thus an Amoeba never dies a natural <>A Vj0.,f.- '*$&: FIG. 120. A multiplication of Amo?ba by simple fission. death, or, as has been said, "no Ama-ba ever lost an ancestor by death." It may be killed outright, but in that case it leaves no descendants. If it is not killed before it produces new Amoebce it never dies, although it ceases to exist as a single individual. The Amoebce and other simple animals which multiply by direct binary fission may be said to be immortal, and the " immortality of the Protozoa ' : ' is a phrase which will GENERATION, SEX AND ONTOGENY 215 FIG. 121.- -Stentor reproducing by fission. (After Stein.) often be met with in the writings of Weismann and certain other modern philosophical biologists. There is a fallacy, however, in the phrasing, because, as a matter of fact, the protoplasm of a given protozoon gradually loses its vitality with con- tinued division until it ultimately is unable to divide further or indeed to perform the other life functions: it dies of old age. Hardly less simple is generation by budding, which in its simplest character is the breaking off from one individual of a part smaller than a half, often, indeed, only a very small fractional part, which budded off part has the capacity of growing and developing into a new individual like its parent. A still other mode of generation of simple type is that of sporulation, or where the body of one individual subdivides into more than two parts (as in binary fission) , these parts, each of which is usually subspherical FKJ. 122,Holophrya imiltifiliis, an infusorian parasitic Or ellipsoidal, milli- on fishes reproducing by sporulation. beHng perhaps many hundreds. \j A condition known as parthenogenesis is found among certain of the complex animals. Although the species is repre- sented by individuals of both sexes, the female can produce 216 EVOLUTION AND ANIMAL LIFE A young from eggs which have not been fertilized. For example, the queen bee lays both fertilized and unfertilized eggs. From the fertilized eggs hatch the workers, which are rudimentary females, and other queens, which are fully developed females; from the unfertilized eggs hatch only males the drones. Many generations of plant lice are produced each year parthe- nogenetically that is, by unfertilized females. This subject will be discussed at greater length later B (BH1 ^-^ in this chapter. The modes of generation, or re- production, or mul- tiplication, as this making the begin- nings of new indi- viduals may be variously called, so far referred to, may be grouped into a category called asex- u a 1 generatio n. In an examination of the lives of the simplest and but slightly complex kinds of animals we find that even among almost the very simplest of or- FIG. 123. Gregarinida*. A, A gregarinid, Actinocephalus oligacanthus, from the intestines of an insect (after Stein); B and C, spore-forming by a gregarinid, Coc- ritlium oriforme, from liver of a guinea-pig (after Leut- kart); D, E, and F, successive stages in conjugation of spore-forming by Gregarina polytnorpha. (After Kol- liker.) ganisms another mode of reproduction obtains, at least occasionally, which demands for its carrying out the mutual action of two distinct individuals. The essential thing in this mutual action is the exchange of nuclear material from one of these individuals to the other ; with some of the simplest organisms there is a mutual exchange of nuclear material. Paramcecium, for example, reproduces itself for many gen- erations by fission, but a generation finally appears in which a different method of reproduction is followed. Two individu- als come together and each exchanges with the other a part of GENERATION, SEX AND ONTOGENY 217 its nucleus. Then the two individuals separate and each divides into two. The result of this conjugation is to give to the new Param&cia produced by the conjugating individuals a body which contains part of the body substance of two distinct individuals. The new Paramoecia are not simply halves of a single parent, they are parts of two parents. Among the colonial Protozoa the first differentiation of the cells or members composing the colony is the differentia- tion into two kinds of reproductive cells. Reproduction by simple division, without preceding conjugation, can and does take place, to a certain extent, among all the colonial Protozoa. Indeed, this simple method of multiplication, or some modi- fication of it, like budding, persists among many of the com~ plex animals, as the sponges, the polyps, and even higher and more complex forms. But such a method of single-parent reproduction cannot be used alone by a species for many gener- ations, and those animals which possess the power of multiplica- tion in this way always exhibit also the other more complex kind of multiplication, the method of double-parent reproduc- tion. Conjugation takes place between different members of a single colony of one of the colonial Protozoa, or between members of different colonies of the same species. These conjugating individuals in the simpler kinds of colonies, like Gonium, are similar; in Pandorina they appear to be slightly different, and in Eudorina and Volvox the conjugating cells are readily seen to be very different from each other. One kind of cell, which is called the egg cell, is large, spherical, and inactive, while the other kind, the sperm cell, is small, with ovoid head and tapering tail, and free-swimming. In the simpler colo- nial Protozoa all the cells of the body take part in reproduction, but in Volvox only certain cells perform this function, and the other cells of the body die. Or we may say that the body of Volvox dies after it has produced special reproductive cells which shall fulfill the function of multiplication. Beginning with the more complex Volvocince, which we may call either the most complex of the one-celled animals or the simplest of the many-celled animals, all the complex animals show this distinct differentiation between the repro- ductive cells and the cells of the rest of the body. Of course, we find, as soon as we go up at all far in the scale of the animal world, that there is a great deal of differentiation among the 218 EVOLUTION AND ANIMAL LIFE cells of the body: the cells which have to do with the assimila- tion of food are of one kind ; those on which depend the motions of the body are of another kind; those which take oxygen and those which excrete waste matter are of other kinds. But the first of this cell differentiation, as we have already often repeated, is that shown by the reproductive cells; and with the very first of this differentiation between reproductive cells and the other body cells, appears a differentiation of the re- \ FIG. 124. Spermatozoa of different animals: 1, Man; 2, Vesperugo; 3, pig; 4, rat; 5, finch; 6, triton; 7, ray; 8, beetle; 9, mole cricket; 10, snail. (After Ballowitz, Kolliker, and Rath.) productive cells into two kinds. These two kinds, among all animals, are always essentially similar to the two kinds shown */ / by Volvox and the simplest of the many-celled animals- namely, large, inactive, spherical egg cells, and small, active, elongate or " tailed " sperm cells. In the slightly complex animals one individual produces both egg cells and sperm cells. But in the Siphonophora, or colonial jelly fishes, certain members of the colony produce only sperm cells, and certain other members of the colony produce only egg cells. If the Siphonophora be considered an indi- vidual organism and not a colony composed of many individ- GENERATION, SEX AND ONTOGENY 219 uals, then, of course, it is like the others of the slightly com- plex animals in this respect. But as soon as we rise higher in c FIG. 125. Fertilization of Petromyzon fluviatilis: A, Sperm nucleus in periphery of the egg plasm; B, sperm nucleus in periphery of the egg plasm, and egg nucleus ap- proaching; C-E, fusing of the egg and sperm nuclei, and appearance of the asters; F, cleavage of nucleus. (After Herfort.) the scale of animal life, as soon as we study the more complex animals, we find that the egg cells and sperm cells are almost 220 EVOLUTION AND ANIMAL LIFE always produced by different individuals. Those individuals which produce egg cells are called female, and those which produce sperm cells are called male. There are two sexes. Male and female are terms usually applied only to individ- uals, but it is evidently fair to call the egg cells the female reproductive cells, and the sperm cells the male reproductive cells. A single individual of the simpler kinds of animals produces both male and female cells. But such an individual cannot be said to be either male or female, it is sexless- that is, sex is some- thing which appears only after a certain degree of structural and physiological differentiation is reached. It is true that even among many of the higher or complex animals certain species are not represented by male and female in- dividuals, any indi- vidual of the species being able to pro- duce both male and female cells. But this is the. exception. Among almost all the complex animals it is necessary that there be a conjugation of male and female reproductive cells in order that a new individual may be produced. This neces- sity first appears, we remember, among very simple animals. This intermixing of body substance from two distinct individuals and the development therefrom of the new individual is a phenomenon which takes place through the whole scale of animal life. The object of this intermixing seems to be the production of variation; at least it would seem that variation must result from such a mode of generation. By having the beginnings of an organism's body, the single cell from which this whole body develops, composed of parts of two different individuals, a difference between the offspring and the par- FIG. 126. Conjugation of Noctiluca, a one-celled ani- mal: A, Two individuals just fusing; B, the same with cytoplasm wholly fused and nuclei lying closely together; C, the two nuclei in closer fusion; D, the be- ginning of fission. (After Ischikawa.) GENERATION, SEX AND ONTOGENY 221 ents, although it may be slight and imperceptible, is in- sured. Sex is a condition of nature which is one, at least, of the causes of variation. FIG. 127. Conjugation of the infusorian, Ynrti>eUa nebulifera; the smaller individual at the right may be regarded as the male. (After Weismann.) As we have seen, almost every species of animal is repre- sented by two kinds of individuals, males and females. In the case of many animals, especially the simpler ones, these two FIG. 128. Male bird of paradise. kinds of individuals may not differ in appearance or in structure apart from the organs concerned with multiplication. But with many animals the sexes can be readily distinguished. 222 EVOLUTION AND ANIMAL LIFE The male and female individuals often show marked differences, especially in external structural characters. We can readily tell the peacock, with its splendidly ornamental tail feathers, from the unadorned peafowl, or the horned ram from the bleating ewe. There is here, plainly, a dimorphism the existence of two kinds of individuals belonging to a single species. This dimorphism is due to sex, and the condition may be called sex dimorphism. Among some animals this sex dimorphism, or difference between the sexes, is carried to ex- traordinary extremes. This is especially true among polyga- FIG. 129. Cankerworm moth: the winged male and wingless female. mous animals, or those in which the males mate with many females, and are forced to fight for their possession. The male bird of paradise, with its gorgeous display of brilliantly colored and fantastically shaped feathers (Fig. 128), seems a wholly different kind of bird from the modest brown female. The male golden and silver pheasants, and allied species with their elaborate plumage, are very unlike the dull-colored females. The great, rough, warlike male fur seal, roaring like a lion, is three times as large as the dainty, soft-furred female, which bleats like a sheep. Among some of the lower animals the differences between male and female are even greater. The males of the common cankerworm moth (Fig. 129) have four wings; the females are wingless, and several other insect species show this same difference. Among certain species of white ants the females grow to be five or six inches long, while the males do not ex- ceed half an inch in length. In the case of some of the para- sitic worms which live in the bodies of other animals the male has an extraordinarily degraded, simple body, much smaller GENERATION, SEX AND ONTOGENY 223 than that of the female and differing greatly from it in structure. In some cases even as, for example, the worm which causes "gapes " in chickens the male lives parasitically on the female, being attached to her body for its whole lifetime, and draw- ing its nourishment from her blood (Fig. 130). Some of the complex animals are hermaphroditic that is, a single individual produces both egg cells and sperm cells. The tapeworm and many allied worms show this con- dition. This is the normal condition for the simplest animals, as we have already learned, but it is an exceptional condition among the complex animals. However the beginnings of the new T organisms are produced, whether asexu- ally or bisexually (whether, that is, by simple division, budding, sporulation, or as true but unfertilized eggs, or as eggs with a nucleus made by the fusion of two germinal nuclei from male and female in- dividuals respectively, or from an her- maphroditic individual) , this new r organism in embryo has a shorter or longer course of development and growth to undergo, before it, in turn, is in condition to pro- duce new individuals of its kind. Certain phenomena are familiar to us as recurring inevitably in the life of every animal which we familiarly know. Each individual is born in an immature or young condition; it grows (that is, it in- creases in size) and develops (that is, changes more or less in structure) and dies. These phenomena occur in the succession of birth, growth, and development, and death. But before any animal appears to us as an independent individual that is, outside the body of the mother and outside of an egg (i. e., before birth or hatching, as we are accustomed to call such appearance) it has already undergone a longer or shorter period of life. It has been a new living organism hours or days or months, perhaps, before its appearance to us. This period of life has been passed inside an egg, or as an egg, FIG. 130. The parasitic worm, Syngarnus trache- alis, which causes the gapes in fowls. The male is attached to the female and lives as a parasite on her. 224 EVOLUTION AND ANIMAL LIFE or in the egg stage, as it is variously termed. The life of an animal as a distinct organism begins in an egg. And the true life cycle of an organism is its life from egg through birth, growth and development and maturity to the time it produces new organisms in the condition of eggs. The life cycle is from egg to egg. Birth and growth, two of the phenomena readily apparent to us in the life of every animal, are two phenomena Fir;. 131. Leptodera hyalina, showing sex dimorphism: A, Head of male; B, head <>f female. in the true life cycle. Death is a third inevitable phenomenon in the life of each individual, but it is not a part of the cycle; it is something outside. The single cell formed by the fusion of two germ cells is called a fertilized egg cell, and its subsequent development results in the formation of a new individual of the same species with its parents. Now, in the development of this cell into a new animal, food is necessary. 80 with the fertilized egg cell there is, in the case of most animals that lay eggs, a greater or less amount of food matter food yolk, it is called gath- ered about the germ cell, and both germ cell and food yolk are inclosed in a soft or hard wall. Thus is composed the egg as we know it. The hen's egg is as large as it is because of the great amount of food yolk it contains. The egg of a fish as large as a hen is much smaller than the hen's egg; it contains less food yolk. Eggs (Fig. 132) may vary also in their external appearance, because of the different kinds of membrane or shells which may inclose and protect them. Thus the frog's eggs are inclosed in a thin membrane and GENERATION, SEX AND ONTOGENY 225 imbedded in a soft, jellylike substance; the skate's egg has a tough, dark-brown leathery inclosing wall; the spiral egg of the bullhead shark is leathery and colored like the dark-olive seaweeds among which it lies; and a bird's egg has a hard shell of carbonate of lime. But in each case there is the essen- tial fertilized germ cell; in this the eggs of hen and fish and butterfly and crayfish and worm are alike, however much they may differ in size and external appearance. There is great variation in the number of young produced by different species of animals. Among the animals we know familiarly, as the mammals, which give birth to young alive, FIG. 132. Eggs of different animals showing variety in external appearance : a, Egg of bird; 6, eggs of toad; c, egg of fish; d,egg of butterfly; e, eggs of katydid on leaf; /, egg case of skate. and the birds, which lay eggs, it is the general rule that but few young are produced at a time, and the young are born or eggs are laid only once or perhaps a few times in a year. The robin lays five or six eggs once or twice a year; a cow may produce a calf each year. Rabbits and pigeons are more 226 EVOLUTION AND ANIMAL LIFE prolific, each having several broods a year. But when we ob- serve the multiplication of some of the animals whose habits are not so familiar to us, \ve find that the production of so few young is the exceptional and not the usual habit. A lobster lays ten thousand eggs at a time; a queen bee lays about five million eggs in her life of four or five years. A female termite of a certain species, after it is full grown, does nothing but lie in a cell and lay eggs, producing eighty thousand eggs a day steadily for several months. A large codfish was found on dissection to contain about eight million eggs. If w r e search for some reason for this great difference in fertility among different animals, we may find a promising clew by attending to the duration of life of animals, and to the amount of care for the young exercised by the parents. We find it to be the general rule that animals which live many years, and which take care of their young, produce but few young; while animals which live but a short time, and which do not care for their young, are very prolific. The codfish produces its mil- lions of eggs; thousands are eaten by sculpins and other predatory fishes before they are hatched, and other thousands of the defense- less young fish are eaten long before attaining maturity. Of the great number produced by the parent, a few only reach maturity and produce new young. But the eggs of the robin are hatched and protected, and the help- less fledglings are fed and cared for until able to cope with their natural enemies. In the next year another brood is carefully reared, and so on for the few years of the robin's life. Under normal conditions in any given locality the number of individuals of a certain species of animal remains about the same. The fish which produces tens of thousands of eggs and the bird which produces half a dozen eggs a year main- tain equally well their numbers. In one case a few survive of many born ; in the other many (relatively) survive of the few FIG. 133. Eggs of lace-winged fly, Chrysopa. The eggs are fastened sepa- rately, for pro- tection from predaceous in- sects, on the tipsof erect slender pedi- cles. GENERATION, SEX AND ONTOGENY 227 born ; in both cases the species is effectively maintained. In general, no agency for the perpetuation of the species is so effective as that of care for the young. Some animals do not lay eggs, that is, they do not deposit the fertilized egg cell outside of the body, but allow the develop- ment of the new individual to go on inside the body of the mother for a longer or shorter period. The mammals and some other animals have this habit. When such an animal issues from the body of the mother, it is said to be born. When the developing ani- mal issues from an egg which has been deposited outside the body of the mother, it is said to hatch. The ani- mal at birth or at time of hatching is not vet fully devel- / *.' oped. Only part of its development or period of im- maturity is passed rio. 134. First stages in the embryonic development of Within the egg Or the pond snail, Lymnceus: a. Egg cell; b, first cleavage; withill the bodv Of c ' seconc ^ cleavage; d, third cleavage; e, after numerous , , ^, cleavages; /, blastula in section; g, gastrula just form- ing in section; h, gastrula completed in section, part Of its life thus (After Rabl.) passed within the egg or mother's body is called the embryonic life or embryonic stages of development; while that period of development or immaturity from the time of birth or hatching until maturity is reached is called the postembryonic life or postembryonic stages of development. The embryonic development is from the beginning up to a certain point practically alike, looked at in its larger aspect, for all the many-celled animals. That is, there are certain principal or constant characteristics of the beginning develop- ment which are present in the development of all many-celled animals. The first stage or phenomenon of development is the simple fission of the germ cell into halves (Fig. 1346). These two daughter cells next divide so that there are four cells 228 EVOLUTION AND ANIMAL LIFE (c) ; each of these divides, and this division is repeated until a greater or lesser number (varying with the various species or groups of animals) of cells is produced. These cells may not all be of the same size, but in many cases they are, no struc- tural differentiation whatever being apparent among them. The phenomenon of repeated division of the germ cell is called cleavage, and this cleavage is the first stage of develop- ment in the case of all many-celled animals. The germ or embryo in some animals consists now of a mass of few 7 or many undif- ferentiated primitive cells lying together and usually forming a sphere (Fig. 134 ; c), or perhaps separated and scattered through the food yolk of the egg. The next stage of development is this: the cleavage cells arrange themselves so as to form a usually hollow sphere or ball, the cells lying side by side to form the outer circumferential wall of this hollow sphere (/). This is called the blastula or blastoderm stage of development, and the embryo itself is called the blastula or blastoderm. This stage also is common to all the many-celled animals. The next stage in embryonic development is formed by the bending inward of a part of the blastoderm cell layer, as shown in (g) (or the splitting off inwardly of cells from a special part of the blastula cell layer). This bending in may produce a small depression or groove; but whatever the shape or extent of the sunken-in part of the blastoderm, it results in distinguish- ing the blastoderm layer into two parts, a sunken-in or inner portion called the endobhist and the other unmodified portion called the ectoblast. Endo- means within, and the cells of the endoblast often push so far into the original blastoderm cavity as to come into contact with the cells of the ectoblast and thus obliterate this cavity (h). This third well-marked stage in the embryonic development is called the gastrula stage, and it also occurs in the development of all or nearly all many- celled animals. In the case of a few of the simple many-celled animals the embryo hatches that is, issues from the egg at the time of or very soon after reaching the gastrula stage. In the higher animals, however, development goes on within the egg or within the body of the mother until the embryo becomes a complex body, composed of many various tissues and organs. Almost all the development may take place within the egg, so that when the young animal hatches there is necessary GENERATION, SEX AND ONTOGENY 229 little more than a rapid growth and increase of size to make it a fully developed, mature animal. This is the case with the birds: a chicken just hatched has most of the tissues and organs of a full-grown fowl, and is simply a little hen. But in the case of other animals the young hatches from the egg before it has reached such an advanced stage of development; a young starfish or young crab or young honeybee just hatched looks very different from its parent. It has yet a great deal of development to undergo before it reaches the structural condition of a fully developed and fully grown starfish or crab or bee. Thus the development of some animals is almost wholly embryonic development that is, development within the egg or in the body of the mother while the development of other animals is largely postembryonic, or larval develop- ment, as it is often called. There is no important difference between embryonic and postembryonic development. The development is continuous from egg cell to mature animal, and whether inside or outside of an egg it goes on regularly and uninterruptedly. The cells which compose the embryo in the cleavage stage and blastoderm stage, and even in the gastrula stage, are ap- parently all similar; there is little or no differentiation shown among them. But from the gastrula stage on, development includes three important things: the gradual differentiation of cells into various kinds to form the various kinds of animal tissues ; the arrangement and grouping of these cells into organs and body parts ; and finally the developing of these organs and body parts into the special condition characteristic of the species of animal to which the developing individual belongs. From the primitive undifferentiated cells of the blastoderm, develop- ment leads to the special cell types of muscle tissue, of bone tissue, of nerve tissue; and from the generalized condition of the embryo in its early stages, development leads to the special- ized condition of the body of the adult animal. Development is from the general to the special, as was said years ago by von Baer, the first great student of development. A starfish, a beetle, a dove, and a horse are all alike in their beginning that is, the body of each is composed of a single cell, a single structural unit. And they are all alike, or very much alike, through several stages of development; the body of each is first a single cell, then a number of similar un- 36 230 EVOLUTION AND ANIMAL LIFE differentiated cells, and then a blastoderm consisting of a single layer of similar undifferentiated cells. But soon in the course *> of development the embryos begin to differ, and as the young animals get further and further along in the course of their development, they become more and more different until each finally reaches its fully developed mature form, showing all the great structural differences between the starfish and the dove, the beetle and the horse. That is, all animals begin development apparently alike, but gradually diverge from each other during the course of development. There are some extremely interesting and significant things about this divergence to which attention should be given. While all animals are apparently alike structurally J at the beginning of development, so far as we can see, they do not all differ noticeably at the time of the first divergence in de- velopment. The first divergence in development is to be noted between two kinds of animals which belong to different great groups or classes. But two animals of different kinds, both belonging to some one great group, do not show differences until later in their development. This can best be understood by an example. All the butterflies and beetles and grass- hoppers and flies belong to the great group or class of animals called Insecta, or insects. There are many different kinds of insects, and these kinds can be arranged in subordinate groups (orders), such as the Diptera, or flies, the Lepidoptera, or butter- flies and moths, and so on. But all have certain structural characteristics in common, so that they are comprised in one great class the Insecta. Another great group of animals is known as the Vertebrata, or backboned animals. The class 1 We can say that they are alike structurally, only when we consider the cell as the unit of animal structure. But, that the egg cells of different animals differ in their fine or ultimate structure, seems certain. For each one of these egg cells is destined to become some one kind of animal, and no other; each is, indeed, an individual in simplest, least developed con- dition of some one kind of animal, and we must believe that difference in kind of animals depends upon difference in structure in the egg itself. Indeed Wilson, the foremost American student of egg structure, believes himself able to perceive in many eggs a structural differentiation within the egg protoplasm itself, corresponding, in some measure, with the struc- tural differentiation of the embryonic animal as revealed in early develop- mental stages. GENERATION, SEX AND ONTOGENY 231 Vertebrata includes the fishes, the batrachians, the reptiles, the birds, and the mammals, each composing a subordinate group, but all characterized by the possession of a backbone, or, more accurately speaking, of a notochord, a backbonelike structure. Now, an insect and a vertebrate diverge very soon in their development from each other; but two insects, such as a beetle and a honeybee, or any two vertebrates, such as a frog and a pigeon, do not diverge from each other so soon. That is, all vertebrate animals diverge in one direction from the other great groups, but all the members of the great group keep together for some time longer. Then the subordinate groups of the Vertebrata, such as the fishes, the birds, and the others, diverge, and still later the different kinds of animals in each of these groups diverge from each other. That the course of development of any animal from its beginning to fully developed adult form is in all its essentials -fixed and certain is readily seen. All rabbits develop in the same way ; every grasshopper goes through the same de- velopmental changes from single egg cell to the full-grown, active hopper as every other grasshopper of the same kind- that is, development takes place according to certain natural laws: the laws of animal development. These laws may be roughly stated as follows: All many-celled animals begin life as a single cell, the fertilized egg cell; each animal goes through a certain orderly series of developmental changes which, ac- companied by growth, leads the animal to change from single cell to the many-celled, complex form characteristic of the species to which the animal belongs; this development is from simple to complex structural condition; the development is the same for all individuals of one species. While all animals begin development similarly, the course of development in the different groups soon diverges, the divergence being of the nature of a branching, like that shown in the growth of a tree. In the free tips of the smallest branches we have represented the various species of animals in their fully developed con- dition, all standing more or less clearly apart from each other. But in tracing back the development of any kind of animal we soon come to a point where it very much resembles or becomes apparently identical with the development of some other kind of animal, and, in addition, the stages passed through in the developmental course may very much resemble the 232 EVOLUTION AND ANIMAL LIFE fully developed, mature stages of lower animals. To be sure, any animal at any stage in its existence differs absolutely from any other kind of animal, in that it can develop into only its own kind of animal. There is something inherent in each developing animal that gives it an identity of its own. Al- though in its young stages it may be hardly distinguishable from some other kind of animal in similar stages, it is sure to come out, when fully developed, an individual of the same kind as its parents were or are. A very young fish and a very young salamander are almost indistinguishably alike, but one is sure to develop into a fish and the other into a salamander. This certainty of an embryo to become an individual of a certain kind is called the law of heredity. View r ed in the light of development, there must be as great a difference between one egg and another as between one animal and another, for the greater difference is included in the less. The significance of the developmental phenomena is a matter about which naturalists have yet very much to learn. c FIG. 135. Stages in the development of the prawn, Peneus potimiriitm: A, Nauplius larva; B, first zoea stage; C, second zoea stage. (After Fritz Miiller.) larva; B, first zoea stage; C, second zoea stage It is believed, however, by practically all naturalists that many of the various stages in the development of an animal cor- respond to or repeat, in many fundamental features at least, the structural condition of the animal's ancestors. Naturalists believe that all backboned or vertebrate animals are related to each other through being descended from a common ancestor, the first or oldest backboned animal. In fact, it is because all GENERATION, SEX AND ONTOGENY 233 these backboned animals -- the fishes, the batrachians, the reptiles, the birds, and the mammals have descended from a common ancestor that they all have a backbone. It is believed that the descendants of the first backboned animal have in the course of many generations branched off little by little from the original type until there came to exist very real and obvious differences among the backboned animals- E FIG. 136. Later stages in the development of the prawn, Peneus potimirium : D, Mysis stage; E, adult stage. differences which among the living backboned animals are familiar to all of us. The course of development of an in- dividual animal is believed to be a very rapid and evidently much condensed and changed recapitulation of the history which the species or kind of animal to which the developing in- dividual belongs has passed through in the course of its descent through a long series of gradually changing ancestors. If this is true, then we can readily understand why a fish and a salaman- der, a tortoise, a bird and a rabbit, are all much alike, as they really are, in their earlier stages of development, and gradually come to differ more and more as they pass through later and later developmental stages. A crab has a tail in one of its 234 EVOLUTION AND ANIMAL LIFE developmental stages, so that at that time it looks like and really is like the mature stage of some tailed crustacean like a crayfish. A barnacle, which looks little like a crayfish or crab in its mature stage, is hardly to be distinguished in its immature life from a young crab or lobster. Sacculina, which is a still more degenerate crustacean, is only a sort of feeding sac with rootletlike processes projecting into the body of the host crab on which it lives as a parasite, but the young free- swimming Sacculina is essentially like a barnacle, crayfish, or crab in its young stage. However, it is obvious that this recapitulation or repetition of ancestral stages is never per- fect, and it is often so obscured and modified by interpolated adaptive stages and characters that but little of an animal's ancestry can be learned from a scrutiny of its deA T elopment. The fascinating biogenetic law of Miiller and Haeckel summed up in the phrase, " ontogeny is a recapitulation of phylogeny," must not be too heavily leaned on as a support for any specula- tions as to the phyletic affinities of any species or group of species of organisms. " Embryology is an ancient manuscript with many of the sheets lost, others displaced, and with spurious passages interpolated by a later hand." While a young robin when it hatches from the egg or a young kitten at birth resembles its parents, a young starfish or a young crab or a young butterfly when hatched does not at all resemble its parents. And while the young robin after hatching becomes a fully grown robin simply by growing larger and undergoing comparatively slight developmental changes, the young starfish or young butterfly not only grows larger, but undergoes some very striking developmental changes; the body changes very much in appearance. Marked changes in the body of an animal during postembryonic or larval de- velopment constitute what is called metamorphic development, FIG. 137. Metamorphosis of a bar- nacle, Lepas: a, Larva; 6, adult. GENERATION, SEX AND ONTOGENY 235 or the animal is said to undergo or to show metamorphosis in its development. This metamorphosis is familiar to all in insects; to zoologists, it is familiar among numerous other kinds of animals. Fig. 138 FIG. 138. Metamorphosis of the Monarch butterfly, ^4 nosia plexippus: a, Egg; b, larva; c, pupa; d, imago, or adult. shows the different stages in the metamorphic development of the common large red-brown milkweed butterfly, Anosia plexippus. From the egg hatches a crawling, wormlike larva, wingless, without compound eyes, and with strong jaws and other mouth parts fitted for biting. This creature develops into the winged butterfly with different eyes, different antenna 1 , different mouth parts, different almost everything. And, by the intervention of a curious quiescent stage called the pupal 236 EVOLUTION AND ANIMAL LIFE or chrysalid stage, the changes seem to be made by sudden leaps. Of course, this is not so. It is all done gradually, although there are certain periods in the course of the develop- ment when the changing is more rapid and radical than at other times. The changing is masked by the outer covering of larva and pupa, and although it is indeed startlingly radical in its character, it is wholly continuous. The metamorphosis of frogs and toads also is familiar. FIG. 139. Stages in development of silkworm moth. The eggs of the toad are arranged in long strings or ribbons in a transparent jellylike substance. These jelly ribbons with the small, black, beadlike eggs in them are wound around the stems of submerged plants or sticks near the shores of the pond. From each egg hatches a tiny, wriggling tad- pole, differing nearly as much from a full-grown toad as a caterpillar differs from a butterfly. The tadpoles feed on the microscopic plants to be found in the water, and swim easily about by means of their long tails. The very young tadpoles remain underneath the surface of the water all the time, breath- ing the air, which is mixed with the water, by means of gills. But as they become older and larger they come often to the surface of the water. Lungs are developing inside the body, and the tadpole is beginning to breathe as a land animal, although it still breathes partly by means of gills, that is, as GENERATION, SEX AND ONTOGENY 237 an aquatic animal. Soon it is apparent that although the tadpole is steadily and rapidly growing larger, its tail is grow- ing shorter and smaller instead of longer and larger. At the same time, fore and hind legs bud out and rapidly take form and become functional. By the time that the tail gets very short indeed, the young toad is ready to leave the water and live as a land animal. On land the toad lives, as we know, on insects and snails and worms. The metamorphosis of the toad is not so striking as that of the butterfly, but if the tad- pole were inclosed in an unchanging opaque body wall while it was losing its tail and getting its legs, and this wall were to be shed after these changes were made, would not the meta- morphosis be nearly as extraordinary as in the case of the FIG. 140. Metamorphosis of the toad: At left, strings of eggs; in water, various tad- pole or larval stages; and on the bank, the adult toads. (Partly after Gage.) butterfly? But in the metamorphosis of the toad we can see the gradual and continuous character of the change. Many other animals, besides insects and frogs and toads, undergo metamorphosis. The just-hatched sea urchin does not resemble a fully developed sea urchin at all. It is a minute 238 EVOLUTION AND ANIMAL LIFE wormlike creature, provided with cilia or vibratile hairs, by means of which it swims freely about. It changes next into a curious bootjack-shaped body called the pluteus stage. In the pluteus a skeleton of lime is formed, and the final true sea- urchin body begins to appear inside the pluteus, developing and growing by using up the body substance of the pluteus. Starfishes, which are closely related to sea urchins, show a similar metamorphosis, except that there is no pluteus stage, the true starfish-shaped body forming within and at the expense of the first larval stage, the ciliated free-swimming stage. A young crab just issued from the egg (Pig. 141) is a very different appearing creature from the adult or fully developed crab. The body of the crab / in its first larval stage is composed of a short, globular portion, fur- nished with conspicuous long spines and a rela- tively long, jointed tail. This is called the zoea stage. The zoea changes into a stage called the megalops, which has many characteristics of the adult crab condition, but differs especially from it in the possession of a long, seg- mented tail, and in having the front half of the body longer than wide. The crab in the megalops stage looks very much like a tiny lobster or shrimp. The tail soon disappears and the body widens, and the final stage is reached. In many families of fishes the changes which take place in the course of the life cycle are almost as great as in the case of the insect or the toad. In the ladyfish (Albula vulpes) the very young are ribbonlike in form, with small heads and very loose texture of the tissues, the body substance being jelly- FIG. 141. Metamorphosis of a crab: a. The zoea stage; 6, the megalops; c, the adult. GENERATION, SEX AND ONTOGENY 239 like and transparent. As the fish grows older the body oecomes more compact, and therefore shorter and slimmer. After shrinking to the texture of an ordinary fish, its growth in size begins normally, although it has all the time steadily increased in actual weight. Many herring, eels, and other soft-bodied fishes pass through stages similar to those seen in the ladyfish. Another type of development is illustrated in the swordfish. The young has a bony head, bristling with spines. As it grows older the spines disappear, the skin grows smoother, and, a FIG. 142. Three stages in the development of the swordfish, Xiphias glcuiius: a, Very young; b, older; c, adult. (After Liitken.) finally, the bones of the upper jaw grow together, forming a prolonged sword, the teeth are lost and the fins become greatly modified. Fig. 142 shows three of these stages of growth. The flounder or flatfish (Fig. 143) when full grown lies flat on one side when swimming or when resting in the sand on the bottom of the sea. The eyes are both on the upper side of the body, and the lower side is blind and colorless. When the flounder is hatched it is a transparent fish, broad and flat, swimming verticallv in the water, with an eve on each side. As its de- */ / velopment goes on it rests itself obliquely on the bottom, the eye of the lower side turns upward, and as growth proceeds it passes gradually around the forehead, its socket moving with it, until both eyes and sockets are transferred by the twisting 240 EVOLUTION AND ANIMAL LIFE of the skull to the upper side. In some related forms, called soles, the small eye passes through the head and not around it, appearing finally in the same socket with the other eye. Thus in almost all the great groups of animals we find certain kinds which show metamorphosis in their postem- bryonic development. But metamorphosis is simply develop- ment ; its striking and extraordinary features are usually due to the fact that the orderly, gradual course of the development is revealed to us only occasionally, with the result of giving FIG. 143. Young stages of a flounder, Platophrys pedas. The eyes in the young flounder are arranged normally; that is, one on each side of the head. (After Emery.) the impression that the development is proceeding by leaps and bounds from one strange stage to another. If metamorphosis is carefully studied it loses its aspect of marvel, although never its great interest. After an animal has completed its development it has but one thing to do to complete its life cycle, and that is the pro- duction of offspring. When it has laid eggs or given birth to young, it has insured the beginning of a new life cycle. Does it now die? Is the business of its life accomplished? There are many animals which die immediately or very soon after laying eggs. Some of the May flies ephemeral insects which issue as winged adults from ponds or lakes in which they have spent from one to three years as aquatic crawling or swimming larvae, flutter about for an evening, mate, drop their packets of fertil- ized eggs into the water, and die before the sunrise are ex- treme examples of the numerous kinds of animals whose adult life lasts only long enough for mating and egg-laying. But elephants live for two hundred years. Whales probably live longer. A horse lives about thirty years, and so may a cat or toad. A sea anemone, which was kept in an aquarium, lived sixty-six years. Crayfishes may live twenty years. A queen bee was kept in captivity for fifteen years. Most birds have GENERATION, SEX AND ONTOGENY 241 long lives the small song birds from eight to eighteen years, and the great eagles and vultures up to a hundred years or more. On the other hand, among all the thousands of species of insects, the individuals of very few indeed live more than a year; the adult life of most insects being but a few days or weeks, or at best months. Even among the higher animals, some are very short-lived. In Japan is a small fish (Solaux) which prob- ably lives but a year, ascending the rivers in numbers when young in the spring, the whole mass of individuals dying in the fall after spawning. Naturalists have sought to discover the reason for these extraordinary differences in the duration of life of different animals, and while it cannot be said that the reason or reasons are wholly known, yet the probability is strong that the dura- tion of life is closely connected with, or dependent upon, the conditions attending the production of offspring. It is not sufficient that an adult animal shall produce simply a single new individual of its kind, or even only a few. It must produce many, or if it produces comparatively few it must devote great care to the rearing of these few, if the perpetuation of the species is to be insured. Now, almost all long-lived animals are species which produce but few offspring at a time, and reproduce only at long intervals, while most short-lived animals produce a great many eggs, and these all at one time. Birds are long-lived animals; as we know, most of them lay eggs but once a year, and lay only a few eggs each time. Many of the sea birds which swarm in countless numbers on the rocky ocean islets and great sea cliffs lay only a single egg once each year. And these birds, the guillemots and murres and auks, are especially long-lived. Insects, on the contrary, usually produce many eggs, and all of them in a short time. The May fly. with its one evening's lifetime, lets fall from its body two packets of eggs and then dies. Thus the shortening of the period of reproduction with the production of a great many offspring seem to be always associated with a short adult life- time; while a long period of reproduction with the production of few offspring at a time and care of the offspring are asso- ciated with a long adult lifetime. At the end comes death. After the animal has completed its life cycle, after it has done its share toward insuring the perpetuation of its species, it dies. It may meet a violent 242 EVOLUTION AND ANIMAL LIFE death, may be killed by accident or by enemies, before the life cycle is completed. And this is the fate of the vast majority of animals which are born or hatched. Or death mav come \/ before the time for birth or hatching. Of the millions of eggs laid by a fish, each egg a new fish in simplest stage of develop- ment, how many or rather how few come to maturity, how few complete the cycle of life! Of death we know the essential meaning. Life ceases and can never be renewed in the body of the dead animal. It is important that we include the words "can never be rene\ved," for to say simply that "life ceases/' that is, that the perform- ance of the life processes or functions ceases, is not really death. It is easy to distinguish in most cases between life and death, between a live animal and a dead one, yet there are cases of apparent death or a semblance of death which are very puzzling. The test of life is usually taken to be the performance of life functions, the assimilation of food and excretion of waste, the breathing in of oxvgen, and breathing out of carbonic-acid gas, movement, feeling, etc. But some animals can actually suspend all of these functions, or at least Deduce them to such a minimum that they cannot be perceived by the strictest exam- ination, and yet not be dead; that is, they can renew again the performance of the life processes. Bears and some other animals, among them many insects, spend the winter in a state of deathlike sleep. Perhaps it is but sleep; and yet hibernat- ing insects can be frozen solid and remain frozen for weeks and months, and still retain the power of actively living again in the following spring. Even more remarkable is the case of certain minute animals called Rotatoria and of others called Tardigrada, or bear animalcules. These bear animalcules live in water. If the water dries up, the animalcules dry up too; they shrivel into formless little masses and become desiccated. / They are thus simply dried-up bits of organic matter; they are organic dust. Now, if after a long time years even one of these organic dust particles, one of these dried-up bear ani- malcules, is put into Avater, a strange thing happens. The body swells and stretches out, the skin becomes smooth instead of all wrinkled and folded, and the legs appear in normal shape. The body is again as it was years before, and after a quarter of an hour to several hours (depending on the length of time the animal has lain dormant and dried) slow movements of GENERATION, SEX AND ONTOGENY 243 the body parts begin, and soon the animalcule crawls about, begins again its life where it had been interrupted. Various other small animals, such as vinegar eels and certain Protozoa, show similar powers. Certainly here is an interesting problem in life and death. When death comes to one of the animals with which we are familiar, we are accustomed to think of its coming to the whole body at some exact moment of time. As we stand beside a pet which has been fatally injured, we wait until suddenly we say, " It is dead ! ' As a matter of fact, it is diffi- cult to say w r hen death occurs. Long after the heart ceases to beat, other organs of the body are alive that is, are able to perform their special functions. The muscles can contract for minutes or hours (for a short time in warm-blooded, for a long time in cold-blooded animals) after the animal ceases to breathe and its heart to beat. Even longer live certain cells of the body, especially the amoeboid white blood corpuscles. These cells, much like the Amoeba in character, live for days after the animal is, as we say, dead. The cells which line the tracheal tube leading to the lungs bear cilia or fine hairs which they wave back and forth. They continue this movement for days after the heart has ceased beating. Among cold-blooded ani- mals, like snakes and turtles, complete cessation of life func- tions comes very slowly, even after the body has been literally / */ > cut to pieces. Thus it is essential in defining death to speak of a complete and permanent cessation of the performance of the life processes. CHAPTER XIII FACTORS IN ONTOGENY AND EXPERIMENTAL DEVELOPMENT Many biologists find their greatest triumph in the doctrine that the living body is a "mere machine/' but a machine is a collocation of matter and energy working for an end, not a spinning toy, and when the living machine is compared to the products of human art the legitimate deduction is that it is not merely a spinning eddy in a stream of dead matter and mechanical energy, but a little garden in the physical wilderness. What the distinction (between vital and non vital) may mean in ultimate analysis, I know no more than Aristotle or Huxley, nor do I believe that anyone will know until we find out. BROOKS. WHILE in the foregoing chapter there is outlined in some detail the general facts and processes and so-called 'laws" of ontogenetic development, we purposely omitted any reference to what is known or guessed concerning the causes and control of this development. Only less wonderful than life itself is the unfolding and changing of a single tiny apparently homo- geneous speck of life substance (a fertilized egg cell), into a great myriad-celled, extraordinarily heterogeneous, but per- fectly organized fully developed plant or animal body. And only second in point of insistence to man's queries about the whence and whither of life itself are his demands to be informed concerning the causes and control of development. It is indeed strongly felt by most biologists that the study of development, that is, the study of the initiating and guiding factors of de- velopment, is more likely to reveal to us the basic factors and mechanism of evolution than any other kind of study. It is plain that evolution, its causes and method, are intimately bound up with the general primary phenomena of life, as 244 FACTORS IN ONTOGENY 245 assimilation, growth, differentiation, adaptation, heredity, variation, etc., and it is also plain that these fundamental life phenomena are to be most effectively studied in their relations to the development of individual organisms. The most casual analysis of development shows that nu- merous and various influences play their parts in determining its course; it satisfies no one any longer to say that the course and character of an animal's development is determined by heredity. No influence or " force r ' of heredity can make up in any degree in the case of the development of a chick, for example, for the absence of a proper temperature. This purely external factor of heat is as indispensable to the development of the new chick creature as is the mysterious inherent capacity of the tiny protoplasmic mass to unfold or change so radically that it (and what it adds to itself) may become a peeping chicken. And temperature is but one of a number of other external factors that contribute to the creation of the new chicken, as indeed the inherent capacity of the protoplasm of a hen's egg cell to rearrange itself chickwise and no other wise during development is but one among a number of neces- sary intrinsic factors whose correlated influence or working is part of the developmental mechanism. The influences or factors which determine the initiation, course, and outcome of development, then, may be roughly classified into intrinsic and extrinsic factors. And as in our search for rational mechanical explanations of vital phenomena we look on factors as causal, we may use the word " causes ' ; in place of "factors" or "influences" if we like. The intrinsic causes we must believe to be dependent on or incident to the protoplasmic structure of the germ stuff and to be largely the guiding and determining factors in development, while the extrinsic causes are largely such as supply stimulus and energy for the development. Among intrinsic developmental factors are included assimilation, growth, division, differentiation, etc., all constituting what His calls the "law of growth"; under extrinsic factors may be listed heat, light, moisture, food, gravitation, osmosis, etc., composing, according to His, the conditions under which the "law of growth" operates. In order to understand just what part each one of the vari- ous developmental factors or causes plays, there is necessary a most thorough analytical study of development, and an 246 EVOLUTION AND ANIMAL LIFE attempt o determine in measurable or quantitative degree just what specific effects each factor produces. Obviously the most reliable wav to effect this analvsis and this determination of */ */ the specific cause and effect relations is to appeal to experi- ment. But biology has always been looked on as, and until recently has actually been, almost wholly a science of observa- tion. It is now becoming, in part at least, a science of experi- ment as chemistry and physics have long been (these are now becoming more and more sciences of calculation, that is ; exact sciences like mathematics), and this change and advance for it is truly an advance when a science formerly relying for its facts on observation begins to base its foundations on the results of experiment- -is due primarily to the modern interest and Avork in the problem of developmental causes. The search for a rational, causomechanical explanation of the complex and at first sight wholly baffling phenomena of development has been a great stimulus to the bold questioning of many other vital phenomena heretofore looked on as to be explained only by the assumption of a mystic vital force or capacity wholly beyond and foreign to the physicochemical world of matter and force. Mechanism versus vitalism is one of the greatest present-day battles in biology, and nowhere is the struggle keener or are the mechanists more bold in their posi- tion than in the particular field of the processes and factors of development. To the mechanists the play of familiar physico- chemical forces through the complex and unique structure of germ plasm and living tissues has for result all the extraor- dinary outcome of developmental course and outcome; to the vitalists this course and outcome are far too complex and pur- poseful to be explicable without the assumption of an extra- physicochemical force, with a capacity beyond any single or any combination of several physico chemical forces, which thev call vitalism. mJ There is little need of discussing the great mechanism versus vitalism problem here: it is too difficult a subject, and one as yet too little illuminated by known facts, to introduce into any elementary discussion of evolution matters. But it mav not be amiss to call the attention of even the most / elementary student of evolution and general bionomics phenom- ena to the obvious fact, that the moment one indulges a penchant for assuming a mystic, extra-physicochemical force FACTORS IN ONTOGENY 247 to explain a particularly hard problem, one has simply removed his problem from the realm of scientific investigation. It is no longer a problem. It is explained that is, it is explained for whoever accepts the vitalistic assumption. The varying behavior of things in the inorganic world, the functions and capacities of these things, depend on the varying physical and chemical make-up of these things acted upon by the various kinds of energy, such as heat, motion, electricity, and what not, which we are more or less familiar with as a part of the physicochemical world. Varying energy acting upon, or better, through varying structure: this is the causomechanical explanation of all the phenomena in the inorganic world. Should we not in any open-minded consideration of the phe- nomena in the organic world strongly incline to hold to this same explanation until it is definitely proved incompetent, untenable? Answering the question with a hearty "Yes," the mechanists look first of all in their study and analysis of the so-called vital phenomena to the matter of structure of the vital masses and to the play of energy through the masses, to discover, if possible, a tangible clew to the " mysteries " of the life process. In the study of development, then, we strive first to see and to understand the intimate structure of the germ plasm, this protoplasmic stuff with its wondrous endow- ment of potentiality. In Chapter III we have already stated summarily what is known of the chemical and physical make-up of protoplasm. What is actually known, by chemical analysis and earnest microscopic peering, of this structural make-up is wholly in- sufficient to serve as a satisfactory basis of any causomechani- cal explanation of protoplasmic properties. Although some of the simpler capacities of protoplasm, as its motion, its taking up of outside substances (feeding), etc., have been to some degree explained by seeing in them direct physicochem- ical reactions to external stimuli or conditions, practically nothing has been really accomplished as yet toward a mechani- cal explanation of such more complex or unusual capacities as irritability, assimilation, and reproduction. This last func- tion of protoplasm is in a way its most apparently hope- lessly inexplicable property. And this is especially so when the reproduction is of the sort peculiar to the germinal proto- plasm; that is, where the reproducing protoplasmic mass does 248 EVOLUTION AND ANIMAL LIFE ''.- not simply divide and thus make two masses each capable of the growth and change necessary to make it like the parent mass, but where the parent mass (a fertilized egg cell, or a sexual egg or bud cell) can grow and develop into a highly complex many-celled new organism of type like that from which the parent germ plasm was derived. The special capacities, there- fore, of germ plasm have furnished for centuries, and do to-day, the great problem of biology (next to that provided by the existence of life itself). If we cling to a belief that in some way, after all, the ex- planation of the general proto- plasmic and special germ plasm capacities lies in an unusual combination of structure and play of familiar form of energy through the structure, we are at once forced to assume a structural make-up of proto- FIG. 144. Egg ceil of a sea urchin, TOXO- plasm and germ plasm beyond pneustes lividus, showing cytoplasm. the highest powers O f OUr mi- nucleus, and nucleolus;, and network or alveolar appearance of the proto- CFOSCOpCS to detect. And thlS plasm. (After Wilson.) assumption actually is made by most biologists. No agree- ment, however, exists among biologists as to this assumed structure. Biology does not have its atomic theory as chemistry does, to explain the ultramicroscopic make-up of the sub- stances with which it has to deal, but has its atomic theories, a score of fairly well-marked theories as to the ultimate struc- ture of germ plasm having been advanced in the last couple of centuries of biologic study. Almost all of these theories assume a micromeric structure of protoplasm; a few are antimicromeric. By micromeric is meant simply that the plasm which appears to us as a viscous colloidal substance, somewhat differentiated into denser and less dense parts, appearing as fibrils or grains or alveoles in a ground substance of different density, is assumed to be com- posed of myriads of minute, ultramicroscopic units of the general nature of combinations of chemical molecules. These unit combinations are given ; in the theories of various authors, FACTORS IN ONTOGENY 249 various names, endowed with various particular properties, and attributed, as to their origin, to varying sources. In the seventeenth century and early part of the eighteenth century, before the time of the microscope, many naturalists and physicians believed that in each germ cell (or, according to some, in each egg cell, according to others, in each sperm cell) there existed, preformed and almost complete, a new organism in miniature, and that development was simply the expanding and growing up of this tiny embryo man, or monkey, or chick. Also they were forced to believe, if this first assumption were true, that in each preformed embryo still smaller replicas of their particular kind must exist to be the children of this child, and so on, ad infinitum. Like the nests of Japanese boxes, the outer one encasing a smaller and this still a smaller, and this yet a smaller and so on, the young and future young of any kind of organism were, according to this encasement theory of the germ cell structure, nested in the egg and sperm cells of any organism. But the invention and use of the microscope soon put this theory aside. The germ cells were found to contain no pre- formed embryo. Indeed, they seemed to the earlier micro- */ J scopists to be utterly homogeneous little specks or masses of protoplasm, and the pendulum of speculative explanation tended to swing well away from any preformation theory toward the speedily formulated epigenetic theories, which assumed that all germ cells were practically alike except as to their paternity and maternity, and that the development of these homogeneous specks of protoplasm must be determined chiefly by external conditions and influences. However, it was obvious that there was no logical or even fair reason for believing that the lack of structural differentiation in the germ plasm revealed by the micro- scope was a proof of the actual absence of such organiza- tion. The first microscope magnified but a few hundred diameters, revealing structure invisible to the unaided eye; but later microscopes, magnifying objects a thousand and more diameters, revealed structure and organization which were quite invisible to the lower-powered instruments. And so, although to-day we examine germ plasm with lenses magnifying three thousand times, and yet fail to discover more than threads, rods, grains, or droplets in a viscous 250 EVOLUTION AND ANIMAL LIFE ground substance, we do not believe at all that this struc- tural differentiation is the ultimate physical make-up of the mysterious substance protoplasm. We readily believe there may exist an ultramicroscopic structure of great complexity. Buffon suggested that the living stuff is composed ultimately of tiny structural units, which he called organic molecules', these molecules are universal and indestructible; they do not increase in number or decrease; when united in groups they form organisms; when an organism dies its organic molecules are freed but not destroyed, and later may help compose other organisms. Bechamp believed in similar living micromeric units called microzymes, created directly by the Supreme Being, indestructible and strewed everywhere in earth, air, and water. Herbert Spencer postulated the existence of so-called phys- iological units: living units all of the same structure, active because of their polarity of form and of molecular vibrations, in size and character midway between molecules and cells, small but complex and possessed of a delicate and precise polarity analogous to that of the molecules of crystalline sub- stances, a polarity which gives them the capacity to group themselves into organic parts and wholes. Other theories similar to Spencer's assume a special physicochemical en- dowment of the chemical molecules in the organic body (Ber- thold), or a special electrical endowment of the life units (Fol), or a special chemical one (Altmann and Maggi), or, finally, a special vital one (Wiesner). Darwin proposed a theory to explain how the germ plasm could unfold into the whole body, called the theory of the pangenesis of gemmules. Darwin postulated the existence in the body of a host of life units called gemmules to be found in all the various body cells, capable of rapid self-multiplica- tion and of a migratory movement through the body, the direc- tion and goal of which movement is determined by delicate affinities existing among the various gemmules. When a gem- mule enters an undifferentiated or developing cell, as yet gemmuleless, it controls the development of that cell. Thanks to the delicate and precise affinities of the gemmules, they always get to just where they should, to produce harmonious development; but in the germ cells lodge gemmules from all over the body, so the development of these cells results in a new whole body. FACTORS IN ONTOGENY 251 Niigeli, a philosophical botanist,, proposed a theory of germ- plasm structure and behavior which may be called the theory of micelhc, nutritive plasm and idioplasm. When the complex, life-characterizing albuminous substances took their birth in an aqueous liquid, they were precipitated as tiny particles called micella 1 , which attracted other micella? to themselves and thus produced aggregates of primitive life stuff, or protoplasm. The micelke are all separated from each other by thin envelopes of water, thus making water an integral part of protoplasm, and making growth by intercalation of new micelhe possible; this primitive protoplasm becomes arranged in two ways, resulting in producing two kinds, one called nutritive proto- plasm, and the other idioplasm or germ plasm, extending all through the nutritive protoplasm as a fine network. Finally, the most recent micromeric theory of germ-plasm structure is that of Weismann, the modern champion of natural selection. According to him the protoplasm of the nucleus is made up of units called biophors, which are the bearers of the individual characters of the cell; the biophors are complex groups of molecules, capable of assimilating food, growing, and reproducing; the number of biophors is enormous, as it must equal the possibilities of cell variety. The biophors are united into fixed groups called determinants, each determinant containing all the biophors necessary to determine the whole character of any one cell; in each specialized cell there need be but one determinant, but in the germ cells every kind of determinant must be represented. In connection with the postulation concerning the ultimate make-up of the plasm of the germ cell, Weismann has formu- lated a theory of germinal selection to account for the obvious fact that a certain cumulation of variation of a certain kind or along fixed lines may take place without the aid of natural selection: this variation cumulation often being indeed of a degree too slight to give any opportunity for interference by natural selection. To account for this fact, which has been much used by adverse critics of natural selection, Weismann assumes a competition of the determinants in the germ cells for food, hence for opportunity to grow, to be vigorous, and to multiply; the initially slightly stronger or more favorably situated determinants will get the most food, lessening, at the same time, the food supply of others. Now, when the germ cell 252 EVOLUTION AND ANIMAL LIFE begins development the kind of cells or tissues or organs will be best developed whose determinants happen to be the better fed, stronger ones, while other parts of the body may be made smaller or even not appear at all on account of the starvation of their determinants; also the stronger determinants in the better developed parts of the body will produce by multiplica- tion more and stronger daughter determinants for the germ cells of the new individual than the weak determinants in the ill-developed body parts, and thus this disparity in develop- ment of body parts will be passed on, cumulatively, to suc- cessive generations: which is nothing more nor less than de- terminate variation. All the speculations about the ultimate structure of the germ plasm are interesting, but none of them of course is really convincing. As Delage has w r ell said, the chances are too many to one against the probability of anyone's guessing correctly the actual facts concerning the complex structural detail of the protoplasmic make-up. The structural or inherent factors in ontogeny, then, are to be understood only in so far as ob- vious results or effects may reveal them. Now there is one set of phenomena in ontogeny, to which we have not as yet called attention, which does seem to throw some light on certain essential features or facts of germ-cell structure which other- wise would not be obvious to us. This set of phenomena is that called mitosis or karyokincsis, and occurs in connection with each division or cleavage of the egg cells, and of their daughter cells or blastomeres. It occurs also in the division or multiplication of cells in all the tissues of the body, and is a phenomenon normal to cell increase anywhere in the body at any time in the life of the organism. Direct or amitotic cell division is much less common and seems to be restricted to certain kinds of tissues or to certain periods in the history of the life of certain tissues. However, the recent investigations of Child and others show that cell di- vision without mitosis is more common than is usually thought. In this kind of division, the process consists simply of the con- striction and equal (or unequal) splitting of the cell body into two parts, the dividing of the nucleus usually being slightly in advance of that of the cytoplasm. Each half of the parent cell has then but to increase in size to become the counterpart of its progenitor. In the mitotic or indirect division, on the FACTORS IN ONTOGENY 253 contrary, the process is more complex. It has been described by F. M. McFarland l as follows: "One of the earliest results of the study of cell multiplication was the discovery that division of the nucleus precedes the division of the cell body. Furthermore, a careful examination of the different phases of the process offers the strongest proof that the most important feature of this division, an end to which all the other processes are subsidiary, is the exact halving of a certain nuclear substance, the chromatin, between the two daughter cells which result from the division. To gain a clear conception of this process of indirect cell division, called 'mitosis' or 'karyokinesis/ let us consider the changes which take place in typical cell multiplication. Two parallel series of changes occur nearly simultaneously, the one affecting the nucleus, the other the cytoplasm. In the so-called 'resting' nucleus i. e., the nucleus not in active division the chromatin, as we have seen, exists usually in the form of scattered granules arranged along the linin network, and does not color readily with nuclear stains. As division approaches, these chromatin granules become aggregated together in certain definite areas, forming usually a convoluted thread or skein, which now readily takes up the nuclear stains which may be used. In some nuclei this skein is in the form of a single long filament, in others the chromatin is divided up from the first into a series of segments, a condition which soon follows in the case of a single fila- ment. By transverse fission the latter breaks up into a series of seg- ments, the 'chromosomes,' the number of which is constant for each species of animal or plant. Thus in the common mouse there are twenty-four, in the onion sixteen, in the sea urchin eighteen, and in certain sharks thirty-six. The number may be quite small, as, for example, in Ascaris, a cylindrical parasitic worm inhabiting the alimen- tary canal of the horse. Here the number is either two or four, depending upon the variety examined. In other forms the number may be so large as to render counting exceedingly difficult or im- possible. In all cases, however, one fact is to be especially noted, viz., the number is always an even one, a striking fact which finds its explanation in the phenomena of fertilization to be discussed later on. "While the chromatin is collecting into the form of the chromo- 1 Most of the discussion in the following twenty pages, whether indicated by quotation marks or not, is taken from McFarland's essay on ' The Physical Basis of Heredity " in Jordan's " Footnotes to Evolution" (1902). 254 EVOLUTION AND ANIMAL LIFE somes the nuclear membrane has disappeared. The chromosomes soon reach their maximum staining capacity, and appear usually as a collection of rods or bands of deeply staining substance lying free in the cytoplasm. " While this is taking place in the nucleus, another series of changes has been gone through by the centrosome and the cytoplasm im- mediately surrounding it. We have already indicated the presence of the centrosome as a minute spherical structure lying at one side of the nucleus. This body assumes an ellipsoidal form, constricts transversely into a dumbbell-shaped figure, and divides into two daughter centrosomes, which at first lie side by side but soon move apart. Around each of them is gradually developed a stellate figure composed of a countless number of delicate fibrils radiating out in all directions from the centrosome as a center. This 'aster' or 'astro- sphere ' is at first small in extent, but grows in size progressively as the two centers move apart, apparently being derived from a rearrange- ment and modification of the threadlike network of the cytoplasm under the influence of the centrosomes. ' Between these two asters, which lie a short distance apart and at one side of the nucleus, a spindle-shaped system of delicate fibrils may often be made out, stretching from the center of one aster to that of the other. This fusiform figure is termed the 'central spindle.' The two asters, together with the central spindle, form what is termed the 'amphiaster' or the 'achromatic' portion of the karyokinetic figure. The two series of changes in nucleus and cytoplasm, which have thus far gone on apparently independently of each other, now become closely interrelated in that, as the nuclear membrane dis- appears, a system of fibrils grows out from each astrosphere, which attach themselves to the individual chromosomes. These 'mantle fibers' insert themselves along the chromosomes in such a way that each segment receives a series of fibrils from each pole of the amphi- aster, the two series being attached along opposite sides of the chromo- somes. Under the influence of these fibers, probably by direct pulling, the chromosomes, now bent into V- or U-shaped loops, tend to place themselves in a circle around the center of the spindle, transversely to its long axis, and form the 'equatorial plate.' 'The changes thus far constitute the 'prophases' of the division. The ' metaphases ' following these consist primarily in the longitudinal splitting of each chromosome and the moving apart of the halves. This longitudinal splitting of the chromosome into two equivalent parts forms the most important act of the whole cell division, and is FACTORS IN ONTOGENY 255 ft FIG. 145. Cell fission in the salamander: A, Resting nucleus stage, centrosome partly developed; B, skein stage, chromatin visible as a convoluted hand, the centrosomes having separated; C, the nuclear membrane having disappeared, and a few of the chromosomes lying free in the cytoplasm; D, central spindle complete, the chro- mosomes on splitting being drawn to the spindle; E, metaphase; F, anaphase, the chromosomes being drawn to the poles. (After Driiner.) 256 EVOLUTION AND ANIMAL LIFE of the greatest theoretical significance. By it the chromatin substance of the original nucleus is equally distributed between the two daughter nuclei, so that each receives a half of each original chromosome. The elaborate mechanism and consequent expenditure of energy involved in this careful longitudinal division of each chromosome, rather than a simple mass division, such as might be brought about by far less com- plicated means, indicates clearly that the distribution of the definite organization of the chromatin to the daughter cells is of primary importance, a conclusion which is further strengthened by much evidence too extended to be entered upon here. "In the 'anaphases' and 'telophases/ which include the closing stages of division, the daughter chromosomes migrate along the fibers of the central spindle toward its poles, perhaps through the direct contraction of the mantle fibers under the influence of the centro- some, though this and many other points regarding the forces at work must be left for future investigation to decide. Arrived at the poles, V-shaped chromosomes become grouped in a star-shaped figure, the 'aster,' their outer ends become again joined together in the form of a tangled skein, the individual chromatin granules separate somewhat along the threads of the limn network, their deeply staining quality is decreased, and a new nuclear membrane develops around each group of chromosomes. Simultaneously with this the cytoplasm constricts across the middle of a somewhat elongated cell, resulting in complete division in the equatorial plane of the spindle, and two separate daughter cells result. Each of these is made up of cytoplasm containing a centrosome and a nucleus, similar in all respects to the parent cell from w r hich it has arisen. "A simple tabulation of the changes just described is as follows: PHASES OF CELL DIVISION BY KARYOKINESIS ( 1. Resting nucleus. I. Prophases -j 2. Skein stage of chromatin. (.3. Segmented skein. ( 4. Equatorial plate and splitting of II. Metaphase ( chromosomes. f 5. Movement of chromosomes to poles III. Anaphases - and formation of I 6. Segmented daughter skeins. ( 7. Reconstruction of nucleus. IV. relophases < -_. . . . ( 8. Division or cytoplasm. FACTORS IN ONTOGENY 257 "It is readily seen that the culmination of the process lies in the splitting of the chromosomes and the separation of their component halves to form the two new daughter nuclei." The obvious distinction in capacity of development shown by the various cells which compose an animal's body leads us to ask whether we can distinguish differences associated with these different potentialities in the fine structure of the cells themselves, and especially in their behavior during the process of multiplication. For the fate or future character of any cell must largely depend on the nature of its origin, the character of its inheritance. Now in some cases this difference in poten- tiality of the undifferentiated dividing cells is plainly shown by differences in the details of the process of division. A conspicuous and important instance of this, and one bearing directly on our subject of the relation of the structure and character of the germ plasm to the fully developed organism, is the distinction, usually easy to make, between the body or so-called somatic cells and the reproductive or germ cells of any organism. "Every multicellular organism arises by a process of division from a single cell, the fertilized germ or egg cell, which in turn has been cut off from the cells of a preexisting individual. Out of the group of cells which result from the continued division of the germ cell and its descendants are differentiated the various tissues and organs of the body through which the vital functions are carried on. Those tissues and organs which perform functions pertaining directly to the existence of the individual have been termed 'somatic/ and their constituent cells the 'somatic' or body cells, in contradistinction to the repro- ductive tissues or cells whose function concerns the continuance of the species. In some forms these groups of cells, the somatic and the reproductive, become isolated from each other quite early in develop- ment; in one case, indeed, the differentiation of reproductive cells from the somatic ones has been traced by Boveri back to the first division of the egg. This case of Ascaris mcgalocephala is so striking and of such fundamental theoretical importance that it must not be passed without notice, for in it we find marked differences between the somatic and reproductive cells in their nuclear structure, their relative amount of chromatin, and mode of division. The egg of Ascaris has been the classical object for cytological studies on account 258 EVOLUTION AND ANIMAL LIFE of its small number of chromosomes (two in variety univalens, four in bivalens), their large size, and the diagrammatic clearness of the FIG. 146. Reduction of the chromatin in the cleavage of the egg of A scaris mcgalo- cephala var. univalens. (After Boveri.) changes which take place in division. In the division of the fertilized egg cell we have two (in univalens) long chromosomes handed over to each daughter cell. As these two cells in turn divide, a striking FACTORS IN ONTOGENY 259 difference is seen in the karyokinetic figures. In Fig. 146, A, such a two-celled stage is seen from the pole; in R, a slightly later stage in side view of the spindle. In the upper cell of A, the division is of the usual form, the two chromosomes split longitudinally, and their two halves travel to opposite poles of the spindle (B}. But in the lower cell this is not the case. The central portion of the two chromosomes is broken up into a large number of minute chromatin granules which divide, and, as shown in B, form the only portion of the chromosomes drawn up to the poles and entering into the structure of the resting nuclei after the division is complete. The large swollen outer ends of the chromosomes are cast off into the cytoplasm and are eventually absorbed, playing no further part as nuclear structures. C shows the four-celled stage, in which a marked difference in the size of the nuclei of the upper and lower cells is visible. Lying near the margins of the lower cells are the remnants of the ends of the chromosomes which have been cast off in the division. In D the four-celled stage is shown with the karyokinetic figures of the next division. In the lower cells the spindles are seen from the pole, the chromatin is present in the re- duced amount, in the form of small granules. In the upper left-hand cell the two full chromosomes are seen, each split longitudinally, while the upper right-hand cell shows a repetition of the reduction phenome- non viz., the central portion of the two chromosomes, broken up into granules, alone enters into the spindle figure, the outer ends being cast off into the cytoplasm, where they suffer a similar fate to those of the lower cell in the previous division. The next division repeats the process, one cell retaining the two full chromosomes, while all the others have the reduced amount. This takes place for five successive divisions and then ceases; from the one cell having the two full chro- mosomes the reproductive tissues develop, the others with reduced chromatin form the somatic tissues. Thus is accomplished a visible structural differentiation of the nuclei of the reproductive cells which distinguishes them sharply from all the somatic tissues in Ascaris. We shall see further on that there is abundant evidence in favor of the theory that the nucleus i. e., the chromatin is the bearer of hereditary influences from one generation to the next, and that the specific development and functions of each individual cell are de- pendent upon the specific changes which take place in the chromatin of its nucleus. In this light the almost isolated case of Ascaris pos- sesses a value and interest that cannot be overestimated. 'While in the higher forms of animals and plants we find a sharp differentiation of their tissues into somatic and reproductive or germ 260 EVOLUTION AND ANIMAL LIFE cells, we must bear in mind that not in all forms is this power of the reproduction of the whole organism so sharply limited to the germ cells alone. The familiar propagation of plants by cuttings, the re- generation of complete animals from small portions of their somatic tissues in many lower forms, and numerous other considerations such as these, show clearly that the difference between the powers of somatic and germinal cells is but one of degree; that while in higher organisms the two seem sharply defined from each other, a series of lower forms may be taken which will show the intermediate steps in this gradual specialization of function. "In the unicellular organisms we have most interesting examples of the fundamental facts of reproduction, and through an examina- tion of these we may gain an insight into the more complicated processes of the Metazoa. Each of these lowest forms consists of a single cell in which are carried out in a generalized way the complex physiological functions which, in many-celled animals, are divided up among cell groups. In reproduction the animal simply divides into two, the division of the nucleus preceding that of the cytoplasm, and the method is usually a more or less modified karyokinetic one. This mode of multiplication continues in most forms for a certain number of genera- tions, and then the necessity for conjugation i. e., a temporary or permanent fusion with another individual sets in. If this conjuga- tion be prevented, the animal soon shows increasing signs of de- generation which result in death. This 'senescence 7 of the powers of growth and multiplication can only be checked by the admixture of new nuclear substances from an entirely different individual by con- jugation. In its simplest terms this process is found in Chilodon, according to Henneguy. Chilodon is a minute fresh-water infusorian, which multiplies for a considerable period of time by transverse divis- ion. After a time, however, the physiological necessity for conjugation ensues. The animals having placed themselves side by side in pairs and partly fused together, the nucleus of each individual divides into two portions, one of which passes from each infusor into the other to unite with the half remaining stationary. The two then separate, each having received a half of the nucleus of the other. After thus trading experiences, as it might be termed, a period of renewed vigor and activity for each sets in, manifested in rapid growth and multi- plication by division, producing a large number of generations, which continues until weakening vital activities indicate the periodically recurring necessity for conjugation. In general, among the Infusoria we find the same process taking place in regular cyclical order, with FACTORS IX ONTOGENY 261 more or less complicated variations of the phenomena just outlined for Chiloflon. In all of them the aim of the conjugation is the same, the exchange of a certain amount of nuclear substance between the two conjugating individuals, and the same physiological effect is reached, a rejuvenescence, as it were, of the two organisms which manifests itself in renewed vigor of growth and multiplication. "In some of the lowest forms of unicellular life for example, the Schizomycetes or bacteria and their allies this necessity for con- jugation does not appear to exist, but "\\ A / / D for the vast ma- jority of forms this cyclical law of de- velopment holds good. In the Pro- tozoa no division into somatic and germinal cells is found, both func- tions being united in the one cell which forms the whole body of the or- ganism. In the Met- azoa, however, this differentiation has taken place ; the germinal cells are set apart for the preservation of the race; the somatic cells carry on their various functions for a time, grow old, die, and disappear, certain of the germ cells alone surviving in the production of new individuals. On the borderland between the unicellular and the multicellular organisms, however, stand cer- tain colonial forms, which show an exquisitely graded series of steps, from the conditions of unicellular multiplication to those of the multi- cellular forms." (McFarland.) In the many-celled animals the egg is a single cell laden with a large amount of food yolk, and made up of nucleus and cytoplasm as the living elements. For the normal development of this egg, conjugation with another germ cell, derived from a different individual, is usually necessary. This germ cell is the spermatozooid, a minute cell consisting of nucleus and centro- 18 FIG. 147. Gonium perforate, a simple colonial Protozoan, composed of sixteen cells holding together in a single layer or plate: A, The whole colony; B, a single cell; e, eye spot; c/, chloroplast; n, nucleus; v, vacuole. (After Campbell.) 262 INVOLUTION AND ANIMAL LIFE some with a small amount of cytoplasm modified primarily into an organ of locomotion, the tail. A physiological division of labor is here met with which admirably meets two diametrically opposed requirements. The one of these demands that the conjugating cells be highly motile, and consequently small, in order that they may be able to come together in the water in which they are usually set free. The second requires that there be furnished a sufficient amount of nutritive material for the nourishment of the embryo until it arrives at a stage of growth in which it can shift for itself. These two necessities have been met by the physiological division of labor between the two conjugating cells. The one, the sperm cell, has become reduced in size with a corresponding gain in motility, the other, the egg cell, has had food yolk stored up in it, and its consequent increased size prevents any more than a very slight degree of independent movement, if any. Different stages of these modifications may be met with among unicellular forms, as illustrated in Pandorina, Eudorina, and Volvox, to which might be added many others. In Pandorina the conjugating cells are of nearly equal size, in Eudorina an intermediate con- dition is reached, while in Volvox the egg and sperm cells are sharply differentiated in size and motility. Again, in the first two and their allies all of the cells are at first vegetative and afterward reproductive, while in Volvox the definite separation into vegetative or somatic, and reproductive or germinal cells makes its appearance. We arrive then at the conclusion, from the consideration of these and many other lines of evidence, that the germ cells were primitively exactly alike, and that the differences between them have arisen in the process of differentiation along two separate lines. Furthermore, it is clear that the differences between the two sexes, which become strongly characterized in the higher vertebr