THE PROBLEMS OF EVOLUTION . THE MACMILLAN COMPANY BBW YORK • BOSTON ' CHICAGO • DAIXAS ATLANTA • SAN FRANCISCO MACMILLAN & CO., Limited LONDON • BOMBAY • CALCUTTA MELBOURNE THE MACMILLAN COMPANY OF CANADA. Limited TORONTO L 6>f 3- THE PROBLEMS OF EVOLUTION BY ARTHUR WARD LINDSEY FROFEBSOB OF ZOOIX>GT IN DENIBON trmVEBSITT Nrm fork THE MACMH^LAN COMPANY 1931 Copyright, 1931, i By the MACMILLAN COMPANY ALL RIGHTS RESERVED — NO PART OF THIS BOOK MAY BE REPRODUCED IN ANY FORM WITHOUT PERMISSION IN WRITING FROM THE PUBLISHER Set up and electrotyped Published February, 1931 SET UP AND ELECTROTYPED BY T. MOREY & SON PRINTED IN THE UNITED STATES OP AMERICA TO EDWIN GRANT CONKLIN PREFACE The preparation of this small volume has been the result of long dissatisfaction with the existing treatment of the processes of evolution. Published theories and discussions representing a wide range of opinion are almost universally weak in the treatment of certain problems which are clearly fundamental. The logical solution of these prob- lems is of the greatest importance, and even though a final solution is at present impossible, the prob- lems commend themselves to the attention of bi- ologists. The problems of evolution arise mostly from the interpretation of observed facts. While the facts themselves cannot be a subject of serious disagreement, their significance in this field is often a matter of dispute. Philosophical inquiries here, as elsewhere in scientific fields, are often sufficiently removed from the facts to give a misleading plausi- bility to theories of doubtful or limited value, and a plausible theory is all too likely to assume such importance in the minds of its supporters that it may obscure other factors with which the problem is associated. This difficulty may be met in two ways. On the • • vu viii PREFACE one hand we may attempt to deal with theories in close connection with observed facts, although the body of facts which can be gathered in support of a theory is limited and the capacity of the human mind for their interpretation is often less than we might wish. It is possible to draw thor- oughly sound conclusions from apparently suffi- cient evidence in many cases, only to find them worthless when extended ever so slightly beyond the field in which they were formulated. On the other hand one may admit the futility of present attempts at detailed correlation of our enormous store of observations and resort to logical funda- mentals as a possible source of useful working hypotheses. The former method must be followed; without it the entire fabric of evolutionary thought must remain forever without adequate scientific foundations. But if the foundations are to be sound when they are finally completed, our efforts must be properly directed; this they have not always been, and it is the purpose of this work to inquire into the trend of evolutionary thought and to seek the proper background for future study. It has long been evident that we are making little progress in our absolute knowledge of evolu- tion, and since our experimental methods have undergone very little change they seem to be the most promising field for correction. Experimental evolution has been so much concerned with en- PREFACE ix vironmental relationships that any attempt to criticize or correct leads inevitably into Lamarckian problems and terminology. We must continue to investigate environmental relationships, but they may be studied in a spirit wholly different from the controversial approach of the past, and here lies the chief claim of this study. It is presented as a non-partisan discussion of the existing state of evolutionary theory in the hope of finding a more logical approach for experimental work. It has a Lamarckian flavor, but is neither Lamarckian nor Darwinian in intent; I am interested only in the sound investigation of evolutionary processes, regardless of the names by which they may be called. Such conclusions as I present here must neces- sarily be the result of years of thought and of many men's work. It is impossible to acknowledge or even to realize the extent of one's obligation to the many who collaborate through the medium of their published works, but that the debt is great I both realize and gratefully acknowledge. I wish to extend my thanks for permission to use quotations from copyrighted works to the pub- lishing houses of Henry Holt and Company, Charles Scribner's Sons, Charles C. Thomas, and the University of Chicago Press. My gratitude is also due once more to my wife, Winifred Wood Lindsey, for assistance in reading proof and for the X PREFACE many less tangible but infinitely valuable services which she has rendered in this project, as in all of my work. The final stages of organizing this book and the actual preparation of the text were carried on at Princeton University during a semester's leave of absence from Denison in the fall and winter of 1929. During this period the members of the staff of the department of Biology at Princeton were uniformly willing and generous in giving any as- sistance which I cared to ask. To them I extend my sincere thanks. To Dr. E. G. Conklin, who was at all times actively interested in my project and ready to share his time, his wealth of informa- tion and experience, and his keen appreciation of the facts and problems of this field, I cannot ade- quately express my gratitude in thanks; in appre- ciation of his generous help it is my pleasure and privilege to dedicate this volume to him. A. W. LiNDSEY. Granville, Ohio, January, 1931. CONTENTS PAGE CHAPTER I. Introduction 1 Scope of work. Heritage and environment. Life: vitalistic and mechanistic concepts. Necessity of mech- anistic point of view in scientific inquiry. Environment as initial source of change. Present autonomy of heritage. Complexity of environment. Direction of inquiry. CHAPTER II. Individuals and Species . . 26 The species problem. Nature of species as a unit. Species and evolution. Heritage and environment in species and individuals. Individual characters and the characters of species. CHAPTER III. Sources of Change . . .47 Materials and forces in natural entities. Internal and external forces. Change of environment. Fluctuations of heritage. Scientific attitude toward variation: Darwin, Lamarck, Bateson. Osborn's tetrakinetic theory. Modi- fications, recombinations, and mutations. Adaptation: process and result. Adaptability a fundamental property of protoplasm. Mutilations. Fundamental similarity of all acquired characters. Individual characters a part of species complex. Difference between modifications and hereditary characters. Adaptation of species. Selection. Permanence of modifications. Cause of mutations. CHAPTER IV. Genetics 76 Basic concepts of genetics. Chromosomes, nucleus, and cytoplasm in heredity. Action of genes. Cytoplasmic in- heritance. Germ plasm and soma. The body as environ- ment. Relations of body and external environment. En- vironment and germ cells. Stability of genes. Modern xi 37140 xii CONTENTS PAGE attitude to this question. Duration of gene activity. Sphere of gene activity. Use and disuse. CHAPTER V. Theories of Evolution . . .108 Faith the ultimate basis. VitaHsm and mechanism. Nat- ural selection. Germinal selection. Intraselection. Coin- cident selection. Validity of selection theories. Isolation, geographic and biological. Results of isolation. Impotence of selection and isolation to explain evolution. Mutations. Mutations and selection. Orthogenesis. Cause of mu- tations. Preadaptation. Inadequacy of germinal theories. Lamarckian theory. Kinetogenesis. Parallel induction. Insufficiency of all theories. CHAPTER VI. Evolution in Nature . . .143 Usefulness of mutations. X-ray mutations. Mutations and environment. Acquired characters. Hypothetical analysis of trend of evolution. Existing species based on acquired characters, on hereditary characters. Experi- mental results: eye defects in rabbits; Daphnia; mice; Simocephalus; flax. Possible methods of evolution in a complex species. Difficulties of interpretation of natural cases. Palaeontology. What is a new character? Emer- gent evolution. Complexity of evolutionary processes. CHAPTER VII. The Organic Tendency . . 177 Autonomy of the individual. Opportunism in evolu- tion. Mind. Results of mental control. Favorable orientation. Instinct. Self -maintenance the tendency of living things. Changes the outcome of interacting herit- age and environment. Environment a perpetual source of change. Effects of environment on organism. Tend- ency of organism to become independent. Universal necessity for adaptive adjustment. Necessity for experi- mental evidence. CHAPTER VIII. Our Future Course . . .201 Both heritage and environment essential. Environ- ment may be only directive but is a possible source of hereditary change. Some factors of heritage depend on CONTENTS xiii PAGE external conditions for expression. Nature of individual response. Possibility of chromosomal modification. Use and disuse in chromosomes. Somatic and germinal chromosomes. Continuity of germ plasm. Modifications and the internal environment. Modifications and pre- adaptation. Reversibility. Conditions for experiment. Index • • • • ^29 |lu{LII8RARY . v^\ - /i THE PROBLEMS OF EVOLUTION INTRODUCTION We are reminded by a recent writer that "the problems of evolution are much broader than the origin of biological species," ^ and with this warn- ing in mind I must confess to a limitation of my subject and of my present interest. Admittedly evolution is inclusive of universal processes, not all of which are concerned with living things, but the origin of species has a peculiar interest to the biologist. This particular problem still offers prom- ise of solution on a basis of established scientific principles, and even though a fragment of the whole subject, it is a fragment of the utmost im- portance. Its solution would, if nothing else, re- lease a vast amount of energy for the pursuit of other scientific inquiries ! The scientific world has been thoroughly inter- ested in problems of organic evolution for approxi- mately three-quarters of a century. During that * Conger, G. P., New Views of Evolution, p. 77, 1929. 1 2 THE PROBLEMS OF EVOLUTION time it has been necessary to devote a considerable amount of effort to the adequate estabhshment of the fact of its occurrence, but the method of evolu- tion has also been the subject of extensive discus- sion. The matter has been handled controversially to such a degree that expressions of opinion are much more plentiful than sound discussions, but here and there in the literature, especially in re- cent years, a new and valuable trend in the discus- sion of phylogenetic processes may be noted. Ap- parently our knowledge has reached a point where rational analysis is being diligently attempted. The expressed conclusions of writers on this subject are so different that they can hardly re- sult in an early agreement, but they strike at the heart of existing difficulties in so many cases that the prospect of progress is bright. We have been concerned for years with the gathering of new data and have clung so tenaciously to the old theories and the old theoretical controversies that any departure from the established mode of treatment is promising, granting only that it partakes of an open-minded point of view and does not introduce a new bias among those already existing. Such a work as this cannot pretend to be an ultimate solu- tion, any more than the contributions of many other scientists, but the time seems ripe for a soundly logical analysis of the problems of phylo- genesis and this is an attempt at such analysis. In INTRODUCTION 8 most ways it is not novel; most of the ideas con- tained in it have been expressed in some form else- where. But novelty is not the only valid reason for writing. The reason for this work is a sincere belief that it approaches fundamental problems of evolution without prejudice, that it paves the way for a rational examination of details, and that it furnishes a rational basis for experimentation which has hitherto been obscured by mistaken attitudes. At the outset a word should be said of the possi- ble fields of inquiry in evolution, for it is possible to find unnecessary prejudice even in methods of approach. One writer ^ summarily dismisses the results of philosophical inquiry when conducted without close association with laboratory investi- gation. To the extent that an inquiry may be purely philosophical I heartily agree with his atti- tude, but it suggests to me that there are two dis- tinct courses to be followed in phylogenetic studies. We may, on the one hand, confine ourselves to the examination of various lines of descent, determin- ing the spatial and chronological relationships of the various included species and the environmental conditions under which they arose, and construct phylogenetic trees of great practical value and interest in the field of science. On the other hand, we may interest ourselves in the broader aspects 2 Sewertzoff, A. N., Acta Zoologica, Vol. X, pp. 59-141. 1929, 4 THE PROBLEMS OF EVOLUTION of phylogeny in an effort to determine the general laws which govern the occurrences in the organic world as evolution progresses. In the latter case, of course, we risk departing rather too widely from the solid ground of a specific problem in phylogeny but any inquiry worthy of attention must be founded upon a background of biological facts which should serve as an adequate guaranty of its scientific soundness and protect it against the stigma of philosophic insecurity. It is unnecessary in a study of this kind, which is avowedly an attempt to approach the problems of species formation and general organic evolution in the broadest possible way, to delve into the past history of the subject in detail. Anyone who is interested in it will undoubtedly be thoroughly acquainted with that history so that a general survey may be dispensed with. One conspicuous controversy has come down to us from the past which, in final analysis, colors every consideration of our problem that has yet appeared. This controversy is best labelled with the familiar term, inheritance of acquired char- acters. Of it so much has been written that again it is unnecessary to go into the details of past treatment, but it establishes the point of depar- ture for the critical portion of our study of evolu- tionary processes. It has raged for many years through the proposal of all existing theories of INTRODUCTION 5 evolutionary method, and all existing theories may be disposed on one or the other side. They neither decide nor abate the controversy. If this quarrel between the proponents of en- vironment as the important shaping factor in the origin of species and those of the heritage appealed to me as valid, I should have nothing further to say, for in that case our present knowledge would be insuflficient to decide the matter and further argument would be as profitless as that of the past. Rather, it appears that we have been barking up the wrong tree and that the promising field for investigation is not the relative merits of these two factors but their universal and exceedingly intri- cate coordination. This point of view is not orig- inal, but since it came to me without direct sug- gestion from others I derive a firm conviction of its soundness from the similar conclusions that I find scattered through the literature. It presents difficulties of its own. Perhaps it will not lead us yet to an ultimately tenable conclusion, but as an initial step it seems inevitable. This point of view is slow to gain a hearing. Conklin ^ years ago emphasized the inseparability of heritage and environment. Rabaud states an undeniable truth in saying: "Des maintenant, nous pouvons aflSrmer que ni le milieu ni I'orga- nisme consider^s isolement ne portent en eux de ' Heredity and Environmeni, First Edition, 1915. 6 THE PROBLEMS OF EVOLUTION r^sultat predetermine,"^ yet the Hagedoorns^ find it necessary to warn us that scientists have not approached the problem with open minds in apply- ing the results of their researches and show little evidence of doing so now. And still when we have recognized and admitted that the heritage and the environment are inseparable, we are scarcely nearer to a solution of our problem than before. We wit- ness their interaction only in the individual and the diflSculty has always been to determine if, and how, the transition might occur from the somatic characters admittedly produced in the individual to the hereditary characters which appear nor- mally in all individuals of a species. We are left with several fundamental concepts to clarify; among them the relationship of individual and species, and the consequent association of heritage and environment in the two entities, are prominent. Once we acquire a satisfactory attitude toward these fundamental questions, the difficulties pre- sented by the apparently sound theories of evolu- tionary method are lessened. It becomes obvious that whether we deal with orthogenesis, mutation, natural selection, or any of the minor methods of change, a uniform analysis on the basis of interact- ing heritage and environment is possible. Whereas these theories have overemphasized one or another « Biologica, Vol. Ill, p. 171, 1913. * The Relative Value of the Processes Causing Evolution^ 1921. INTRODUCTION 7 factor, a rigidly logical interpretation makes it impossible to fall into such an error, although it leaves us ultimately the difficult course proposed by Rabaud: to investigate that which results in the interaction of heritage and environment. At this point we come dangerously close to the eternal question, what is life? It would be folly to add to the discussions of this subject which have already appeared; beyond it there is still a profit- able field for inquiry on the basis of known chemi- cal and biological principles. Without knowing exactly what life is, the complexities of heritage and environment furnish an adequate background for the realization that their interaction is suffi- cient to explain the orderly phenomena of life and the complexity of the organic world. This, in itself, is neither a unique nor a difficult concept, but when we introduce the principle of evolutionary change we encounter the difficulties with which the sub- ject of evolution is fraught. It is easier to see that a thing occurs than to determine how it occurs, be it life or evolution. Fortunately biological investigations provide an abundance of material from which to draw conclu- sions relating to our problem. In addition to obser- vations and experiments relating directly to the problems of evolution, other branches of biological science disclose many cases in which definite con- ditions produce definite results and changing con- 8 THE PROBLEMS OF EVOLUTION ditions produce changed results. In physiology, particularly in the field of endocrinology, there are striking evidences of the exactness of cause and effect in the organism and in its environmental relations. In experimental embryology there are many disclosures of hereditary response to modifi- cations of environment, both external and internal. In grafting experiments there are still further sig- nificant results to be considered, both in embryos and in adult organisms. All of these things bear definitely upon the problem of evolutionary method and aid in the establishment of a sound foundation upon which to approach the less satisfactory ac- cumulation of evidence with regard to evolutionary processes themselves. It is rare to find a distinctly original attempt to explain the problems of life, possibly because hun- dreds of men working over a long period of years are not likely to cling to concepts of no value be- yond the acquisition of the facts necessary to prove their uselessness. One such attempt is now prominent in the literature, viz., Augusta Gaskell's What is Life? ^ This is a brilliant and thought- provoking work. It paves the way to a more defi- nite inquiry into fundamentals than has yet been made. But unfortunately it is entirely without assurance that the inquiry will be more successful than those of the past. When Mrs. Gaskell makes « What is Life? 1928. INTRODUCTION 9 such statements as this: "The new verdict is that it would seem almost certain that the strain which resulted in 'man' was a separate one from the beginning. It is extremely improbable that there should have developed a common ancestor from which (as many think, at a late date) both man and the apes evolved — ," she evades an over- whelming mass of significant facts which neither biologist nor palaeontologist can agree to discard. Apparently two points of view are possible, the one conditioned by an accumulation of laboriously ac- quired information which must be considered in reaching a conclusion, the other free from any pre- conceived notions which might exert control over its results. One is reminded of Osborn's statement about the ancient Greeks: "Not pausing to test their theories by research, they did not suffer the disappointments and delays which come from our own efforts to wrest truths from Nature. Com- bined with great freedom and wide range of ideas, independence of thought, and tendencies to rapid generalization, they had genuine gifts of scientific deduction, which enabled them to reach truth, as it were, by inspiration." ^ If the principle of an elan vital should some day be established, whether as an independent force comparable to energy in the physical world or as the Z-system of electrons postulated by Mrs. Gas- ' From the Greeks to Darmn, p. 30, 1894. 10 THE PROBLEMS OF EVOLUTION kell, the problems of evolution will be greatly mod- ified, but with the knowledge at present available I fail to see how it could overthrow completely the intricate web of relationships now so convincingly demonstrated in the field of evolution. For the present everything points toward the adequacy of a physico-chemical concept of the organism as a basis for inquiry into this subject. Whatever life may be, the living being and its functions are almost infinitely complex. Seem- ingly organisms act by the exercise of intangible forces resident within them, but it is increasingly evident as time goes on that the foundation for every action is tangible, and that only our igno- rance of details conceals from us the whys of be- havior. The skin of the frog darkens or becomes pale, but it is not due to some mysterious power of the animal. Darkening is brought about by cold and dampness and is due to a specific stimu- lant from certain parts of the pituitary gland which causes expansion of the melanophores.^ The digestive glands respond to the stimulus of food under normal conditions, but secretin extracted from the lining of the intestine will induce secre- tion of pancreatic juice regardless of other condi- tions.^ Far from being a completely discriminat- ing unit, the organism is very largely a result of complex interaction of substances governed by a ^ Hogben, L. T., The Comparative Physiology oj Internal Secretion, 1927. INTRODUCTION 11 likewise complex association of conditions. This is true even of intelligent man with his relatively- great power to control the conditions of his own life. It must be admitted that the physico-chemical basis has not yet been proved the sole explanation of life, but we may confidently predict that it is a universal foundation for the manifestation of life. Vital processes have never yet been demonstrated independently of such a foundation and, until they are, the problems of evolution should be susceptible of solution to the same extent as most of our prob- lems of science. When we explain a natural phe- nomenon, we have merely resolved it into its com- ponent simpler phenomena; we push back the barriers of ignorance but never wholly remove them. In the case of evolution we first had to recognize its occurrence. Then it was necessary to bolster it up with adequate evidence in the vari- ous fields of science. And all the while we have been struggling to learn how it proceeds, but here our footing crumbles and long before we get to why, we have such a difficult task on hand that we are still struggling. The problem of life itself is very near this why. Given life, be it the result of an elan vital, a Z-system, or merely of a peculiarly intricate physico-chemical organization, we have a satisfactory foundation for the investigation of the how of evolution. 12 THE PROBLEMS OF EVOLUTION A number of phenomena are characteristic of living matter. Ordinarily we think of its ability to appropriate to itself selectively the materials of its environment and to shape them definitely into its own body, its ability to adjust itself to the conditions of its environment, its ability to repro- duce itself, and the physiological properties by which these things are brought about. Yet in- organic matter affords a number of phenomena which show us that these properties may, after all, be only a result of intricacy which makes possible the association of a certain complex of actions. We may mix substances dry without result, moisten them and secure a reaction. We may mix other things and secure reactions only by heating them or striking them. Association of elements may be accidental to the extent that they are distributed by natural forces beyond our ken. One may be wholly independent of another until the proper conditions obtain and they may then unite. The resulting compound is different from what existed before. It may be able to avail itself of other con- ditions and other neighboring materials to form still other things. Nor must a complete change always result. The mere presence of a substance may enable others to react or may prevent reac- tion; this phenomenon of catalysis is apparently a marked acceleration or slowing of reactions already possible, but under natural conditions INTRODUCTION IS they may actually not occur to a significant extent without its intervention. Through autocatalysis, moreover, a catalyst may even produce more of itself. And so in the organism we start with a chemical complex of great intricacy, associated with environmental conditions which are no less essential than the matter in the body. It carries on processes fundamentally like those of any chemical system, governed by similar conditions. It differs conspicuously in its maintenance of or- ganized units — cells, individuals, colonies — and in its ability to reproduce those units. Here lie problems of life at present insurmountable, but they occur like all other vital phenomena as a reaction of the complex organism to the complex conditions of life, and so do not destroy our concept. A reasonably full discussion of the complexities of living things would be a liberal education in biology, but there are a few principles which indi- cate a definite trend, not to say purpose, in the development of organisms to their present state. The primordial living substance which is a neces- sary concept in our modern ideas of evolution must have been very simple as compared with the sim- plest cell of the present. We must assume that it was a very labile protein compound; as to its organization we may safely predict an absence of the differentiation evident in existing cells, a closer approach to homogeneity. Since life, according 14 THE PROBLEMS OF EVOLUTION to modern concepts, probably resulted from the proper association of inorganic materials at a re- mote period in the history of the world when a pecu- liar complex of conditions prevailed, this primordial living material was evidently a product of in- organic factors entirely. In other words, if the idea is valid that life had a beginning, barring the doctrine of special creation and vitalistic theories, it must have been a product of things which we now recognize as factors in its environment. In order to fall within the category of living things, this substance must necessarily have had some capacity to maintain and reproduce itself, the former ability including a simple expression of the various phenomena of adaptation and metabo- lism. Maintenance and reproduction need not have been more than a complex form of autocatalysis. Thus the origin of a partially independent sub- stance as a product of non-living factors entirely is not difficult to conceive. With its origin, the activity of living things began, consisting of an interaction between the living substance and its surroundings. Since then we have a source in liv- ing matter to consider as well as the inorganic fac- tors associated with its existence, for the spon- taneous origin of life has not been demonstrated and most living things have probably arisen from a single ultimate source. Life itself, so far as we have been able to determine, is the manifest activ- INTRODUCTION 15 ity resulting from the interaction. The organism is a mechanism for the storage and transformation of energy, but the source of its energy is the world about it; without frequent replenishment it soon runs down and dies. The idea of a vital principle may be superimposed upon this interpretation but in the present state of our knowledge may not dis- place it; such an idea is, moreover, fatal to a scientific inquiry. Obviously the changing world environment which brought about the conditions favorable to the origin of life must, in time, have given rise to other conditions less favorable for its maintenance. The whole history of the world is one of changing climates and geological events which have exposed its various regions to a succession of different con- ditions. The partially independent living sub- stance, if it were to persist through changing en- vironmental conditions, must necessarily have had adequate latitude of response to enable it to meet them, and likewise the environment surrounding an organism could not change beyond the organ- ism's capacity for adjustment without destroying it. These facts still apply to all existing organisms. But according to our ideas of evolution the com- plex population of the modern world has evolved from a simple primordium. As the years have passed the living substance has gained tolerance for conditions which would have been fatal to it 16 THE PROBLEMS OF EVOLUTION in a previous state, and changes forced upon living things have met with a response in ever-increasing diversity of life. It is primarily a tale of gradually increasing independence of environment; a change of acidity in the surrounding medium which would be fatal to many Protozoa is incidental to others, and a change of temperature which stiffens the most active insect into immobility merely stimu- lates homoiothermal man. As fixed conditions have persisted, many organisms have developed a narrowly limited dependence upon them, but the major tendency in evolution has been toward greater and greater freedom. This does not mean that true independence is even a possibility. Ter- restrial organisms are as dependent upon adequate water as aquatic creatures, but they have devel- oped the ability to conserve their more limited supply enough to invade the driest parts of the earth. They encounter a much greater fluctuation of temperature than aquatic organisms, and in their highest groups, the mammals and birds, they have met this condition by developing the ability to maintain a constant temperature regardless of that of their surroundings. The organism has gained complexity. If we could have been on hand to see its entire history the later steps might be intelligible in the light of the earlier, but since we are a late product of the entire chain of events, and a very complex one, we INTRODUCTION 17 may only attempt to resolve the aggregate into its simpler components in a logical way. The cell, the simplest unit now available as a living thing, is, itself, complex, and when it lives in the free state it is scarcely less so than other organisms. The simplest multicellular organism seems much more intricate but it accomplishes the same ends as its simpler brother; its elaborate structure is a matter of method rather than result. All living things of the present share this quality, hence the heritage, that which reacts with the environment in the maintenance of the phenomenon called life, is complex in proportion to the degree of self- determination that it has wrested from its sur- roundings and in relation to the nature of its ul- timate dependence upon them. The details of organic complexity are, as has been said, the substance of biological science. It is characteristic of past treatment of evolution that we have recognized their importance in the existence and transformation of living things even, in many cases, to the exclusion of any serious con- sideration of the other factors necessary to their existence, namely, environmental conditions. But we cannot expect to understand the organism or its evolution without giving adequate attention to every significant fact. The complexity of the environment is no less than that of the organism, but certain of its char- 18 THE PROBLEMS OF EVOLUTION acteristics are more important for the purposes of our inquiry than any details. The recognition of three phases of environment is by no means new,^ and by no means difficult to grasp. In its life the organism encounters a number of physical and chemical factors which cannot fail to influence it. The importance of water and carbon dioxide, for example, is beautifully expressed by Henderson. ^° Since the perfection of the hydrogen ion method of determining acidity, this condition has been shown to exert an important influence on the dis- tribution of organisms. Temperature, especially daily and seasonal fluctuations thereof, is an unavoidable part of any environment which must be of importance to the organism. The nature of the soil, air or water pressure incidental to bathy- metric distribution, the inorganic constituents of the environment, motion of the air, all of these things influence life and constitute the physical or non-living environment to which organisms must adjust themselves. The physical environment is very important to the green plants, since they depend for their food upon inorganic materials. To them water and carbon dioxide are the fundamental necessities of life. Together with inorganic salts and the radiant ^Osborn, H. F., "Tetraplasy," Jn. Acad. Nat. Sci. Phil., Special Volume, pp. 275-309, 1912; Osborn, H. F., Origin and Evolution of Life, p. 18, et seg., 1917. *° The Fitness of the Environment, 1913. INTRODUCTION 19 energy of sunlight they incorporate these com- pounds into the food supply of most of the organic world and any factors tending to modify the avail- able supply of these several materials inevitably bring about adjustments of the flora. Trees do not grow on prairie areas, in some places because the prevailing winds are too dry to permit their de- velopment, favoring only the growth of low xero- phytic vegetation. Elsewhere great altitude brings about rapid fluctuations of temperature and a short growing season; here again a characteristic flora prevails. Animals respond to the physical environment no less certainly than plants, although their response involves less fundamental needs. A conspicuous evidence of animal response is the adjustment of the circulation to low pressure at high altitude and the peculiar effect of the sudden release of the human body from high pressure. Anyone who has attempted normal activity soon after moving to an altitude of ten thousand feet above sea level will realize that he is not adjusted to the prevail- ing conditions, but after a few weeks the increase of red corpuscles compensates for the low concen- tration of oxygen in the air. And after the appear- ance of a popular account of the last submarine disaster few people can be unaware of the neces- sity for bringing divers gradually back to surface pressure after they have worked at a considerable 20 THE PROBLEMS OF EVOLUTION depth. Although pressure is not the only physical factor which affects animals, its effects are among the most striking. The animals and such plants as depend upon other organisms for food give striking evidence of another phase of their surroundings which may be called the organic environment. Green plants are not independent of organic associations, but their dependence seems less vitally associated with other organisms than that of animals. Many green plants live normally in woods, but any environ- ment which provides shade, moisture, and proper soil conditions is favorable to their existence. Trees are not essential beyond the fact that in nature forests alone usually guarantee the proper conditions. Animals and colorless plants, on the other hand, must have organic food. They are utterly dependent upon other organisms, ulti- mately on the green plants, and so far even man has failed to abolish this dependence in any case. A host of adaptations are associated with this dependence. Aggressive and defensive structures, concealing adaptations, and even many adapta- tions directly associated with the physical environ- ment have to do with an animal's food-securing habits or his defense against those of other crea- tures. Organic environment also leads to the bene- ficial associations, as symbiosis and social organi- zation, and to the one-sided benefits of parasitism. INTRODUCTION 21 in all of which the relations of individuals are conspicuous. Finally, in that environment provides the sur- rounding conditions for a given action, we recog- nize that the body itself maintains the proper environment for many of its processes. Instead of depending upon fluctuating atmospheric tempera- tures, birds and mammals maintain a constant body temperature which is favorable for their vital processes and maintains a constant rate of metabolism, other things being equal. When ex- ternal conditions or individual activities demand greater loss of heat, the sweat glands become more active, more blood reaches the surface, and the body cools itself. The accumulation of waste prod- ucts in the tissues results in more rapid beating of the heart and more rapid respiration, so that elimination is hastened. Every organ of the body responds best to certain conditions, and the normal body provides optimum conditions for the per- formance of its various organs, thus constituting an internal environment. It is impossible, of course, thus rigidly to separate organic responses, for everything that the organism does is a product of the coordinated body; the internal environment plays a part in every action and the activities of every organism are closely linked with all three phases of its environment. From the moment of conception an organism 22 THE PROBLEMS OF EVOLUTION exists by virtue of the power of its heritage of liv- ing substance to react to the factors of the com- plex environment. To the extent that heritages are similar and the conditions of their environ- ments coincide, we may expect the resulting organ- isms to resemble each other. The mechanism of heredity guarantees to each individual of a species approximately similar heritage, but within the limits characteristic of the species it also guaran- tees diversity. With the exception of identical twins, the chance that two individuals will receive identical heritages is exceedingly remote. And the infinite variation of conditions on the surface of the earth makes it practically impossible for two individuals to live under exactly the same condi- tions. With even slightly different heritages and slightly different environments, the products of interaction, living individuals, cannot fail to be different. Variation is, to this extent, axiomatic. Identical organisms are a practical impossibility although in theory they may exist. When we undertake a consideration of the species and of the problems of phylogenetic change, our difficulties multiply. The considerations already stated are not obscure; it is increasingly evident to biologists that individuals must be looked upon as a reaction product, and the principle stated above applies chronologically as well as spatially. If an individual of one generation receives a heritage INTRODUCTION 23 identical with one of a previous generation, it will be identical with its ancestor only if it encounters an identical environment. If the environment undergoes fluctuation, the individuals of succes- sive generations will differ. But since we have never been able to witness such a change, the ques- tion still confronts us : can such variable responses bring about a change in the mass of individuals which constitute a species, beyond the initial range of variation? Logically, since the species is the sum of all its individuals, this must depend upon the ability of individual heritages to pass beyond the bounds of previous possibilities. What occurs in the individual belongs to the species, regardless of whether it belongs to all other individuals or not. In view of these indubitable facts it is not strange that the acquisition by species of the adaptations which appear in individuals in response to the environment was long taken for granted. The idea is exceedingly plausible. Nevertheless the investi- gations of biologists have continued negative. We still do not know with absolute certainty what is the relation of individual adaptations to the char- acters of species. In pursuing an inquiry into this subject, it is possible to find some precedent for a method of treatment. The sources of the antagonistic points of view are generally regarded as Lamarck's and Darwin's theories of evolutionary method, the one 24 THE PROBLEMS OF EVOLUTION emphasizing environment and the other heritage in the evolution of species. But since Darwin, in his assumption that characters must be useful or harmful in order to have evolutionary value, recog- nized environmental relationships, and since La- marck, in that he regarded environment as influ- encing the animal through its nervous system, recognized the activity of the heritage, I have never yet been able to find the fundamental disa- greement in their views. However, there are differ- ences, and the work of neither man is above criti- cism, so they may well continue to stand as the exponents of the opposed schools. A more promising approach by far seems the inquiry, first of all, into the nature of species and individuals, their association, and the relationship in each of heritage and environment. The possi- bility of change, both in individuals and in species, then engages our attention. Sources of change, methods by which it is brought about, the nature of change in organisms in relation both to organ- ism and to environment, are all pertinent to our problem. When once these things are disposed in an orderly manner we have a sound basis for the consideration of the various theories of evolution already proposed and for inquiry into the nature of evolutionary processes from the point of view adopted in this work. As I have already said, I do not regard it as a final solution, but I feel that INTRODUCTION 25 it points the logical way for scientific investigation of the problem, not in terms of physics and chem- istry, but in terms of biology. When we have re- duced our knowledge to biological essentials there will still be room for the solution of these essentials in terms of other sciences. n INDIVIDUALS AND SPECIES The species problem has had a place in most studies of evolution, in addition to the numerous discussions in which it has occupied the center of the stage. Fortunately it has been so thoroughly handled that the usual points need scarcely be considered in detail; a few characteristics alone will be adequate for our needs. The many recorded attempts to reach an agree- ment on the distinctive characteristics of species as biological units have said, as a rule, one of two things: that the species is a natural entity,^ or that only individuals exist in nature and the species is merely a concept for human convenience. ^ We gain nothing by pursuing the inquiry. If entity means to the individual a material object which can be perceived, then obviously he will take the latter point of view, but if the broader idea of reality permitted by the accepted meaning of the word satisfies him, then nothing need prevent his 1 Powers, J. H., Am. Nat, Vol. XLIII, pp. 598-610, 1909; Bateson, Wm., Problems of Genetics, pp. 16, 21, 1913; Lindsey, A. W., Denison U. Bull., Jn. Sci. Lab., Vol. xx, pp. 289-305, 1924. 2 Montgomery, T. H., Proc. Acad. Nat. Sci. Phil., 1902, p. 193; Bessey, Chas. E., et al.. Am. Nat., Vol. XLII, p. 218 et seq., 1908; Williston, S. W., Am. Nat., Vol. XLII, p. 187, 1908. 26 INDIVIDUALS AND SPECIES 27 taking the former. Whichever course may be followed, the difficulties of systematic treatment which have inspired most discussions remain the same. Species differ in their nature. The same criteria are frequently inadequate for their separa- tion in two unrelated groups. Morphological and physiological differences, fertility, mutability, all play a part in taxonomy. Their relative values must depend upon their effectiveness in special cases, and the taxonomist's idea of species is likely to be influenced by the useful characteristics of the group in which he works. Attempts have been made from time to time to clarify the term species by simplifying its applica- tion. Elementary species, which are approximately the same as the pure lines and pure-line equiva- lents of biology, are possibly of some use in experi- mental work, but they are more definitely limited than the species of taxonomy only in degree. In certain characters they are uniform. The recogni- tion of elementary species does not, moreover, solve the species problem. They are not independent natural entities but only components of natural species, which remain to be explained. For this reason the term species is used in these pages to designate natural species. The relationship of individual and species has another aspect which can best be brought out by a broader consideration of living units. 28 THE PROBLEMS OF EVOLUTION The simplest biological unit which is known to carry on independent life is the cell. Regardless of the fact that bacteria present certain character- istics which distinguish them from the vast ma- jority of cells, this statement still applies to living matter generally. Until proof to the contrary is available we may regard bacteria as true cells of a special, perhaps a more primitive, type. The cells with which we are concerned elsewhere in the organic world, however, are definitely organ- ized and complex in themselves. They are made up of various subsidiary units. The nucleus is one, but it in turn includes lesser units such as the chromosomes, and beyond these limits we are justified in believing that the genes may also be definite entities. The cytoplasm presents a variable host of minor units, granules, mitochondria, and others. Nevertheless, since these minor particles are incapable of existing independently, we may accept the cell as the ultimate living unit. Were cells always to exist as independent organ- isms, as in the Protophyta and Protozoa, we would lack one valuable bit of evidence, but when they band together intimately and inseparably in the multicellular organism the case is different. Here we have a concrete entity, a unit which can be seen, heard, felt, smelled, and tasted. There is no obstacle to its acceptance as a natural entity, re- gardless of one's interpretation of the word, al- INDIVIDUALS AND SPECIES 29 though it is composed of lesser units whose reaHty is also generally admitted. The relation of a species to included individuals is, of course, not identical with the relation of an individual to its component cells, but are individ- uals an end in themselves? They may exist alone, to be sure. An asexual or parthenogenetic indi- vidual is even a sufficient unit for a time. If per- petual life were possible it might remain so, but the life of the individual under any circumstances is limited. Asexual or sexual, it must sooner or later die or merge its existence with that of the offspring which it produces. It is inevitably asso- ciated in some way with other individuals of its kind. These may be very different from itself, whence the difficulties arising from polymorphism, or they may be essentially the same, but the asso- ciation always occurs. The aggregate of these asso- ciated individuals is the species. The reality of the species is necessarily different from that of the individual, yet it, too, is an aggre- gation of units. The unit individuals are con- stantly changing, and they are not usually con- nected with each other or wholly dependent upon each other like the cells of the body. From year to year and from generation to generation no species ever embraces exactly the same constituents, yet it continues to exist. Until reduced to a very lim- ited compass, such as the last dozen passenger so THE PROBLEMS OF EVOLUTION pigeons, it is diflScult to comprehend, but this is no proof that it is imaginary. The old analogy of the stream is sound. We have no doubt of the existence and reality of the Missis- sippi River, yet it is beyond our power to see it as a whole. We may see a small part of it by standing on its banks, but at no successive moments does that small part have exactly the same character- istics. So too the species exists as an ever changing aggregation of individuals, their limits beyond our view, their changes constant and usually elusive, but always within a boundary which our fragmen- tary knowledge permits us to realize, as we map the course of a stream. Which is more real, the Mississippi River or a pint of water dipped from it.^ It does not matter that it differs in flood and in drought, nor that it is always in motion; the river exists. Likewise the species, although it is a troublesome thing to fix, exists always as an aggregation of individuals, sometimes of various kinds in form or functions, fluctuating both in space and in time, but always associated either structurally or functionally and always part of a reproductive sequence. Since we accept the reality of evolution, we must recognize the inconstancy of species through time. In this respect there can be no definite boundaries of species, save only the abrupt limit established by extinction. Consequently the requirements of INDIVIDUALS AND SPECIES 31 systematic science are of a special kind. We must catalogue our materials. We must develop a rea- sonably definite idea of the various species making up the organic world, and in doing so we cannot conveniently express a fluctuating unit. It is this need for stability which has caused most of the trouble with the idea of species. Gregory has attempted the expression of an un- defined taxonomic unit by proposing the term cir- culus for the groups of individuals centering about a definite association of characters.^ The concept is valid, but it amounts to no more than another definition of the species concept of all time, hence a new term is unwelcome. For the purposes of taxonomy Montgomery's proposal that the species is a mental cross-section of a line of evolution appeals to me as the most valuable idea yet ex- pressed.^ Elsewhere ^ I have considered more fully the practical aspects of this matter in tax- onomy; this much is adequate for my present purpose. Given an entity, the species, made up of many lesser entities, the individuals, we have an ade- quate foundation for the consideration of problems of evolution, but it is desirable to clarify these con- cepts still further by the consideration of another step in the association of organisms. In a number of cases, notably among the insects, the relation of ' Mason, F., Creation by Evdvtion, 1928. * Op. dt. •* Op. cit. 32 THE PROBLEMS OF EVOLUTION individuals within a species has taken a new trend in the formation of societies. This matter is ex- ceedingly complex; in brief it shows that the individual may give way as the unit to a more complex association. The honey-bee colony is a familiar and clear-cut example. In it we find three types or castes, the queens, drones, and workers. The queens and drones are incapable of securing food or rearing young, yet they alone can produce young; the workers are incapable of normal repro- duction, but they alone can rear young, supply food to the colony, and carry on the many neces- sary accessory tasks. The colony here is the unit of which the species is formed. No one would question its reality or call it a mere concept for the convenience of the beekeeper. The relationships of the individuals within it are no more like those of cells in a metazoon than like those of the indi- viduals comprising a species, but they make up a unit suflSciently restricted to be seen at a glance, to be appreciated without the necessity of associ- ating isolated facts. The limited existence, the dependence, of individual honey-bees is reminis- cent of the condition of fragments of cells or of isolated tissue cells, but the fact that the colony is an aggregation of structurally separate individ- uals more closely approaches the nature of species. The series of units here described show important and significant characteristics. Cells may live as INDIVIDUALS AND SPECIES 33 individuals, but those of most organisms exist only as components of individuals. Individual honey- bees may exist only as members of colonies. Col- onies may be permanent, and may reproduce them- selves as definitely as do individuals, yet they exist only as parts of the species. In most species the association is simpler: individuals exist only as parts of species. In any case we have species only through the existence of individuals. In such asso- ciations it is useless to argue the relative impor- tance of the various categories. They are different, but interdependent. An individual cannot exist without belonging to a species nor a species without including individuals, and when two factors are essential, neither can be said to surpass the other in importance. Their importance, obviously, must differ in kind, but evaluation of degree is not ad- missible. Herein lies an important point of view for the evolutionist. Species were long regarded as fixed and invariable beyond certain limits. The idea of organic evolution arose from the realization that such was not the case, but that species were vari- able to the extent of merging with each other, and that they had undoubtedly arisen in the past by development from preexisting species. This reali- zation led even to denial of the importance of species formation as a problem of evolution; one writer, at least, has recognized evolution as a con- 34 THE PROBLEMS OF EVOLUTION tinuous process of change inherent in living things and species as merely stations along its course.® The point of view is easily harmonized with all sound ideas of evolution and need not affect our approach. Species are the units of evolution, and as such must be the chief concern of the evolution- ist, but in the very nature of their existence he must appreciate their impermanence. He must work also with individuals and must appreciate to the full their significance in his problem. Here is a vital point in his progress. He must not fall into the sociological error of opposing individ- uals to species in estimating values. He must not fall into the biological error of regarding the phe- nomena of individual existence as independent of the characteristics of species. And he must not fall into the philosophical error of satisfying him- self with concepts whose reality cannot be demon- strated within the bounds of science. If he succeeds in observing this trio of don'ts we may expect his results to be sound, even though they may be meager in the present state of our knowledge. The nature of living beings has been briefly stated in the preceding chapter. We are unable to say that life is produced from non-living materials at the present time, or even to predict with reason- able certainty the possibility or impossibility of such additions to the living world. The old princi- « Cook, O. F., Ann. RepL Smiths, Inst, 1904, pp. 397-412, 1905. INDIVIDUALS AND SPECIES 35 pie that all life arises from preexisting life is suffi- ciently established to condition our studies of living things. Consequently we may conclude that all organisms begin life with a heritage character- istic of their ancestral line. From the moment of conception they have certain possibilities. Grant- ing that the reproductive body meets a favorable environment, be it spore, fertilized ovum, or any of the numerous other forms in which the organ- ism may first become independent of its parents, the heritage will proceed to develop according to its inherent possibilities. Once developed into an adult organism, it continues to exist by virtue of the constant interchange which it is able to main- tain with its environment. This idea has been especially well expressed by Dendy ^ in the follow- ing words: "It appears, then, that an organism really inherits from its parents two things: (1) a certain amount of protoplasm loaded with poten- tial energy, with which to begin operations, and (2) an appropriate environment. . . . Therefore, when we say that an organism inherits a particular character from its parents, all we mean is that it inherits the power to produce that character under influence of certain environmental stimuli." The essential nature of both factors is evident throughout all stages of development, although it is impossible in many cases to say what stimulus 7 Am. NaU Vol. XLIX, p. 161, 1915. 36 THE PROBLEMS OF EVOLUTION IS responsible for a given response, and in every case to say just how the hereditary substance is able to respond. We know, for example, that a temperature of approximately 39° C. is a neces- sary condition for all of the development of the unhatched chick. We know that a surrounding medium containing oxygen is essential to both life and development of most organisms, and that water and carbon dioxide are also generally neces- sary. We know that exact relationships with a host species are necessary for the development and existence of many parasites. We know that the failure of one step in development may be fatal to the organism or may render it abnormal. Here are conditions representative of every phase of envi- ronment: a physical condition, chemical factors, organic surroundings, and internal associations within the body of the individual. All are essential, responsible for some phase of development. Rosen- heim has shown that some conditions at high alti- tude, probably the intense light, result in the pro- duction of flavones in the sap of edelweiss which appear in greatly reduced quantities in plants raised at London.^ Henderson's thorough analysis of environment shows many ways in which water and carbon dioxide are important to organisms ; ^ it is sufficient to note that they are fundamental ^ 8 Biochem. Joum., Vol. XII, pp. 283-289, 1918.' ^Fitness of the EnvirouTnent, 1913. INDIVIDUALS AND SPECIES 37 constituents of the food supply of a vast majority of living things. Petrunkevitch ^° calls attention to the fact that a number of parasitic worms, in- cluding Taenia solium, while they must tolerate a considerable variation of chemical environment in their normal hosts, cannot be raised in other animals. And it is commonly known that con- genital thyroid deficiency so interferes with normal development that cretinism results. When we examine some of the discoveries of experimental embryology a certain independence of the heritage also becomes evident. At the outset of development in sexual organisms, with the ex- ception of a few cases of natural parthenogenesis, union of the ovum with a spermatozoon is a neces- sary preface to the formation of a new individual. But Loeb ^^ found many years ago that a slight increase in salt concentration in sea water was adequate to produce artificial parthenogenesis in the eggs of the sea urchin, development continuing to the production of swimming larvae, and in 1916 he wrote that he had "seven parthenogenetic frogs over a year old, produced by merely punctur- ing the eggs with a fine needle." These frogs are said to have been as large as normal frogs of the same age and in no way different from frogs pro- duced in the natural way. " Organic Adaptation^ 1924. ^^ The Mechanistic Conception of Lijct p. 7, 1912. 38 THE PROBLEMS OF EVOLUTION Harrison ^^ reports that limb buds of the sala- mander, Amblystoma punctatum, when transplanted to parts of the body other than their normal loca- tion, frequently express their potentiality as they would under normal conditions, by developing into legs of normal appearance. Such observations indicate that there is some latitude in the conditions which may activate a given factor in the heritage, but we need only note the extensive modifications produced in developing embryos by relatively slight abnormalities of envi- ronment to realize that the correlation is, in general, very delicately adjusted. Morgan's monumental volume. Experimental Embryology, records more cases than are necessary to establish the point. ^^ Development which appears to proceed nor- mally under abnormal conditions is relatively rare, and in such cases as those cited above, close scru- tiny shows that the degree of normality attained is due to the persistence of adequate normal en- vironment in spite of the artificial modification. The salamanders' transplanted limb buds may continue their development into limbs correspond- ing to their original destiny, but not to the extent of taking the place of normal legs in the individual body. This much may be accomplished through their reception from the blood stream, and from " Jn. Exp. Zool, Vol. XXXII, pp. 1-136, 1921. 13 Experimental Embryology, pp. xi + 766, 1927. INDIVIDUALS AND SPECIES 39 other than the usual nerves, of the same things which would normally enable them to complete their development. Bones, muscles, and nerves are not peculiar to any one part of the body, hence the substances required for their development are everywhere present. Those features of their de- velopment which are associated with a given loca- tion, however, may be abnormal in transplanted limbs; they receive an abnormal innervation, if any, and have abnormal external relationships, hence they have accomplished only a part of the normal course of development. And it is worthy of note that a relatively small percentage of the transplanted limb buds are free from conspicuous abnormality. We may accept as fact the conclusion that everything which appears in the organism is a product of definite interacting factors in the herit- age and in the environment, and if either factor departs from the normal, we may expect the result likewise to depart from the normal. Since the individuals thus brought into existence constitute species, it is an accurate conclusion also to say that the species is the result of interacting heritage and environment, but here additional com- plexity enters. Taxonomically we may accept the idea of a "mental cross-section of a line of evolu- tion" and so limit our species to a fixed time level in the present or to definite geological horizons 40 THE PROBLEMS OF EVOLUTION for the convenience of classification, but phyloge- netically the species includes not only the individ- uals extant at a given time, but all of the succes- sion of generations occurring before it loses its identity in the production of other species. In this succession there can be no sharp boundaries. We are dealing with entities which may be sharply limited in space but are inevitably diffuse in time. Genetics has taught us that the mechanism of heredity guarantees a maximum diversity of indi- viduals constituting a species. The assemblage of unit characters for which determiners are carried by the chromosome complex of the species includes many different expressions of the same fundamen- tal character. Illustrations are common enough. Hair color in man may be black, brown, red, or blonde; skin varies in pigmentation from African black to Caucasian fairness; eyes may be anything from deep brown to pale blue. In the normal course of sexual reproduction the various characters are redistributed in such a thorough way that the chance of two individuals receiving identical herit- ages is negligible. Twins of the identical or mono- chorial type apparently do so because they are developed from equipotential portions of a single developing zygote, separated at an early stage in embryonic life. Their approximate identity em- phasizes the more common differences of individ- ual heritage. INDIVIDUALS AND SPECIES 41 Not only do the individuals of a given genera- tion receive an extensive assortment of character combinations, but also, because of their distribu- tion over a considerable area, they are certain to encounter different environmental conditions. The species embraces all of the variety of individuals resulting from the combination of these diverse fundamentals. It is a product of heritage and en- vironment, but in such a degree of diversity as cannot be exemplified by a single individual or even by a group of moderate size. In a given location at different seasons the en- vironment may vary so much that individuals of the same species may resemble each other no more than individuals of closely related species. These differences may be associated with conditions of light, moisture, or temperature. Henneguy, in speaking of this phenomenon, mentions Pieris octavia-seramus, a butterfly of the Transvaal, as one striking case. Of its wet and dry season forms he says: "La forme seramus, de la saison humide, presente une paire d'ailes anterieures dont la face superieure est coloree en rouge, souvent avec bor- dures et taches noires, et dont la face inferieure est presque aussi vivement coloree. La forme octavia, de la saison seche, a les ailes d'un bleu vif tache de rouge en dessus et d'un noir verdatre en dessous." ^^ This type of variation is of relatively little im- ** Les insectes, p. 516, 1904. 42 THE PROBLEMS OF EVOLUTION portance in our consideration of species save in showing that variable conditions in nature may be definitely responsible for the appearance of differ- ent individual characteristics. The fact that dif- ferent conditions obtain in different regions is more important. When the gradual dispersal of a species brings it into contact with these variable condi- tions they may play a part in the production of geographic races. The part which they play must be considered later; it is a disputed one, but the races concerned are real and important. Many species of wide distribution encounter diflferent environmental conditions in various parts of their range, and may show marked diversity of individual characters correlated with regional oc- currence. John B. Smith, one of the early students of North American Noctuidae, described many species of these obscurely colored moths which have since been shown to associate with each other by gradual transition through the intervening re- gions. A fairly simple case of this nature is that of the common checkered skipper, Pyrgus tessellata. Skinner ^^ named the pale western form of this species ocddentalis, Williams ^^ later discovered that the two differed in genitalic structure and called them good species. Barnes and Lindsey " examined material from various localities scattered " Ent. News, Vol. XVII, p. 96, 1906. " im.. Vol. XXX, p. 38, 1919. " Ibid., Vol. XXXII, p. 79. 1921. INDIVIDUALS AND SPECIES 43 across the continent and found a gradual transition from one extreme to the other both in superficial characters and in genitalia. This is only one ex- ample among many, but it is an excellent illustra- tion. The individuals of California are never di- rectly associated with those of Iowa or New York. In any of the three states they are as independent as if they were actually distinct species, conse- quently they respond to the conditions of their own limited environments and hand down through heredity such peculiarities as their heritages dis- play, maintaining distinctive characters of geo- graphic races. It is possible to demonstrate their relationship by tracing the gradual modification of the species through the intervening regions, where even reproductive association maintains an indirect connection between the extremes. MacDougal *^ has produced a similar effect by raising Scrophularia leporella^ which grows nor- mally in the mountains of Arizona, at the Desert Laboratory near Tucson, and near the sea. The plant usually has a "strict, scarcely branching shoot with a few fleshy, succulent roots," but at the Desert Laboratory it branched more profusely and formed more roots, and in the maritime loca- tion it formed such extreme growth that some shoots died early for lack of nourishment, and the roots became a great mass. " Am. Nat., Vol. XLV, pp. 5-40, 1911. 44 THE PROBLEMS OF EVOLUTION The species must embrace all of these forms and the many others which appear as minor variations. It is characterized for our convenience by the lim- ited assemblage of characters common to all indi- viduals, but it consists in reality of every character produced by all individuals, wherever they may occur in its entire range. Obviously the intricacy of this association of many individuals, widely scattered, with an ex- treme variety of environmental conditions, offers an opportunity for the development of almost infinite variety in the species. A high degree of constancy is not impossible; some species vary to a scarcely appreciable extent, no matter how wide their dispersal and the variability of the conditions to which they are exposed. But within our powers of observation a species may be very constant as a rule and still include many individual variations from time to time, apparently because the proper association of heritage and environment for their production is rarely attained. There is a possibility even that existing species may encounter conditions with which they have never before come into con- tact. Such things have happened. We are told by geologists that the evolution of the horse was ac- companied by increasing dryness in North America and that the onset of the glacial periods changed the entire climate of this continent. Granting the ability of the organism to live in spite of the new INDIVIDUALS AND SPECIES 45 conditions, such changes of environment could not fail to produce characters hitherto not realized by any of the individuals constituting the species. In view of the facts which have been considered regarding the association of individuals and spe- cies, it is a necessary conclusion that whatever occurs in the individual is a part of the species. If the onset of a new condition brings about the de- velopment of new characters, they are as much a part of the species as of the individual, for the individual exists only as a component of the species. If they disappear when the associated environ- mental conditions disappear, they are none the less a part of the species, for environment is as much an essential in its existence as heritage. The great problem of evolution has hitherto been treated as a question of the importance or insignificance of these various factors. If a char- acter appears only in the individual in response to external conditions, can it be of importance in the evolution of the species, or are those things which come from within the only significant characters .^^ This attitude seems a mistake. .How can anything which appears in an organism, barring modifica- tions due wholly to accident, such as mutilations, fail to be of potential, if not of immediate and obvious, evolutionary significance.'^ Keeping in mind the facts laid down, that individuals exist only as components of species, that heritage and 46 THE PROBLEMS OF EVOLUTION environment are both inevitable factors in what they are and in what they may become, is not it wiser, more scientific, to inquire into the nature of association of these items in the progress of evolution than to weigh them one against the other and ask which is useful and which impotent? Ill SOURCES OF CHANGE All of the material things in the world, as far as we know, are due to the action of certain forces upon certain materials. Even the rocks may be classified according to the substances of which they are formed and the means whereby they were shaped. They may be granitic or basaltic, lime- stones, sandstones, or shales; they may be igneous or sedimentary or metamorphic. Obviously a mod- ification of the materials composing them or of the forces shaping them must have given rise to different products. So too in the case of man-made things. If we desire a given result we must adhere strictly to the materials and processes which give that result. If we are careless of our specifications we cannot expect a uniform product. The organism is subject to these same condi- tions. It is a product of a certain complex type of matter responding to various conditions of environ- ment. The living matter of which organisms are composed may be of many different kinds and the conditions to which it responds are no less varied, hence the great variety of living things which populate the earth. May we conclude also that a 47 48 THE PROBLEMS OF EVOLUTION modification of either material or forces will pro- duce a change in the organism? It seems wholly probable, but we encounter a specific problem here which is quite different from any associated with inanimate objects. The matter of which living organisms are composed possesses a power of self-determination through which it is protected from passive molding by environmental forces. In the course of evolution, as we have noted, organ- isms tend to secure more and more independence of environmental conditions through the autono- mous activities of their own bodies. Outer forces cannot fail to bear upon living things, but internal forces also affect them. Our problem necessitates inquiry into the effectiveness of these various forces in the shaping of organisms. In approaching such an inquiry it is pertinent to note that the discoveries of modern physics disclose the existence of internal forces in inani- mate things which we cannot fail to consider. Chemical changes are brought about by the action of physical forces, such as X-rays, but modern physics has shown that change may take place as a result of internal forces. Atoms of some of the heavier elements, such as uranium, are constantly undergoing disintegrative radiation which trans- forms them ultimately into lead. The concept of independent internal activity in living matter is therefore not impossible. It leads to the admission SOURCES OF CHANGE 49 that change may occur entirely from within in some cases. But, paradoxically, physics and chem- istry provide us at the same time with an abun- dance of facts in the phenomena of the colloid state which seem to dispense with the necessity for such explanations of organic change. External condi- tions are commonly the cause of chemical and physical transformations, and the usual manifesta- tions of life are inseparably linked with external conditions. In seeking the cause of change, the variability of the environment is so conspicuous that it is every- where to be seen. The variation of the heritage is no less obvious, since variation is a universal char- acteristic of living things, and the determinative powers exercised by the organism are apparently functions dependent upon the nature of its herit- age. Variability of the environment impinges upon organisms in a number of ways. In a given locality marked fluctuations may occur, and in any locality some fluctuation is inevitable, due to the topog- raphy of the land and the meteorological condi- tions prevailing. High altitudes, for example, are characterized by rarefied air. This factor not only conditions the respiration of animals, but also in- fluences temperature. During the day when the sun shines it is a poor insulator, hence the body is exposed to intense heat. I have picked caterpillars 50 THE PROBLEMS OF EVOLUTION from sage brush only a foot or two above the ground and seen them die when dropped onto a surface nearby which was without protection from the rays of the sun. At night the same hmitation of the air permits rapid dissipation of heat, both from the earth and from the body, and contrast- ingly low temperatures result. Water supply fluc- tuates with rainfall, which is conditioned by many factors. In central California and similar regions the summer months find practically desert condi- tions while winter brings abundant rainfall. The organic environment is no less variable. The life cycles of plants and the migrations of animals bring about changes from season to season in food supply and other less direct contacts. And the internal environment varies constantly, but in a degree which is in inverse ratio to the degree of independence from the external. The locomotion of animals and gradual dis- persal of plants may also bring them into contact with different conditions of environment as they invade new regions. Other things being normal, animals probably seek congenial surroundings and plants develop better under favorable conditions, hence their contacts are not likely to differ from those established under fluctuating conditions in one environment while such conditions are avail- able. It is more than likely, however, that dis- persal is accompanied by gradual adjustment to SOURCES OF CHANGE 51 new conditions as a species pushes out farther and farther from its original center. Another source of environmental change whose value must be extreme is gradual modification over long periods of time as a result of geological transformations. Elevation of a land mass may- substitute the extreme diurnal fluctuations of high altitude for the fairly even temperature of low areas, or depression may result in the intolerable climate of Death Valley. The thrusting up of a new mountain range may bring a new abundance of rainfall on one side and reduce the neighboring lands on the other to aridity. The geological his- tory of North America shows the past occurrence of tropical and arctic conditions where it is now temperate. Moreover these are not fleeting changes. As construed by human concepts of time they take place slowly and endure long. Within human experience and no doubt for a vastly longer time our continent has been temperate; the whole world, in fact, has had a fairly constant distribution of climatic conditions during recorded history. Fluctuations of the heritage are more elusive. The heritage, in the first place, is a variable con- cept. In the individual it is limited by the par- ticular assortment of characters for which deter- miners were received from the parents; in the species it is the entire range of characters possible to all of the individuals that may be produced. 52 THE PROBLEMS OF EVOLUTION According to the discoveries of genetics, the indi- vidual components of the heritage are remarkably constant, but we are indebted to the same science for the discovery that a reassortment of these units is possible up to the physical limitations of the reproductive mechanism. This reassortment is apparently due to chance. We can in no case say that a given condition will bring about a certain association of chromosomes during normal meiosis and thus give a particular distribution of potenti- alities in the resulting cells. Nor can we attribute a controlling power to the activities of the cells themselves. To this extent, then, the heritage possesses a definite capacity for variation among the individuals of a given species, and this vari- ability is independent of environmental factors, whether or not its expression in differentiated characters of individuals may be equally inde- pendent. The reassortment of individual determiners dur- ing sexual reproduction is, however, of a nature to guarantee the production of all possible combina- tions, hence it cannot be a determining factor in evolutionary change independently of other condi- tions. It is important in this respect, but only as one factor acting with others to attain a given result. A more significant inquiry is whether or not the heritage of an individual, a fixed association of SOURCES OF CHANGE 5S materials received from its parents, is susceptible to modification from within or from without, and if so, whether the modification is of any importance in the evolution of species. This matter is at the heart of the problem with which this work is con- cerned, and consequently involves more questions than the mere possibility of change in the indi- vidual. It will be considered later from several points of view. Changes in organisms due to any of these factors constitute the individual differences in a species, as well as the variability of the individual through the successive periods of its life. They are the variations which are so universally evident among living things, and regardless of their immediate cause, must be looked upon as potentially of inter- est in any study of evolution. The attitude of scientists to this problem has been an interesting evidence of the degree of ex- planation which may be satisfactory to scientific minds. Darwin ^ recognized the occurrence of fortuitous variations without making any attempt to explain them on a basis of cause and effect; as some writers have expressed his attitude, he re- garded them as axiomatic. Nevertheless he was not unaware of the effectiveness of Lamarckian principles in producing variations. He noted the * (a) The Foundations of the Origin of Species, A Sketch Written in 18I^2, edited by Francis Darwin, Cambridge, 1909; (6) The Origin of Species, 1859; (c) The Descent of Man, 1871. 54 THE PROBLEMS OF EVOLUTION effects of use and disuse, and at one time expressed the belief that the incidence of new conditions over a succession of generations might result in greater variation. He did not, of course, have the knowledge necessary to an appreciation of the re- combination of characters by sexual reproduction which is now known to be so important, but even so we find him writing: "A certain degree of varia- tion seems inevitable effect of process of reproduc- tion." 1" Other writers have taken various attitudes to- ward the causes of variation. They could not fail to recognize the occurrence of variations, even as Darwin did, but recognition is not explanation and the trend of modern science is inquiry into causes to the utmost possible degree. In 1909 Bateson wrote: "In reply to the question so often asked, Has modern investigation given evidence as to the nature of these causes.^ the answer must still be. Almost nothing." ^ In 1913 he inquired at length into the causes of variation,^ but still found no satisfactory explanation. His diflSculty in satis- fying himself on the addition of characters to an organism is a very real one, and if we concern our- selves with specific causes we must admit that his recognition of scientific ignorance is valid. The ^'^See footnote 1, part a. 2 Mendel's Principles of Heredity, p. 279, 1909. * Problems of Genetics, 1913. SOURCES OF CHANGE 55 problem seems, however, to be solved in a general way by Osborn's tetrakinetic theory. Osborn recognized the failure of evolutionary theory to accord with the evident facts of evolu- tionary progress, and logically concluded that the explanation of evolution "must lie somewhere within the actions, reactions, and interactions of the four physico-chemical complexes, namely, the physical environment, the developing organism, the heredity-chromatin, the living environment, because these are the only reservoirs of matter and energy we know of in life history." ^ His last clause is worthy of note. These factors are the very foundation of life, so far as we can determine. How can they fail to be at the same time the source of organic change.^ Yet this does not meet the diflficulty of specific points of relationship and he continues: "While it is possible that the relations of these four energy complexes will never be fathomed, it is certain that our search for causes must proceed along the line of determining which actions, reactions, and interactions invariably pre- cede and which invariably follow those of the body cells (Lamarckian view) or those of the chromatin (Darwin- Weismann view)." ^ This is as far as we can carry an inquiry into the cause of variations at the present time. As long as we pursue our investigations on the mechanistic * Origin and Evolution of Life, p. 145, 1917. ^ Op. cit., p. 145. 56 THE PROBLEMS OF EVOLUTION basis the only admissible source of variations will be recognized as these four factors. Their inter- actions vary in results, but evidences of one or another kind are everywhere available. If fluctu- ations take place in the external environment, the responses of individuals are modified. Such physi- cal stimuli as intense sunlight and activity produce demonstrable, usually useful, results which Conk- lin has classified as follows : ^ Stimulus Beneficial Response Increased light Increased pigmentation Increased friction Increased thickness of epidermis Increased use Increased size or strength Unusual foods Appropriate digestive fluids Unusual temperatures Acclimatization Poisons or toxins Toleration or antitoxins Injury Regulation or regeneration Changes in the organic environment are more likely to lead to a modification of behavior than of visible organization, although they may act upon the organism in the form of a modification of food supply. The organic results of such changes as these are visible to us only in the individual, and have logi- cally been called modifications. The term ac- quired character is satisfactory but it has been so sadly abused that it is better avoided. The possibility of changes occurring in the inter- 6 Rice Institute Pamphlet, Vol. VIII, pp. 322-323, 1921. SOURCES OF CHANGE 57 nal environment, i.e., in the body of the individual, without immediate cause in the other factors of life, must be admitted. The body is exceedingly complex and it is quite probable that reactions may occur in it which are not immediately de- pendent upon external conditions or upon the ac- tion of hereditary units. Changes due to increas- ing age are undoubtedly of this type in part. Since the body is an intermediary between heritage and environment, however, these changes seem less significant in evolution than those correlated with external stimuli. Whatever takes place in the body is possible by virtue of properties received from the heritage and developed in response to external conditions, hence purely somatic actions are more likely to be a result of, and in harmony with, normal activity than to be a source of novelty. Fluctuations of the heritage occur, as we have seen, by chance recombination within the limits of the species, but they are of interest to us only in their relations with other factors. Thus far the appearance of new unit characters in the constitution of a species is limited to one known process, mutation. It has been easy to look upon mutations as fortuitous occurrences. Indeed, they are a part of the heritage and have their basis in the chromosomes, but they cannot be referred to the action of the chromosomes alone for these bodies, like all living matter, exist only 58 THE PROBLEMS OF EVOLUTION by virtue of environmental association estab- lished through the cytoplasm. Even mutations, therefore, must be a product of some phase of the environment acting directly or indirectly on the chromosomes. This becomes increasingly evident in the light of experiments with X-rays, notably the recent researches of Muller ^ on Drosophila, and the discoveries of Babcock and Collins ^ in Cali- fornia concerning the effects of terrestrial radiation on the same insects. The body is a complex phase of the environment, far more complex, indeed, than some of the external conditions which give pronounced results in the individual, and it is quite possible that it may undergo changes from time to time which produce mutations. To what extent such changes are associated with fluctua- tions of the external environment it is impossible to say, but it is at least probable, as we have noted, that independent changes are not likely to modify the heritage. The changes due to increasing age, for example, cannot be referred to external en- vironmental stimuli, save only to the extent that everything in the body is a product partly of these stimuli, directly or indirectly. We have no reason to believe that such changes affect the offspring of an individual in fundamental ways. The importance of these various changes to the f Science N. S., Vol. LXVII, p. 82, 1928. ^ Science Supplement, p. x, Aug. 2, 1929. SOURCES OF CHANGE 59 organism throws additional light on their signifi- cance in evolution. Since organisms exist as reac- tion products of heritage and environment, it is obvious that these factors must be in harmony if life is to continue. A given condition of the en- vironment must be met by a particular power of the organism to respond, a function which is the expression of some structural component of the individual. Since, at the outset of their existence, individuals receive a certain equipment and are produced in a suitable environment, their develop- ment and the major steps of their independent life are fairly uniform throughout any species. Adap- tation is an inherent necessity and a universal phenomenon. Some of the adaptations which have been ob- served in the living world are wonderful, and have given rise to a probably unwarranted emphasis upon the adaptive value of organic structures. Attention has frequently been called to the fact that the mere existence of an organ does not signify adaptive value. The presence of eyes is a valuable adaptation for life in the open, but is of no use in dark retreats. Lack of eyes must be a limitation to an organism which lives in the presence of light, yet many such cases occur. So too, animals with eyes have been found in caves. It is necessary in evaluating characters to recognize adaptation as a twofold phenomenon of process and result. All 60 THE PROBLEMS OF EVOLUTION characters are a result of the interacting factors, heritage and environment, and it is probable that all are potentially useful in relation to conceivable environmental conditions, save possibly during periods of development and atrophy. A structure may become an adaptation to conditions which have had no part in its production, and conditions may bring about the development of structures which do not adapt the individual to respond to them. But regardless of these facts, the things which enable the organism to meet conditions of environment are adaptations. The changes which individuals undergo, on the other hand, are in part evidences of the process of adaptation. Each is provided at birth with a hereditary equipment of adaptations, but as it encounters fluctuating conditions of environment it must adjust itself by the modification of its previous condition. The deposition of pigment in the skin is an obvious adaptive response, since it provides protection against the ultra-violet rays which bring it about. That individual responses are not always adaptive is amply shown by Sum- ner's experiments with white mice (see Ch. VI). Increased size of appendages appears in no way to be of value in relation to the higher temperature which brought it about. It is, of course, difiicult to suggest the causes of existing organs without risk of serious error. The SOURCES OF CHANGE 61 functional "lungs" of the lobe-finned ganoids were obviously not developed in response to the condi- tions of terrestrial life, although they were an im- portant adjunct to the successful attainment of terrestrial habits. Lungs are associated with air and air is available to swimming animals as well as to terrestrial species, yet there appears to be no peculiar need for lungs among the fishes and the air has no peculiar power to cause the develop- ment of lungs. However, many such structures have developed in the past, and, once developed, have been available for use and consequently have become adaptive. It is possible to find other well developed characters which are apparently without adaptive value. Such are the brilliant colors of some butterflies. The transverse ridges on the shells of some lamellibranch molluscs and the sharp keels on others are apparently of no fundamental value to the individual. The fantastic projections on some of the tree hoppers are not known to be adaptive, nor are the spines on the legs of Hesperi- oidea, since they may be present or absent in closely related genera. To the extent that all characters are a product of definite hereditary capacity responding to a definite environmental stimulus, there is a similarity in the origin of everything organic, but it is obvious that a character which bears to the stimulus producing it no relationship of importance to the organism is 62 THE PROBLEMS OF EVOLUTION not just the same as a character which fits the individual to cope with the inciting stimulus. We must admit the truth of Cunningham's statement that adaptive and non-adaptive characters are distinctly different, if only in this matter of origin, whether or not we agree with his conclusion that "their origin and evolution are entirely distinct and different." ^ It is exceedingly risky to general- ize concerning origin on the basis of adaptive value. It is significant that the individual receives a normal complement of adaptive structures as a part of its heritage, while its response to a fluctuat- ing environment involves the adjustment of these structures through its power of individual adapta- bility. It may be called upon during its life to meet many fluctuations of environment; it does so, not by producing new organs, but by the modification of the things already present in its body. The tan- ning of our skin is not a deposition of pigment where none existed, but only an increase of the normal deposit. Adaptation in the individual may be looked upon, therefore, as a process or a variety of proc- esses giving rise to modifications of hereditary equipment in response to the fluctuations of envi- ronment. The nature of the process is obscure. It has been admirably discussed by Conklin,^^ and for " Hormones and Heredity, p. 21, 1921. 10 Rioe Institute Pamphlet, Vol. VIII, pp. 299-380, 1921. SOURCES OF CHANGE 63 the present his conclusion is a wholly satisfactory expression of our knowledge: "Since it is pres- ent ... in living things generally, it may be considered to be one of the original properties of life, and our inability to explain its origin is not different from our inability to explain the origin of metabolism, reproduction, irritability, or of life itself." The nature of its results and its immediate causes are varied, and as a result many errors have crept into the discussion of acquired characters. Mutilations, for example, have been the subject of many pages of wasted effort. They are acci- dents. No thinking scientist would give them serious attention in the critical study of evolu- tionary processes, yet they have had a prominent place in the older literature of evolution. I have previously discussed this point in greater detail, showing that other modifications, such as the flattening or constriction of the skull by artificial means in infancy are of similar nature. ^^ All of these changes are impressed upon the organism by the external environment. The organism is unfortunate enough to get in the way of a train or a sword or an amputating knife and thereby loses something which it cannot replace, or its parents bind its feet, and in spite of a noble struggle its body is prevented from gaining normal expres- " Am. Nat, Vol. LXI, pp. 251-265, 1927. 64 THE PROBLEMS OF EVOLUTION sion. In both cases inherent capacity for normal behavior is evident, either in the actual production of the part which is later lost or in the agony re- sulting from the interference with normal processes. The more significant individual modifications have already been listed. They are due to the re- sponses of the individual to external stimuli; re- gardless of the particular conditions involved, they always include these two factors, stimulus and response, and the result is a product not only of environment but also of heritage. If there is any doubt of this, we have only to consider such con- ditions as obtain in the albino. These individuals may encounter the same stimulus that produces heavy pigmentation in normal individuals but they lack the hereditary ability to respond. There has been a tendency in the literature of evolution to handle these individual adaptations in separate groups. This treatment is not nec- essary. Whether they fall in one of the groups listed or in another they are all the result of increase or decrease of some functional activity which brings about a corresponding change of functional capacity to meet a need arising in the life of the individual. Most of these responses appear to be adaptive. It is easy to look upon them also as positive and negative in that a lessened stimulus results in a lessened response and vice versa, but it seems more SOURCES OF CHANGE 65 accurate to regard these degrees as positive re- sponses to varying conditions. The organism ad- justs itself to fluctuations of environment and any adjustment demands positive action. Some apparently non-adaptive responses of in- dividuals seem more truly negative. Deficiency diseases and less serious organic disturbances re- sulting from dietary deficiencies are of this nature. The lack of a vitamine or of iodine, or insufficiency of either, is obviously a fluctuation of environ- ment, but the human body usually lacks hereditary power to effect an adjustment to it, and if it exists in spite of the unusual condition, it does so in a subnormal state. The thyroid disturbances arising from lack of iodine, for example, result in positive conditions in the body which constitute a clinical picture, but they are no more than the results of an abortive attempt at adaptation for they do not fit the individual to meet the conditions which cause them. When we consider these cases thoroughly we find that they are not different, however, from adaptive responses, even though they are more obscure. Our reaction to fiuctuations of iodine or vitamine intake are not considered until these substances fall below the necessary minimum and disease results. If tanned skin were not visible our response to sun- light would probably be just as much neglected, but let sunlight be reduced below the necessary 66 THE PROBLEMS OF EVOLUTION minimum and disturbances result, such as rickets in the growing child. Here again the body is in- capable of adapting itself; its capacity for adjust- ment is limited. The greater size of mountain plants produced by MacDougal ^^ at the lower level of Tucson, the development of a wider head in Daphnia by Wol- tereck ^^ in response to overfeeding, and Sumner's experiments with mice ^^ in which animals raised at higher temperatures had larger feet, tails, and ears, are distinct evidence that characters pro- duced in individuals need not be adaptive, even though associated directly with an environmental stimulus. There is no reason to suppose that large plants of a given species would succeed better as individuals in the conditions prevailing at low altitude than small ones; their increased size and branching in MacDougal's experiments were appar- ently an outcome of longer growing seasons and better nourishment. Nor can one see how larger feet and ears, and particularly longer tails, would benefit mice in warm regions. Such characters may be termed incidental individual responses in contrast with the adaptive responses previously considered, but they are no less definitely a result of hereditary capacity expressed according to pre- vailing environmental conditions. 12 ^m. Nat, Vol. XLV, pp. 5-40, 1911. " Verh. d. Deutsch. ZooL Ges., pp. 110-172, 1909. " Jn. Exp. ZooL, Vol. VII, pp. 97-155, 1909. SOURCES OF CHANGE 67 Since the species is the sum of its individuals, it is obvious that these individual responses, adap- tive or otherwise, are as distinctly a part of the species as are those characters which invariably appear in the normal course of development. The two have been contrasted in all of the literature of evolution. Why.^ In order to answer it is necessary to consider the nature of the normal specific char- acters. Ordinarily we say that they are hereditary, but careful thought shows that the organism does not inherit characters, but only the ability to pro- duce them during the course of development.^^ Development takes place normally only in the proper external environment, but the immediate condition to which a given hereditary power re- sponds is in many cases established by the body as development proceeds, so that the relationship with external conditions is indirect. All of us have eyes. The development of eyes cannot precede the differentiation of ectoderm and neural tube. All of us attain stature and mental development within normal limits, but thyroid deficiency or persistence of the thymus exerts a modifying effect. These things are quite in harmony with the princi- ple already expressed, that the major tendency of evolution is toward greater independence of ex- ternal environment. The things which character- ize all individuals of a species are not inherited as " Conklin, Sci. Monthly, Vol. VII, p. 499, 1919. 68 THE PROBLEMS OF EVOLUTION such, but they are developed by each individual in response to conditions which its body is nor- mally able to produce in spite of moderate fluc- tuations of the external environment. They are fundamentally in contrast with the modifications constituting purely individual responses not in heritability hut in the source of the stimuli which bring them to expression. The question of change in species is different from that of change in individuals and is closely linked with the selective processes whereby a de- gree of evolution is known to be accomplished. While the individual is a unit made up of insepa- rable components, all necessary to its existence, the species is made up of a large number of individuals, varying according to recombinations of hereditary units, and continues to exist even though a large part of these individuals are destroyed. Change of a species may result, therefore, from any factor which tends to preserve one aggregation within it or to destroy another. The individuals of a species spread in the course of time over great areas. As they spread they en- counter different conditions of environment, but even barring this condition it is improbable that they will remain exactly the same in two separate regions, because of the reassortment of hereditary units in sexual reproduction. Through chance or through some peculiar fitness of a given heredity SOURCES OF CHANGE 69 the original inhabitants of a region may bring in certain hereditary characters which distinguish them from members of the same species in other regions. As they reproduce, their descendants ex- press an aggregate of the predominant characters, and thus perpetuate the difference. Such differ- ences may characterize geographic races or, by the vagaries of taxonomic procedure, subspecies or varieties. They need not be adaptive but may be purely incidental, and they are due solely to the isolation of a given hereditary combination. The sharpness of boundaries in such cases must, of course, go far to establish the definiteness of the subspecific units. Oceanic islands like the Gala- pagos archipelago provide an absolute isolation of long duration and so, quite naturally, afford the best illustrations. The mocking birds of the Gala- pagos are of several closely related species, found on separate islands, their distinctive characters apparently without adaptive significance.^^ The ground finches of the same islands show a remark- able series of species or forms based partly on the size of the beak, which varies from enormously large to rather small without correlation with food habits, but in this case the various species are not sharply isolated. Wheeler notes of two species of Galapagos ants that "each of them is represented '6 Giflford, E. W., Proc. Cd. Acad. Sci. (4), II (2), p. 207, 1919; Beebe. William, Galapagos, World's End, 1924. 70 THE PROBLEMS OF EVOLUTION by distinct varieties on each of several of the larger islands" and lists for Camponotus macilentus ten varieties, including the typical form, occurring on eleven islands. In this remarkable series only two varieties occur on two islands and only one island has two varieties. ^^ Conclusions regarding the origin of the archipelago and its fauna are by no means fixed, but it is at least probable that the islands were long ago isolated from the Americas as a single mass of land which later gave rise to the separate islands of the existing group, thus isolat- ing portions of the parent species. Such a history would account nicely for the varieties inhabiting oceanic islands. The types of selection emphasized by Darwin also tend to preserve a limited range of the he- reditary characters of species by eliminating the individuals which do not possess them. We cannot doubt the reality of these processes, however much they may have been overemphasized in the past. Both natural selection and sexual selection are wholly logical in theory. Like many other prob- lems of evolution, particular cases may be obscure in detail, but the principles involved are too well established to be discarded as ineffective. Useful- ness is the keynote of adaptation. We cannot doubt that organs occur in various hereditary de- grees of development, nor that the individuals " Proc. Cal. Acad. Sci. (4), II (2), p. 263, 1919. SOURCES OF CHANGE 71 with optimum development will be more likely to succeed than those less favored and more likely to perpetuate their characters in posterity. This con- stitutes an isolation through usefulness; the species consists more and more, as generations of pass, that group of individuals which possess the useful character. Instead of embracing the original wide range of variation it embraces a restricted portion of that range. Sexual selection has rather less evident impor- tance. It has not been demonstrated by human standards that any particular kind of excellence is chosen by females in their acceptance of mates, although the possibility seems admissible in the case of higher animals. Certainly the display made by birds during courtship implies that there may be a basis for choice. When males habitually fight for mates, it seems probable that the aggres- siveness and jBghting ability of the species must be improved, but since the male takes the active part in this process it is rather a "survival of the fittest" than a selection by the female. In addition to these processes whereby certain parts of the original hereditary complex are pre- served and certain others eliminated, or the species is split up into races by the simultaneous preserva- tion of various characters in various regions, we must not overlook the effects of external environ- ment in producing changes in entire species. In 72 THE PROBLEMS OF EVOLUTION every case forces outside of the body are involved, but they act in different capacities. We have recognized the abiUty of individuals to respond in various ways to environmental condi- tions, and have concluded that everything belong- ing to the individual belongs also to the species. In a diflferent way entirely, individual responses may become characteristic of the species if a change in the environment is so extensive as to affect all individuals; the resulting development of individuals will then occur as uniformly as the normally inherited complex of characters. En- vironmental changes of this extent are likely to occur only as the results of extensive and important geological events, like the elevation of great land masses. Such events do not come and go rapidly. Once they occur, their effects are likely to be felt for a long time. Generation after generation the individuals constituting our hypothetical species may respond to the same conditions by producing the same expression of characters. The results will be as certain of appearance while the environ- mental conditions persist as will the characters which inevitably develop during ontogeny. If the climate of North America should suddenly become subtropical we might confidently expect the paw- paw butterfly, Papilio ajax, always to occur in its summer form, while now its spring brood is recog- nizably different. And if the rainfall of some trop- SOURCES OF CHANGE 73 ical countries should be evenly distributed through the year, we might expect the wet- and dry-season forms of butterflies to give place to successive generations of similar individuals. The difference would still exist among specific characters that some were due to response of herit- age to factors of the external environment while for the others the internal environment would normally provide the proper stimulus. The same thing is true of characters which we now consider in the establishment of species and their sub- divisions. Sumner's mice demonstrate this fact. The characters which they usually present in size of feet, ears, and tail, are as definitely a response to external conditions as are the changes produced experimentally. It will be noted that all of these changes, be they of the "inherited" or of the "acquired" type, deal with degrees of development. Degrees are important. Most species differ in degree rather than in kind from the nearest related forms and such transitions are to be expected first if we ever succeed in producing new species in the laboratory. Nevertheless, change to the extent of complete loss of an existing structure has been noted in the classical example of the mutant, polled Hereford cattle, and almost complete loss in the wingless mutant of Drosophila. Such losses may well be due to accidents of meiosis, and so products of the 74 THE PROBLEMS OF EVOLUTION heritage primarily, or to gene mutations, and sub- ject to the attendant uncertainty of origin, or they may be due to external stimuli reaching the chro- mosomes directly. It is diflBcult to imagine the addition of a new character by accident to the chromosomes, but there seems to be no obstacle whatever to modifica- tion of characters either in this way or by action of external stimuli on the germ cells. In the former category we may place the results of polyploidy and heteroploidy.^^ Some condition, no doubt, in the organism or in the external environment, is responsible for the appearance of these chromo- somal abnormalities, so that we cannot leave en- vironment entirely out of consideration. De Mol's studies of polyploidy in tulips give striking evi- dence of this association of factors. ^^ In the latter category mutations produced in Drosophila by the action of X-rays aflford abundant examples. These mutations, like the similar but less frequent changes in untreated cultures, are in the nature of modifica- tion or loss rather than the addition of new struc- tures. Change in organisms is by no means difficult to note. It emanates from all phases of the environ- ment and from the heritage, but whatever its source, we find that significant changes invariably " Morgan, T. H., Theory of the Gene, Ch. XI, XII, 1926. " Genetica, Vol. XI, pp. 119-212. 1928. SOURCES OF CHANGE 75 concern both heritage and environment. There is some difference in the various types of change. Species will always continue to display a capacity for individual adjustment to fluctuating environ- ment and there is no reason to suppose that the resulting conditions in individuals must become fixed characters of the species nor that they may never become so fixed. These changes must take their place with the purely hereditary characters as possible materials of evolution and from the en- tire assemblage our conclusions regarding the origin of species must be drawn. Q ?«?x IV GENETICS During the twentieth century the science of genetics has given us a valuable account of the transmission of hereditary characters, and associ- ated cytological studies have disclosed many de- tails of the material basis for these phenomena. The association of genetics with the problems of evolution is not wholly a happy one, for it has set up in the minds of some scientists a new bias, no less enthusiastically received and supported than those of the past. Therefore, while genetics is one of the most promising fields for the investigation of evolution, it is like other fields in presenting material which must be very carefully estimated. The fundamentals of Mendelian heredity are, presumably, known to every serious student of the problems of evolution. The basic concepts of ge- netics are very different. We may regard the exist- ence of genes as definitely established. Certainly there are few if any competent judges who now deny that the chromosomes are responsible for the transmission of hereditary characters, whether or not they accept the modern idea of genes as subordinate units within the chromosomes. Grant- 76 GENETICS 77 ing this much, there are still important matters to be dealt with in the association of these units with the process of evolution. Genes have never been seen, but since their existence is established beyond reasonable doubt, we may judge them by the things which we know of the chromosomes containing them. Their functions in heredity are recognized; they have even been regarded as independent of external conditions in their hereditary capacities. Neverthe- less the succession of generations is always ac- complished by com.plete cells, not by nuclear units or nuclei alone; chromosomes have never yet been shown capable of action independent of the surrounding nuclear and cytoplasmic materials. For this reason the part played by the cytoplasm, as well as the known functions of the chromosomes, is a matter for serious consideration. Opinions of its importance vary widely. Some scientists have concluded that it is responsible for a transmission of characters no less definite and orderly than that carried on by the chromosomes, and have sought in cytoplasmic inclusions the material basis for this transmission. Others have denied that any orderly inheritance proceeds through the cyto- plasm. From the point of view of this study it is necessary to seek the relationship existing between it and the nucleus, rather than to defend any ex- isting view. 78 THE PROBLEMS OF EVOLUTION The relationship may differ somewhat in the germ cells and in the specialized cells of the body. Years ago, before cytological studies disclosed the true facts, it was supposed that the hereditary units of the germ cells were distributed differen- tially to the developing cells of the body, so that a specialized cell not only had characteristic differ- entiations of cytoplasm but also a corresponding nuclear condition. Boveri ^ showed in his studies of Ascaris that the chromatin is diminished in quantity in the cells of the body and that the re- mainder is broken up into small particles, but even in this case the differentiation is not comparable to that of the cytoplasm. In by far the greater number of known cases the full chromosome com- plex of the species seems to persist in all cells of the body which retain nuclei capable of mitotic division. Even the marked contrast between the germinal cytoplasm and that of the most highly specialized cells is not accompanied by comparable nuclear differentiation. The differentiation of the cytoplasm thus becomes one of the great problems of development, and the relations of nucleus and cytoplasm in reproductive cells are part of the general problem of interaction of cellular constit- uents. The nucleus is commonly accepted as a control- * Die Enticicklung von Ascaris megalocephala, etc., Festschr. f. C. von Kupffer. 1899. GENETICS 79 ling center of the cell, and since everything con- tained in it but the chromosomes is repeatedly discharged into the cytoplasm and reformed in daughter nuclei during cell division we may apply the same interpretation to the chromosomes. The additional fact that these bodies alone are univer- sally maintained and perpetuated in a character- istic state throughout the vicissitudes of normal development and reproduction is further evidence of their importance, and the many discoveries of genetics which correlate the development of char- acters with chromosome behavior are perhaps the most convincing tribute of all to the type of control which they exercise. Nevertheless even chromosomes are dependent. The nucleus lies within a cytoplasmic envelope which is necessary to its existence. It may be removed by micro-dissection but cannot live in- dependently so far as is known. Conklin ^ has suc- ceeded by centrifuging eggs in producing daughter cells of which one had a small nucleus, abundant yolk, and a minute amount of cytoplasm. The cytoplasm later increased in quantity; if none had been present, it would be evident that the nucleus alone might produce it, but this condition has not yet been attained. Moreover cytoplasmic ma- terials may pass into nuclei, so that the formation of cytoplasm by a nucleus would not be evidence « Jn. Exy. ZooU Vol. XII, No. 1, 1912. 80 THE PROBLEMS OF EVOLUTION that the chromosomes actually produce the sub- stance. A few cells, notably the spermatozoa, have the cytoplasm greatly reduced, but in every case, natural or experimental, that has been noted, the nucleus is associated with at least a small amount of cytoplasm. Neither does the cytoplasm have the power to exist independently for more than a short time. The red blood corpuscles of mammals lose their nuclei in the course of development, but they are short-lived and are constantly being destroyed and replaced. Protozoa may be deprived of their nuclei artificially and may continue to live for a time as masses of cytoplasm, but they are incapable of normal anabolism. Their activity seems to be due to the residuum of potential energy persisting from their nucleate stage. It is evident that the present state of our knowl- edge permits only the conclusion that nucleus and cytoplasm are coexistent. We do not know the nature of their interaction, but a significant fund of information enables us to establish satisfactory working hypotheses concerning the essential de- tails. In the latest summary of this question known to me, Conklin mentions the uncertainty which pre- vails, but in quoting Morgan's statement: "While, then, we may not be warranted in speaking of the genes as enzymes, the genes may be protein bodies, GENETICS 81 one of whose activities is to produce enzymes which, being set free, act in each cell, and take part in catalytic reaction in the cytoplasm," ^ he further states: "This conclusion seems to coincide in broad outlines, though not in all details, with the views that have been generally held since the hypothesis of De Vries." ^ Indeed, the far-reaching importance of enzymes in the living body makes it very easy to accept such a view, even though de- tails are lacking. The contributions of Troland,^ and lately of Alexander and Bridges ^ carry out some interesting considerations of these matters, and Goldschmidt's work,^ also mentioned at some length in Conklin's paper cited above, is perhaps the most detailed of all published theories. These several works are interesting and suggestive; even though they attempt such detailed treatment that they are unavoidably speculative, the reiteration of interpretations based upon enzymatic action is an indication of its fundamental importance. The perpetuation of the gene is highly suggestive of autocatalysis ; the control exercised by the nu- cleus over cytoplasmic action is equally suggestive of heterocatalytic powers. Whether Goldschmidt's statement: "... die Natur ^m^ <^^<^^ [uj! LIBRARY 1^1 VIII OUR FUTURE COURSE The views expressed in the preceding pages appeal to me as nothing more than a sound inter- pretation and appHcation of the principles of good biology and common sense to the problems of evolution. It sometimes seems that no position is more difficult to establish, for the careful applica- tion of facts demands the discard of all dogma and controversy of the past in its attempt to reduce all factors dispassionately to their proper places in the greater whole. We have had little but con- troversy in evolution for many decades, yet it has not been especially productive; the same contro- versial treatment is still possible. Controversy is much too likely to breed opinion instead of prog- ress, and progress, at present, is greatly to be desired. The outstanding feature of this interpretation of evolutionary processes is the fact that it is neither Darwinian nor Lamarckian nor mutational. I have attempted to show in my first few chapters how utterly impossible it is to separate the factors of environment and heritage in living things, and how illogical it is to debate the relative importance 201 202 THE PROBLEMS OF EVOLUTION of essentials. Nothing of significance in the organ- ism can fail to have its foundations in the heritage; no character or function can fail to be related to the other parts and functions of the body; and nothing in the individual can be without an asso- ciation, direct or indirect but vital none the less, with the factors of external environment. The organism, whether we speak of individuals or of species, is a resultant coordinative mechanism which arises through the harmonious interaction of a self-perpetuating physico-chemical complex, its heritage, and the intricate complex of conditions under which it develops. This essentially epige- netic view is not uncommon; it is ably developed by Child ^ and is accepted by most modern biol- ogists. It is true that the recognition of these facts justifies the opinion held by many evolutionists that the environment is merely directive, but the emphasis that has been given to their point of view seems wholly unwarranted. Even though an acquired character is the product of a basic herit- age and of many factors in the body, brought to expression by an external stimulus, the fact re- mains that the external environmental factor is essential to its production, and that the hereditary basis is due in part to other environmental condi- tions of past life. These contributions from the ^ Child, C. M., Physiological Foundations of Behavior, Ch. Ill, 1924. OUR FUTURE COURSE 203 environment include both substances and stimuli. A true appreciation of these facts demands the complete discard of the older views. We must forget that we have had three-quarters of a century of bickering over the environment and the heritage as alternative causes of evolution and begin anew with a recognition of their essential and coordinated functions, for in no single case have we observed a significant organic change which could be ascribed to either alone. The modification of the heritage, while it may be brought about through the action of penetrat- ing irresistible forces of chemical or physical nature on the germinal chromosomes, as by alcohol or X-rays, may also be sought in the variable environ- mental conditions which give it the opportunity for variable expression of its inherent properties. It is generally recognized that the inherent powers of the specialized tissues which make up the body are governed by the genes which associate these tissues with the preceding generation. If the indi- vidual receives one type of genes it may deposit pigment normally, if another it may freckle, and if a third it may be an albino, but whichever may be its heritage, it was represented in the chromo- somes of the germ cells with which development started. Some of the inherent powers of the chromosomes are realized, as development proceeds, in spite of 204 THE PROBLEMS OF EVOLUTION any slight fluctuations of the environment. Others are expressed only under special conditions. The latter result in the acquired characters which have evolutionary significance. They include the strik- ing responses to mechanical stimuli, light rays, unusual foods, and the various other conditions which have been previously listed, and they may be either adaptive or non-adaptive, according to the part which they play in the life of the individ- ual. According to one possible interpretation of these characters, a given hereditary power is either used or not used during individual life; according to another, a given gene responds positively in various ways to various stimuli. It is satisfactory to use the expression use and disuse in this connec- tion. This interpretation of gene action may be applied equally well to the variable development of heritable characters in connection with fluctu- ating conditions in the internal environment. Elsewhere I have considered in detail the several kinds of acquired characters and have shown that all of them may be explained in this one way.^ Whether the condition is an acquired immunity, a degree of muscular development, or pigmentation of the skin, it is the result of use or disuse of some inherent functional capacity. This is the basis for acquired characters generally as far as they concern us. 2 Am. Nat., Vol. LXI, p. 251, 1927. OUR FUTURE COURSE 205 If the human epidermis is subjected to the rays of the sun it becomes pigmented. If the stimulus is continued the pigment increases, and finally the maximum power of the body to form pigment is expressed or an effective protection against the inciting stimulus is attained. The action is prob- ably cumulative. In the case of tolerance for nar- cotics the cumulative nature of the action is even more evident, for tolerance may be built up by gradually increasing the dosage, to a point which is not found in untreated individuals, and a return to normal must be accomplished in the same way, by gradually decreasing the dosage. According to Wells ^ tolerance for morphine and alcohol must be, in part at least, due to ''a certain refractoriness or cellular immunity," in addition to any possible power to destroy the toxic substance. Whatever may be the mechanism of this tolerance, which is not definitely understood, it indicates a cumula- tive acquisition of a quality present to a much smaller degree in the initial state of the individual. That the response of the heritage is invariably cumulative in individual response to environment cannot be said with certainty. Information on this subject is inadequate. Nevertheless the available facts point to this probability. While with-holding dogmatic assertion on the question, we may work with confidence on the tentative conclusion that 3 The Chemical Aspects of Immunity, Ch. XII, 1925. 206 THE PROBLEMS OF EVOLUTION the living substance in any form is able to respond to conditions demanding the exercise of a given function by the gradual increase of its capacity for that function, and if the type of activity excited by the environmental stimulus results in a visible development, that too may be expected to increase. Conversely, no part of the organism and none of its functions may be expected to develop beyond the degree favored by the conditions of its exist- ence. The possible limits of the cumulative process can only be estimated. When an organ is absolutely unused its reduction may proceed in the individual to a state of atrophy, but the body maintains it in some degree of development. When excessive activity is demanded of an organ, the individual suffers; its capacity for response is apparently not unlimited, but it may be extended to an unsus- pected degree by the cumulative effects of response to a long-continued or gradually increasing stimu- lus. Exact information on this problem is greatly to be desired. At present we can only rest on this significant but incomplete evidence. Granting that cumulative change is possible, even in the individual, we have found a possible source of chromosomal change. Acquired charac- ters are cytoplasmic in their expression, but so also are inherited characters. Cytoplasm is the material of construction in which all characters are wrought. But we have seen that cytoplasm OUR FUTURE COURSE 207 alone is incapable of continued constructive ac- tion; everything that it accomplishes is ultimately referable to the controlling action of the chromo- somes. Two interpretations of the activity of the chromosomes in the development of acquired char- acters are possible: We may conclude that they always exert the same influence and that the cyto- plasmic result is cumulative, or that the chromo- somes also undergo a cumulative change which is responsible for the condition of the cytoplasm, that the ultimate expression of the acquired char- acter is the result of an actual increase, through use, of the functional capacity of the genes involved in its production. It must be admitted that these interpretations are speculative, but the probability of the latter is attractive. There is nothing to show that increased muscular power is purely a result of quantitative modification of cytoplasm or that tolerance for morphine is due to the accumulation in the cells of the body of some product which aids in protecting it against the drug. Again exact information is to be desired, and in its absence we must fall back on a logical interpretation of the available facts. It is commonly known that the material of which the body is composed undergoes frequent renewal under the control of nuclear activity.^ This fact seems much more compatible with the idea of gradual modification of the func- 4 McClung, C. E., Cowdry's General Cytology, pp. 665-667, 1924. 208 THE PROBLEMS OF EVOLUTION tional capacity of the chromosomes than with the idea that fixed chromosomal fmictions may pro- duce a cumulative effect in the cytoplasm. If increased pigmentation is due to cumulative de- posit of pigment, the constant renewal of epider- mal cells would seem inimical to the process; if it is due to an increased capacity for the formation of melanin by the melanoblasts, the newly formed epidermal cells would find an increasing supply of melanin as the organism responds to light. The latter alternative is in harmony with the known facts and indicates an increase in the functional capacity of the melanoblasts, which would involve the nucleus as well as the cytoplasm. This idea is the theory of use and disuse in the chromosomes which I have previously expressed. In brief it is no more than the extension of the principle of use and disuse, which has been an accepted source of modification of living substance in macroscopic units, to the minute bodies which have been found so important in heredity. Its value is limited, like the value of all other theories of evolutionary method, but it has the unique possibility of explaining a chromosomal modifica- tion as a result of organic activity on the basis of a recognized peculiarity of living substance. It suggests a means whereby the existing latitude of organic response may shift in harmony with the demands made upon the organism by environ- OUR FUTURE COURSE 209 mental conditions, whether or not the results are adaptive. The chief limitation of this theory is the old difficulty of associating the responses of somatic structures with the germ cells. Since the struc- tures involved in the development of acquired characters are somatic, the chromosomes directly associated with the response are necessarily so- matic. The possibility of a change in the functional capacity of the chromosomes as an organism re- sponds to environmental stimuli is a distinctly valuable factor in evolutionary theory, but it does not solve the problem of associating these changes with the chromosomes which are handed down to succeeding generations. Weismann's interpretation of the continuity of the germ plasm has had a most unfortunate influ- ence on scientific consideration of this problem. The continuity which he emphasized exists; in- deed, it is so logical and in some cases has been so clearly demonstrated that it stands out as one of the salient facts of heredity. But the fact of ger- minal continuity is a wholly different thing from the isolation and insulation of germ plasm which Weis- mann proposed. We may even admit the validity for certain purposes of his distinction between germ plasm and somatoplasm without attaining the position which he defended. Although we are still ignorant of any mechanism whereby the influences 210 THE PROBLEMS OF EVOLUTION of the soma are impressed upon the germinal tis- sues, the converse is equally true and the known interaction of organic structures through the me- dium of hormones is significantly indicative of possible relationships here. I have previously pointed out that continuity is incidental to the reproductive function and that the continuity of the germ plasm is conspicuous only because this is the normal reproductive substance. Continuity of somatic tissues is well known in plants and lower animals, and discontinuity of germ plasm is on record in a sufficient number of cases to destroy effectively the Weismannian hypothesis. The most significant facts available in the pres- ent unsatisfactory state of scientific knowledge of evolutionary processes are those brought out in the preceding chapters. The organism exists by virtue of an association between heritage and environ- ment. The cytoplasm exists because of the action of a characteristic chromosome complex upon cyto- plasm derived from the preceding generation under conditions determined by all phases of the com- plex environment. The germinal tissues, even though they are safeguarded to the ultimate ability of the individual organism from external influ- ences, are, nevertheless, produced by the chromo- some complex descended from the original repro- ductive unit under conditions which are dependent upon the life of the individual. And since the OUR FUTURE COURSE 211 chromosomes must grow and reproduce like all living things, we cannot avoid the possibility that, in spite of their remoteness, a residuum of influ- ence may reach them from the surrounding body. Successive generations of them are, in their turn, products of their predecessors' reaction to their environment. The nature of this influence may be obscure, but it presents one logical possibility. We know that organic activity or inactivity results in a corre- sponding increase or decrease of functional capacity and, without doubt, in a corresponding physico- chemical development. If we extend this principle to the chromosomes or to the genes, as may logi- cally be done, the exercise of a given function establishes a condition in the body which favors a coordinated development of the genes involved. There is no known reason why this condition should be limited to the cells involved in the expression of the character, for the individual maintains its en- tire chromosome complex normally in all cells of the body, without regard to their specialization and their individual cytoplasmic functions. A chromosome exists in a given state partly because a given condition in the body makes possible the development and perpetuation of chromosomes in that particular state. Since the same kind of chromosome appears in many cells, irrespective of their relation to the character whose somatic 212 THE PROBLEMS OF EVOLUTION expression it governs, it is obvious that the condi- tion governing its development and maintenance is available throughout the body. The significant aspect of this hypothesis is that the result is defi- nitely associated with some condition of individual life. Whatever change may occur is only a reduc- tion or enhancement of something already present, but it takes place according to the demands of the environment. In this the hypothesis is more satisfying and more in harmony with the known facts of evolution than the equally hypothetical idea that evolution has proceeded entirely from the random influences of rays upon the structure of genes. It offers equal possibility that a degree of development, once attained, may find a new importance to the individual under new environ- mental contacts without depending entirely upon this preadaptive value for its eflScacy. Neither idea is, at present, susceptible to proof. This idea of the development of the chromosomes harmonizes with the known facts of embryology and with the emergent interpretation of evolution. We cannot avoid the conviction that the chromo- somes are always the controlling center of the cell, yet the same chromosome complex accomplishes a succession of varying results during the course of ontogeny. Each generation of chromosomes arises from a previous generation which is apparently similar, yet each brings about new stages of growth OUR FUTURE COURSE 213 and differentiation. The potentialities of the whole organism must be present in the fertilized ovum, yet they are realized only through gradual de- velopment, with a gradual change of internal environmental relations. Given a normal oppor- tunity, they express their inherent qualities nor- mally in the adult, but the very nature of their attainment of this degree of expression suggests the possibility of some fluctuation in their later functional capacities, of later emergents. If change through use and disuse may thus affect the chromosomes of the germ cells, the con- clusion does not follow that the next generation will show the same character as its parents. It must still encounter the stimulus which brought out that character previously. If the stimulus is present, or better still, if it is intensified, there is a possibility that the condition of the genes may reach another degree of development, and so, gen- eration after generation, lead to a state which was beyond the immediate possibilities of their pro- genitors. But if the stimulus is lacking, the pos- sibility still exists that the heritage may respond as in previous generations. During the entire period of development of a character in this manner it must maintain a place in the organism as a whole. If the organism exists in a given condition, then the maintenance of each part must conform to that condition, and if one 214 THE PROBLEMS OF EVOLUTION part undergoes change, adjustments must be made elsewhere in the body. It is conceivable that through gradual development a character might ultimately attain a condition of sufficient impor- tance, or a condition sufficiently different from that in which it existed at an earlier stage, that the body, in spite of the cessation of the original stimulus, could not return the character suddenly to its original condition, but must gradually work out an adjustment according to the importance of the internal relationships involved. It is not necessary to limit our analysis of ac- quired characters to this one possible method of perpetuation. While it is conceivable that an organ may attain a degree of importance which would guarantee its persistence beyond its period of active usefulness and make its loss impossible save through the gradual reduction which we ascribe to our own vestigial structures, its develop- ment may also give rise to new environmental associations. Any association with an additional environmental condition would bring new stimuli to bear on its further development, and would also open the possibility of perpetuation of the charac- ter through selection. The latter point is the essen- tial feature of Cuenot's idea of preadaptation, although he does not extend his theory to the origin of the character in the beginning. Any such shift in environmental relations must certainly add OUR FUTURE COURSE 215 to the possibility of continued evolution of a char- acter and to the complexity of interactions involved in its production and maintenance, and the more complex these interactions, the less chance there is that the loss of any single factor may completely eliminate the character. This idea may seem to involve a contradiction in that I have spoken of the character as the result of a certain heritage responding to a certain en- vironmental stimulus. It may be quite true that the character would never have been expressed without the intervention of the external stimulus, but in this as in the development of every organic structure, relationships in the internal environ- ment are involved. No character is a simple result of one or a few genes responding to an environ- mental stimulus of slight complexity; instead the somatic expression of any heritage involves many factors, activated sometimes by external stimuli but always including complex interactions among themselves as components of the internal environ- ment. The importance of new conditions and changing relations in the body can scarcely be overestimated as an evolutionary force, and examples of their action are common among living things. For instance, the fishes have pharyngeal pouches which are of the utmost importance in the development of their respiratory system, and man retains these 216 THE PROBLEMS OF EVOLUTION pouches as a feature of his embryological develop- ment, although he has no such need for them. Dur- ing the ages of his past evolution each step was accomplished in association with these pouches and their derivatives, hence they persist in the development of structures which are now essen- tial. It is unnecessary to write at length of the importance of the endocrine glands developed in this part of the body. They may originally have been incidental, but they existed and they are now indispensable, along with the pharyngeal pouches from which they develop. These points are strictly in harmony with the facts of evolution as disclosed by palaeontology in long phylogenetic series, and with the limited experiments which have demonstrated an appar- ent inheritance of acquired characters. In the former case we find gradual change under a grad- ually changing environment to be a common condi- tion. In the latter we find the gradual develop- ment of a character and its gradual disappearance in succeeding generations convincingly demon- strated, especially in Woltereck's daphnids. Gradual appearance and gradual disappearance, however, introduce the problem of reversibility. If the changes under discussion are evolutionary, one may well ask how they harmonize with the generally accepted principle of irreversibility. It seems, first of all, that this principle is based upon OUR FUTURE COURSE 217 a consideration of major steps in evolution, upon finished characters, rather than upon the process by which they are formed. In other words, we judge the results of evolution as they stand before us, and not the possible steps which preceded their consummation. It is obvious that a hand may not again become a fin, and that a flipper may not be- come a foot, but quite true that an organism which has legs may be the descendant of legless ancestors and the progenitor of legless descendants. Each structure is a positive phenomenon, the product of a given heritage, and because of its positive characteristics it is obviously impossible for it to present the conditions of heritage necessary for the production of a preexisting stage of its develop- ment. As a result it cannot be the cause of its own cause nor of something exactly like it, but within its own latitude of development there is no funda- mental obstacle to its fluctuation. As an example the probable history of the penta- dactyl appendage is excellent. According to the accepted view of the evolution of the terrestrial vertebrates, the initial steps in the development of these appendages occurred in the Crossopterygii of the geological past. The skeletal elements were present in these fishes, which apparently used their pectoral fins as supporting structures when restmg on the bottom. Here are environmental factors common to terrestrial and aquatic habitats: the 218 THE PROBLEMS OF EVOLUTION force of gravity and a solid substratum. But when the ancestral fishes emerged from the water, the stimulus of a dense fluid medium was lost. Pre- sumably the fringe of the fins disappeared, but we have no reason to suppose that the power to pro- duce a complete fin was lost in these early ter- restrial fishes. They were, however, subject to new stimuli on land, stimuli which favored the realization of latent possibilities of their fins which could never have been brought out by an aquatic environment, and the pentadactyl appendage is the result. In most terrestrial vertebrates its im- portance is so great that reversion is unthinkable; it is a positive adjustment to positive environ- mental conditions, many of which could have had no part in its production, and even though subject to the same aquatic environment as the ancestral fin, it presents a different heritage and so gives a different result in the course of evolution, as in the flippers of the seals. But it can be reduced, like the fore limbs of the kangaroos, or lost completely as in the snakes. It seems entirely logical to expect reversibility in the response of a heritage to an environmental stimulus during the initial stages of stimulation, but when the heritage has been subjected to the stimulus for many generations, or to a gradually increasing stimulus, until a cumulative response has been brought to an extreme expression, the OUR FUTURE COURSE 219 condition may well be different. Add to this the possible successive incidence of different stimuli and we have the assemblage of environmental conditions which may be operative in nature. If a heritage can be brought to an expression whose latitude is beyond that of its previous state, it may still be reversible through an equally slow process, but it may have a different and potent effect within the organism while maintained in its new condition, and either it or some results of its new development may respond to an environmen- tal stimulus toward which it was previously inert. The latter condition would make probable the persistence of the character through its relation to the second stimulus even after the cessation of the first. Such an assumption is, of course, highly speculative, but the conditions involved are un- avoidable in the circumstances of life, and when we take into consideration the internal environ- mental factors, it is of great significance. Among the many attempts to demonstrate the inheritance of acquired characters in the past, these fundamental considerations have in no case been met. It is not diflScult to find organisms, both simple and complex, which respond to environ- mental conditions through demonstrable func- tional adjustment or through visible structural change, nature of pigmentation, or habits. Leav- ing out mutilation and mechanical malformation. 220 THE PROBLEMS OF EVOLUTION experiments have dealt with organic responses of these kinds. In every case some hereditary capacity has been involved, and the degree of its expression has been controlled by artificially controlling the environment. The procedure followed has in- volved the subjection of the organisms to a given stimulus through several generations in all of the experiments known to me, and the return of the stock, finally, to the normal ancestral environ- ment. In no case have I found a record of an attempt to determine the effect of a gradual in- crease of a stimulus over a succession of genera- tions, and in no case has the experiment dealt with the usefulness of a character under other condi- tions than those which determined its production. The various conditions suggested here for the possible modification of living things through indi- vidual response to environment present very differ- ent possibilities for experimental verification. The time element is always important in such studies, but probably to the greatest degree in those experi- ments which involve the modification of a simple character in response to a simple environmental condition. Such a response may be adaptive or it may be incidental, but in any case we can scarcely expect it to carry the development of a character quickly beyond its former latitude of response, if at all. It is conceivable that an increasing or de- creasing^ stimulus may ultimately accomplish this OUR FUTURE COURSE 221 result, but probably at such a slow rate that it may not be experimentally demonstrable except in very favorable cases. It is much more probable that an induced change, if it can be brought into relationship with other factors of the environment than those which caused it to appear, may be fixed in the species through experiments of suflScient duration to afford convincing evidence of its im- portance in evolution. The relation of such char- acters to selective processes is a more delicate matter, since any selection makes it necessary to distinguish carefully between the heritage and the somatic characters as the basis for selection. The relationship is a valid field of inquiry in the general process of evolution, but a dangerous one for the study of acquired characters. The limited duration of experiments in this field has been a necessity which will probably continue to be felt. It has been possible to follow organisms of short life through many generations, and for this reason they are desirable material, but un- fortunately experimental material must be sought among species which show some capacity for varied response to environment, whether or not their other characteristics are favorable. Duration is certainly an important factor in evolution, but it may not be an unavoidable obstacle. Certainly we cannot expect to work important changes in a species in a few generations when individuals may 222 THE PROBLEMS OF EVOLUTION be called upon in the natural state to adjust them- selves many times to fluctuations of the environ- ment, so we must avoid this condition as far as possible. The behavior of the oflFspring of animals kept for several generations under unusual conditions has varied in the work of different investigators. It is unfortunate that so many of these experi- ments have been conducted without adequate checks, as, for example, Schroder's studies of the habits of the willow moth,^ for while any room for objection is left by the experimenter he may be certain that it will be forthcoming. Nevertheless, the available experimental data strongly support the points suggested above. The permanence of characters is in proportion to the length of treat- ment in some cases, and a return to normal within one or two generations of the renewal of normal environment is a common result. Apparent per- sistence of a character is reported by Guyer and Smith in their work with cytolysins,^ but, without denying the positive value of these results, I feel that normal responses to artificially modified ex- ternal environments are the most desirable mate- rials for this work. The theoretical possibilities of individual re- sponses have not been adequately tested by experi- 5 Verh. d. Zool. Ges., Vol. XIII, p. 158, 1903. 6 Jo. Exy. Zool, Vol. XXXI, p. 171, 1920. OUR FUTURE COURSE 223 merit. In view of the fact that they have as great logical appeal as the mutations which have de- manded so much of our attention, and even more harmony with the apparent course of evolu- tion, they seem worthy of more attention and more careful and thorough experimentation than have hitherto been accorded them, but we cannot expect greater success in our experiments than has re- sulted in the past unless they are undertaken with a clear idea of the probable part of individual response in evolution. No one factor in so complex a process can be responsible for all of the results, and until we know all factors and recognize their interaction we must remain ignorant of the total process. Any experiment which embodies the essential factors of individual response to environment as stated above will give useful results, but for the thorough investigation of the problem we must first determine the nature of individual response through a reasonably long succession of genera- tions. Response is cumulative in the individual, but is there any possibility of cumulative response through several generations? Guyer and Smith have given us the nearest approach to a sound investigation of this question in their studies of typhoid agglutinins in rabbits.^ "The young of immunized mothers can, without further immuni- ' Jn. Inf. Diseases, Vol. XXX, (6), p. 498, 1923. 224 THE PROBLEMS OF EVOLUTION zation, transmit agglutinating ability to their own offspring," and successive generations are able to build up increasing degrees of immunity. But the possibility that these results are due to "a placen- tal rather than a truly hereditary transmission" is a serious difficulty in a study of evolution. I feel that the experiments have evolutionary signifi- cance in either case, for placental transmission is a normal factor in mammalian life, but for the establishment of individual responses as a factor in evolution generally, the possibility of prenatal influence must be eliminated. Mammals are there- fore barred as experimental subjects, and it is very desirable to confine experiments to species in which no association between parents and offspring exists beyond the production of the fertilized ovum or other reproductive unit. It is further necessary that the individual re- sponse to a fixed stimulus over an extended period of time, and to a gradually increasing stimulus, be determined. In succeeding generations it is desir- able to determine how the young of treated parents respond to a continuation of the initial stimulus and to an increase of it, with adequate provision against the possibility of natural selection. Are the young able to adjust themselves successfully to a more extreme condition than their parents could meet.^ If so, the influence of individual re- sponse upon the reproductive cells is definitely OUR FUTURE COURSE 225 established, and with scarcely less certainty we may say that the germinal chromosomes are affected. There is, however, no more reason to suppose that the condition will persist after a few generations if the inciting stimulus is removed than to conclude a ^priori that such a cumulative change may not occur. If cumulative change does not take place, then studies of individual response are not likely to have value in evolution, but the available experimental evidence does not point to this end. Given a cumulative response of considerable degree, it is desirable next to build it up to a maxi- mum. It is conceivable that the degree of develop- ment attained may become a factor of such im- portance in the internal environment that internal coordination may, after a time, guarantee its per- petuation. The factor of time appears to be important here, and may be an insurmountable obstacle for experimentation if we can judge by the long geological periods which have been oc- cupied by the evolution of known species. There is, however, no reason to rulq this possibility out; directly adaptive responses must be inherited in this way, if at all. Such a result would be the inheritance of acquired characters in a very simple form. It is more probable that the factor of usefulness in relation to other than the inciting stimuli may 226 THE PROBLEMS OF EVOLUTION play a part in successful experimentation. For this reason a second stage of investigation should be undertaken to relate the artificially developed character with some condition of the external environment as an adaptive character. It is more than likely that the attainment of this end will be diflBcult, but it is essential to a thorough study of the problem. With a character developed artificially by modi- fication of the environment through several genera- tions, and an adaptive association established be- tween it and some other environmental condition, the possibility arises of new development through use, in the ordinary sense of the word, of the pre- viously indifferent character. The essential condi- tion to be investigated, however, is the potency of the adaptive value of the character to maintain it even in the absence of the environmental condi- tion under which it originally developed. Mainte- nance of the adaptive association in an artificial environment and the abatement, at the same time, of the original stimulus, would determine this point. These are the conditions necessary for an accept- able and final investigation of the inheritance of acquired characters. Whether they can be com- pletely attained remains to be determined. It is certain that the attainment will be difficult, for the available organisms have had many years in which to adjust themselves to the natural environ- OUR FUTURE COURSE 227 ment which we know and to gain the degree of independence which is characteristic of their kind. The individual responses which are commonly known are of slight degree. Even if developed to a maximum it is possible that they will still lack adaptive value under any available conditions. But stranger things than this have been accom- plished and we may yet see all of these conditions realized. It is certain that the previous method of subjecting organisms to a stimulus for a few gen- erations and then removing them to the original environment is hopelessly inadequate for the in- vestigation of such a complex subject as the process of evolution. Succeed or fail, it is better by far to follow a method which promises success than one which is obviously inadequate. Finally I would note again that this proposal does not deny in the slightest degree the efficacy of other factors in evolution. The inheritance of acquired characters is emphasized because it de- mands investigation more than any other subject, but it is broader than the term usually implies. Natural selection, isolation, and other processes of less importance, are also potential sources of change in species, once we have a variable heritage on which they may work. Our task is largely to de- termine the source of variation in an originally homogeneous heritage, and the response of the individual to its environment is too ligtiificant 228 THE PROBLEMS OF EVOLUTION to be denied investigation. Mutation certainly does not explain; it is a recognition of a type of variation, rather than of the underlying causes, and until we have determined what causes heritable variations we have not exhausted the available field of investigation. INDEX Abnormal abdomen, 94 Acquired characters, 4, 149, 164, 204, 219 Adaptation, 14, 59; process and result, 60; adjustment of, 62; process of, 62; and natural selection, 114; and Lamarck- ian theory, 135 Adjustment, need for, 197 Agar, W. E., 157 Agglutinins, 223 Albino, 64, 203 Alcohol, 203, 205 Alcoholism, 146 Alexander, J., 81, 97 Alimentary tract, 90 Altitude, 19; effect on blood, 19; effect on environment, 49 Amblystoma pundatum, 38 Animals, and organic environ- ment, 20; responses of, 186 Ant-eater, 134 Ants, 69 Appendage, pentadactyl, 217 Ascaris, 78 Autocatalysis, 13, 14, 81 Autonomy, 48, 178 Babcock, E. B., 58 Bacteria, 28 Baldwin, J. M., 116 Barnes, W., 42 Bateson, W., 26, 53 3eebe, W., 69 Bees, bumble-, 187; honey-, 32, 187 Belief, in science, 109 Bessey, C. E., 26 Boas, J. E. v., 168 BoUey, H. L., 158 Bones, 168; development of , 166; sesamoid, 168 Borodin, N. A., 161 Boveri, T., 78 Brain, 184 Bridges, C. B., 81 Butterflies, 61, 95, 119, 180 Camels, 165 Camponotus macilentuSt 70 Carbon dioxide, 36 Catalysis, 12 Caterpillars, 49 Cave animals, 59 Cell, organization of, 28 Centrifuging, 79 Centro-epigenesis, 139 Cephalochordata, 166 Change, in genes, 96; founda- tions of, 147; cumulative, 174, 205, 207 Characters, acquired, 63, 98, 158, 204, 219; addition of, 74; complexity of, 215; degree of development, 73; hereditary, 67; indifferent, and selection, 117, 118; modification of, 74; new, 174; non-adaptive, 61; origin of, 61; stability of, 154 229 230 INDEX Child, C. M., 181, 202 Chromatin, 86; of Ascaris, 78 Chromosomes, 89, 206; as con- trolling center, 79; depend- ence of, 79, 91; and heredity, 77; use and disuse in, 208, 211 Circulus, 31 Climate, and altitude, 49; and rainfall, 50; and geological change, 51 Clupeay 161 Cockerell, T. D. A., 163 Coincident selection, 116 Colonies, 32 Conger, G. P., 1 Conklin, E. G., 5, 56, 62, 79, 80, 82, 85, 97 Constancy of organism, 92 Continuity, of germ plasm, 92, 104, 209; of somatic tissues, 210 Controversy, 201 Cook, O. F., 34 Cope, E. D., 138 Corpuscles, red, 80, 99 Cowdry, E. V., 84 Coyote, 151 Cretinism, 37 Crossopterygii, 134, 166, 180, 217 Cuenot, L., 125, 130, 133, 214 Cunningham, J. T., 62, 91, 97, 141, 149 Cytolysins, 155 Cytoplasm, 87, 206; compared with soma, 87; constancy of, 89; dependence of, 84; differ- entiation in eggs, 85; in hered- ity, 77, 84; necessity of, 82; and nucleus, 78 Daphnia, QQ, 155 Darwin, Charles, 23, 53, 70, 113, 114, 124, 136, 137, 139, 199 Darwinian school, 113 Darwinism, compared with La- marckism, 138 Deer-mice, 153 De Mol, W. E., 74 Dendy, A., 35 Depression and climate, 51 Devaux, E., 120, 175 Development, degrees of, 73; of muscles, 207 DeVries, H., 81, 127, 128, 132 Dispersal, 68, 121, 160; and ad- aptation, 50 Dixey, F. A., 95 Drosophila, 58, 73, 74, 92, 94, 101, 128, 144, 148 Duration of experiments, 221 Edelweiss, 36 Elan vital, 9 Elephants, 165 Elevation and climate, 51 Embryology, 85 Emergents, 169 Endocrine glands, 90, 216 Enucleation, 99 Environment, 5, 35, 87, 178, 181, 201, 203; effect of change, 72; complexity of, 17; control by man, 191; in evolution, 147, 151, 193; external, 58; inter- nal, 21, 57, 215, 225; and mu- tation, 94, 98; organic, 20; phases of, 18; physical, 18; of species, 41; source of life, 14 Enzymes, 81, 97 Epigenesis, 202 INDEX 231 Erythrocytes, 80, 99 Evening primrose, 127 Evolution, emergent, 150, 168, 181, 212; and past environ- ment, 165; not an inherent tendency, 191; scope of, 1; starting point of, 150 Experiment, duration of, 221; Lamarckian, 141; need for, 199 Eyes, 59 Factors, locaHzation, 102 Fell, H. B., 103 Fmches, 69 Fins, 217 Fishes, 215 Fittest, survival of, 71 Flavones, 36 Flax, 158 Flipper, 217 Foods, response to, 56 Foot, 217 Forces, 47 Friction, response to, 5Q Frog, change of color, 10; par- thenogenesis, 37 Functions, latitude of, 197 Galdpagos, 69, 122 Ganoids, lobe-finned, 61; see Crossopterygii Gaskell, Augusta, 8 Genes, 76, 77, 80, 104; action of, 204; duration of activity, 100; sphere of activity, 100; de- pendence of, 91; modification of, 96; perpetuation of, 81; role in acquired characters, 99; stability of, 95; use and disuse of, 105; and X-rays, 144 Genetics, 76, 194 Geological changes, 51 Germ cells, 91, 209 Germinal selection, 115 Germ plasm, 87, 97, 98, 210; continuity of, 92, 209 Gifford, E. W., 69 Glands, 194; endocrine, 90, 216 Goldschmidt, R., 81 Goodrich, E. T., 149 Gopher, 151 Gowen, J. W., 101 Green plants, 18 Gregory, J. W., 31 Guyer, M. F., 136, 155, 171, 222, 223 Hagedoorn, A. L. and A. C, 6 Hand, 217 Harrison, R. G., 38 Henderson, L. J., 36 Henneguy, L. F., 41 Herbert, S., 116 Heredity, Mendelian, 76; and mitochondria, 83; and cyto- plasm, 77, 84 Heritage, 5, 35, 87, 178, 181, 201; in acquired characters, 149; in evolution, 147; fluctuations of, 51; independence of, 37; modification of, 203; reas- sortment in, 52; in species, 41; tendency of, 194 Herrick, C. J., 183, 184 Herrings, 161 Hesperioidea, 61 Heterocatalysis, 81 Heteroploidy, 74 232 INDEX Kingston, R. W. G., 187 Hinny, 84 Hogben, L. T., 10 Honey-bee, 32, 187 Horses, 165 Hybrid interpretation of mu- tants, 128 Hypotheses, see Theories Immunity, 223 Individual, adaptive processes in, 135; and reproduction, 105; and species, 29 Individual response, 67, 98, 155, 172 Induction, parallel, 140 Inheritance, cytoplasmic, 87 Injury, results of, 56 Insects, mental processes of, 186 Instincts, of insects, 186; origin of, 189 Integument, 90 Intelligence, 196, 197; of in- sects, 188; of man, 188; and evolution, 189, 191, 193 Intraselection, 116 Iodine, 65, 181 Irreversibility, 216 Irritability, 184 Isolation, 69, 119, 160, 227; geographic, 120, 122; limita- tions of, 126; organic, 120; results of, 124 Johanssen, W., 127 Johnstone, J., 110, 111 Jordan, D. S., 152 Just, E. E., 84 Kangaroo, 218 Kepner, W. A., 185 Kidneys, 90 Kinetogenesis, 138 Lamarck, J. B., 23, 136, 199; laws of, 136, 137; work of, 138 Lamarckian school, 118 Lamarckian theory, 135; com- pared with Darwinism, 138; failure of, 164; modern atti- tude, 136; experiment, 141; insuflficiency of support, 141 Lawrence, W., 91 Lead, 48 Le Dantec, F., 173 Life, 7, 182; and external con- ditions, 49; origin of, 178 Light, response to, 56 Lillie, F. R., 84 Lindsey, A. W., 26, 42 Linneon, 129 Living substance, characteris- tics, 12; plasticity of, 175 Locomotion, 50 Loeb, J., 37 Lotsy, J. P., 129 Lungs, 61 MacDougal, D. T., 43, 66 Mammals, 224 Man, 192, 195; control of en- vironment, 191 ; intelligence of, 188 Marshall, G. A. K., 95 Mason, F., 31 Materials, 47 McClung, C. E., 207 Mechanism, 111 Meiosis, 73 Melanin, 208 Mendelian heredity, 76 INDEX 233 Metabolism, 14 Mice, 73; California deer-, 153; white, 60, 156 Mind, 67, 183; and thyroid, 181 Mitochondria, 83 Mneme theory, 139 Mocking birds, 69 Modifications, 5Q, 64 Molluscs, 61 Monkeys, Mona, 162 Montgomery, T. H., 26, 31 Moore, J. E. S., 161 Morgan, T. H., 38, 80, 83, 93 Morphine, 205, 207 Moth, willow, 222 Mule, 84 Muller, H. J., 58, 98, 143 Mutant, abnormal abdomen, 94; wingless, 73 Mutations, 6, 57, 127, 143, 163, 175, 223, 228; cause of, 132; and acquired characters, 145; cumulative, 130, 131; degree of, 128; definition, 148; and environment, 74, 146; expla- nation of, 132; importance and effects, 130; of eye, 101; gene, 74; kinds of, 170; re- verse, 128; and selection, 130; somatic, 101; usefulness of, 144 Mutation theory, 127, 164 Mutilations, 63 Nageli, Carl von, 131 Narcotics, tolerance for, 205 Natural selection, 6, 70, 112, 227; criticism of, 126 Neo vitalism, 110 Newman, H. H., 131 Novelty in organisms, 165 Nucleus, as controlling center, 78; and cytoplasm, 78; com- pared with germ plasm, 87; necessity of, 82 Oenothera^ 128 Opportunism, 179, 193 Organic selection, 116 Organism, 47, 202, 210; adjust- ment of, 196; capacity for change, 134; complexity of, 16; constancy of, 194; inde- pendence of environment, 16; limitations of, 206; multi- cellular, 28; product of herit- age and environment, 22; source of energy, 15; tendency to be constant, 92 Orthogenesis, 6, 131 Osbom, H. R, 9, 55, 116, 161, 194 Overproduction, 118 Oxygen, 36 Palaeontology, 164, 216 Pancreas, 10 Pangenesis, 139 Panmixia, 115 Papilio ajax, 72 Paramecium, 192 Parasitism, 20 Parental care, 178 Parthenogenesis, artificial, 37 Patterson, J. T., 101 Pawpaw butterfly, 72 Pentadactyl appendage, 166, 217 Peromyscus municulatusy 153 Petrunkevitch, A., 37, 98 Pharyngeal pouches, 215 234 INDEX Physico-chemical basis of life, 10, 11 Pieris octavia-seramus, 41 Pigeon, passenger, 29 Pigmentation, 40; in butterflies, 95, 180; in man, 94, 204 Pike, F. H., 90 Pituitary gland, 10 Plants, and organic environ- ment, 20; xerophytic, 134 Plastids, 83 Plebeius melissa, 119 Poisons, response to, 56 Polarity, 85 Polled Hereford, 73 Polyploidy, 74 Powers, J. H., 26 Prairie, 19 Preadaptation, 125, 133, 159, 214 Prescience, 185 Pressure, 19 Primates, 195 Primordial living substance, 13 Primrose, evening, 127 Promorphology, 85 Protophyta, 28 Protozoa, 28; enucleate, 80; re- sponses of, 185 Pure lines, 27, 127 Pyrgus centaur eae, 123 Pyrgus tessellatay 42 Rabaud, E., 5, 7 Rabbits, 151, 155, 223 Races, 42, 69; geographic, 152; of deer-mice, 153 Radiation, disintegrative, 48; in evolution, 146, 151; ter- restrial, 58 Rays, 89, 212; see X-rays Reassortment, 68 Recapitulation theory, 167 Recombination, 57 Relationship of nucleus and cy- toplasm, 77 Rensch, B., 152, 162 Response, 56; adaptive, 64, 204; of bumble-bees, 187; through generations, 223; of higher animals, 186; of honey-bees, 187; individual, 67, 155, 172; of Protozoa, 185 Resultants, 168, 169 Reversibility, 218 Rignano, E., 139 River, analogy with species, 30 Roux, W., 36, 116 Ruthven, A. G., 161 Salmo fario, 153 Salmo ferox, 153 Schroder, C, 222 Schwarz, E., 162 Scott, E. L., 90 Scrophularia leporella, 43 Scrotum, 91 Seal, 218 Secretin, 10 Selection, 159, 194; coincident, 116; germinal, 115; and isola- tion, 117; limitations of, 126; and mutations, 130; natural, 70, 113, 227; organic, 116; re- sults of, 118, 124; sexual, 70, 114 Self-determination, 182, 195 Semon, R. W., 139 Sense organs, 90 INDEX 235 Sewertzoff, A. N., 3 Sex reversal, 103 Sexual selection, 70, 114 Simocephalus vetulus, 157 Skinner, H., 42 Smith, E. A., 155, 222, 223 Smith, John B., 42 Snakes, garter, 161 Social organization, 20 Soma, 87, 90, 209 Specialization, 90 Speciation, 161 Species, 22, 26, 39; change in, 68; elementary, 27; and indi- vidual, 29, 135; instability of, 33; Lotsy's, 129; reality of, 29; and reproduction, 105; scope of, 44; uniformity of, 181 Spencer, W. P., 102 Spermatozoa, 80, 91 Spontaneous generation, 14 Stature, 67 Stimulus, 224 Subspecies, 69 Sumner, F. B., 60, Q6, 73, 148, 153, 156 Survival of the fittest, 71 Symbiosis, 20 Taenia solium, 37 Temperature, 36, 56, 89; effect on mice, 60; regulation, 21 Tetrakinetic theory, 55 Theories: centro-epigenesis, 139; coincident selection, 116; ger- minal selection, 115; intra- selection, 116; isolation, 119; kinetogenesis, 138; Lamarck- ian, 127; mneme, 139; muta- tion, 127; orthogenesis, 131; pangenesis, 139; panmixia, 115; parallel induction, 140; preadaptation, 125, 133; re- capitulation, 167; tetrakinetic, 55; use and disuse in chro- mosomes, 208 Thinking, 183 Thyroid, 37, 65, 181 Translocation, 170 Transplantation, 38 Tree hoppers, 61 Troland, L. T., 81, 97 Tropisms, 188, 196 Trout, 152 Tulips, 74 Typhoid, 223 Unit characters, 40 Uranium, 48 Use and disuse, 137; results of, 56 Usefulness, 70 Variation, cause of, 53; of en- vironment, 49; geographic, 42; of heritage, 49, 52; seasonal, 41 Varieties, 69; of C, macilentus, 70 Vertebrates, terrestrial, 218 Vitalists, 108 Vitamines, 65 Wagner, A., 110 Wagner, M., 119 Water, 36, 50 Weismann, A., 92, 103, 115, 125, 209 236 INDEX WeUs, H. G., 205 Wheeler, W. M., 69 Whitman, C. O., 199 WilHams, R. C, Jr., 42 Willow moth, 222 Woltereck, R., 66, 155, 216 Worms, parasitic, 37 X-rays, 48, 58, 74, 92, 101, 144, 203