Proceedings of the Washington Academy of Sciences
Vol. VIII, pp. 197‑403. February 13, 1907.

ASPECTS OF KINETIC EVOLUTION
By O. F. COOK

4. MODES OF EVOLUTIONARY MOTION.

The law of evolution which declares that organic nature has come into existence through a connected and gradual process, and not through millions of separate creations of species, now commands the practically universal adherence of biologists, and [287] has also been applied as a philosophical principle in the elucidation of many facts and problems outside the organic series. After being once adequately presented such an integration of knowledge could scarcely have failed to command respectful consideration, and its general acceptance has already become so much a matter of course that the word evolution is not uncommonly used in a much narrower sense and identified with one or the other of the theories which have been invented to explain the methods and immediate causes of the process of organic change, a subject upon which there is still no lack of differing opinions.

Although the doctrine of the independent creation of species has been set aside, it has proved much more difficult to eliminate, even from the minds of the biologists themselves, what may be called the static view of nature. It is not strange that the stability of species should have first impressed the scientific mind. When closely similar plants and animals, not distinguished by the popular intelligence, were found to differ in minute particulars which were, nevertheless, invariably transmitted to their offspring, a creative pre‑arrangement seemed to be the only explanation, and the apparently gratuitous variety of organic forms was very naturally ascribed to causes outside the reach of human comprehension.

Later, when it was realized that in spite of the wonderful stability of species the component individuals are never identical in all particulars, but differ endlessly among themselves, and that even these minor differences tend to reproduce themselves, the theory of the gradual transformation and subdivision of species became a logical possibility, and the search at once began for a method by which variations of a certain kind could be accumulated instead of cancelling each other and disappearing in a stationary average.

The explanation of evolution is the biological task now receiving the widest and most earnest attention, and is the subject, directly or indirectly, of a literature so vast that even a casual reading of all the books and papers as they come from the press would be a formidable undertaking. Such multiplicity of publications betokens, of course, a corresponding diversity of opinions. [288] Not only is there no common point of view from which evolutionary problems are studied; there is no agreement regarding the nature of the problem or the methods by which a solution is to be expected, nor even a general evolutionary language in which discussion may be made intelligible.

Explanations of such a process as evolution are of many different grades or categories. Literary demands were satisfied by a name and a definition; theologically it was sufficient to substitute the idea of a continuous for an intermittent creation. Philosophy was content with the predication of gradual transformations due to natural causes. Even among biologists there are those who appear to have rested content with similar generalities, though some have not failed to appreciate that when Darwin established the probability of biological evolution he opened a multitude of other questions regarding the nature, causes and significance of the process. Realizing at once the importance of his discovery and the difficulty of securing the confidence of either the scientific or the general public, he expended years of labor in the collection of facts and the contrivance of theories which should increase the plausibility of the main proposition, that plants and animals are variable, both in nature and in domestication, and that the diversity of organic nature was gradually attained through the medium of variations.

When the causes of a phenomenon are known the sequence of events can be predicted. Theory may then out‑run and assist observation. On the other hand, if the causes are out of reach it is obvious that we can not even theorize to advantage without a correct conception of the externals. We must know what takes place before we are in a position to ask why it takes place. In some lines of thought the simple historical conception of continuous evolutionary change greatly assists in the causal explanation of events, but in biology, the home of the evolutionary conception, the sequence is still in doubt and we are still far from the causal stage of knowledge. It is needless, perhaps, to add that the application of false and fictitious biological analogies vitiates much philosophical and sociological literature.

Gravitation was not explained by Newton, its behavior was [289] carefully studied and found to be consistent, and mathematically precise. "Natural laws" are working substitutes for causal explanations. When we understand the why, the 'law' of sequence becomes superfluous.

There is a frequent impression that the principal object and result of scientific study is generalization, but as a matter of fact the progress of science leads much more often to particularization, to the recognition of distinctions between things previously supposed to be alike. The powers, forces and principles which formed the subject of abstract discussions in the earlier history of science are being gradually relegated to the background, as our acquaintance with the facts improves and yields insight into the causal connection of events which formerly appeared mere sequences.

Evolution is not merely a law, but a process. In each species an evolution is going on, in a manner quite analogous to the processes of growth, locomotion and reproduction in the individual. Certain features of similarity there are, no doubt, in all evolutions, as there are in digestion and other general forms of vital activity. These general similarities can be collected, it may be, and formulated as laws if this method of expression be desired, though this would be, after all, only a special method of describing the processes. Laws themselves have to be explained by resolving them into processes. Only hopelessly metaphysical minds are satisfied with abstract statements, or able to imagine that generalizations are explanations.

Evolutionists agree that organisms change, but regarding the nature and causes of change great diversity of opinion still exists. The progress thus far is negative. We have learned that evolution is not a merely mechanical process, or due to merely environmental causes, and that it is not a merely cytological process, due to internal mechanisms of descent. It is a superorganic process accomplished through the association of organisms into large specific groups.

Evolution is, in short, a process of change in organisms, a kind of motion by which plants and animals have advanced from the simple and undifferentiated protoplasm of the lowest types to the highly specialized and complicated structures of the [290] highest. For half a century this probability that the world of organism has come into existence through long series of changes has been the most prominent idea before the scientific public, but we have not yet accepted fully the simplest purport of the idea of evolution and asked ourselves the direct question: By what mode or manner of motion is evolution accomplished?

Some have assumed that the evolutionary causes are resident in the environment, and others that they exist in the organisms themselves. A third alternative is here considered, that evolution arises from the association of organisms into interbreeding groups, or species. Species, in this interpretation, appear to contain the causes of evolution, instead of evolution affording the explanation of species.¹

The first result of Darwin's attempt at establishing the general idea of evolution on a basis of relation to concrete facts was a long and bitter controversy with those who clung to the older theory that the species of nature had arisen by separate creative acts. Biological science made good its escape from the house of theological bondage, but its controversial sins have condemned it to forty years of wandering in the wilderness of species‑formation and environmental adjustments, desert regions often very interesting in themselves, but remote enough from the fertile fields of evolution.

It may well be doubted whether any student of nature, if asked the direct question, whether species are normally at rest or normally in motion, would definitely and dogmatically hold to the static assumption. This appears to have been made quite unconsciously, in the great majority of cases, or taken entirely for granted. Nevertheless, all the current theories and methods of investigating evolutionary problems are based on this assumption of normally stationary species. The influence of the doctrine of special creation was too strong to be overcome at once, even by biologists who were very active in opposing its theological implications.

The idea of environmental causation of evolution has completely [291] pervaded all our forms of thought and expression; it has been the general base and background of evolutionary science. The average of biological opinion remains very nearly in the same place as Darwin's original announcement of a theory of environmental causes of evolution. The environment is supposed to bring about the variations and to select and preserve those having adaptive value, and thus to cause evolution. Though Darwin himself appreciated in later years the tentative character of this inference and sought in every direction for contributing agencies to strengthen and support it, some of his followers have had no such reluctance in crystallizing the idea of environmental causes into definite formulae which are still the shibboleths of evolutionary orthodoxy. President David Starr Jordan not long ago quoted an interesting paragraph from the evolutionary creed of the late Dr. Eliot Coues:

"Every offspring tends to take on precisely the structure or form of its parents, as its natural physical heritage; and the principle involved, or the law of heredity, would, if nothing interfered, keep the descendants perfectly true to the physical characters of their progenitors; they would breed true and be exactly alike. But counter influences are incessantly operative in consequence of constantly varying external conditions of environment; the plasticity of organization of all creatures rendering them more or less susceptible of modification by such means, they become unlike their ancestors in various ways and to different degrees. On a large scale is thus accomplished by natural selection and other natural agencies, just what man does in a small way in producing and maintaining different breeds of domestic animals."¹

It should be needless to say that this formula, like many statements of similar import which might be collected from biologists of a former generation, and even from those of the present day, involves a complete misrepresentation of the facts No such species has been found in nature, and no species has been made uniform by any refinement of artificial conditions. It is possible through selective inbreeding to eliminate a large part of the normal individual diversity of organisms, but at the [292] expense of vitality, and at the ultimate cost of extinction, wherever such experiments are continued for a sufficient period of time.

More recently still, a son of Charles Darwin, speaking as President of the British Association for the Advancement of Science, has reflected the conclusion which the scientific world has drawn from his father's doctrine of natural selection, that it is the cause of evolution.

"The fundamental idea in the theory of natural selection is the persistence of those types of life which are adapted to their surrounding conditions, and the elimination by extermination of the ill‑adapted types. The struggle for life amongst forms possessing various degrees of adaptation to slowly varying conditions is held to explain the transmutation of species."¹

It may be doubted whether Charles Darwin himself would ever have ventured upon so direct and so generalized a statement. He was anxious always that his readers should take a favorable view of the feasibility of evolution through natural selection, but at the same time he could not forget the immense improbability of the claim that all characters are adaptive and useful. This caution was not shared by Wallace, who has never hesitated to proclaim selection as the cause of evolution, alike efficient and sufficient. With Darwin, natural selection remained a theory, and he never ceased to seek additional evidence to support or supplement it, but with Wallace and many others it soon became an undoubted fact, or at least an unquestioned formula.

"Suffice it to say here that this theory of natural selection — meaning the elimination of the least fit and therefore the ultimate 'survival of the fittest' — has furnished a rational and precise explanation of the means of adaptation of all existing organisms to their conditions, and therefore of their transformation from the series of distinct but allied species which occupied the earth at some preceding epoch. In this sense it has actually demonstrated the 'origin of species,' and, by carrying back this process step by step into earlier and earlier geological times, we [293] are able mentally to follow out the evolution of all forms of life from one or a few primordial forms. Natural selection has thus supplied that motive power of change and adaptation that was wanting in all earlier attempts at explanation, and this has led to its very general acceptance both by naturalists and by the great majority of thinkers and men of science."¹

But notwithstanding the categorical certitude of these and many similar statements which might be collected, it is still very doubtful whether any naturalist, that is, any careful and experienced student of plant or animal species in nature, would definitely claim or undertake to prove that isolation or natural selection is, or could be, a true, actuating cause of evolution. Nevertheless, many such students have permitted themselves to use expressions which can be so interpreted, and the philosophical, and especially the unbiological part of the scientific community, has not hesitated to repeat and elaborate this idea as though it were an ascertained and undeniable fact.

Primitive peoples are ever ready to personify nature and inanimate objects and to ascribe to them the ability to grow and to put forth other spontaneous actions. Modern science has gone to the other extreme. It has denied to the species of plants and animals the powers of development which they really possess, and has sought for the causes of organic evolution among the inanimate objects of the environment. It has done this quite gratuitously and as a matter of course, without taking the trouble to raise the question whether there might be any alternative worthy of consideration.

The primitive theory of a flat earth, with its various childish explanations of the sun's whereabouts during the night, endured for thousands of years, but finally gave place to the conception of a spherical earth, about which the luminary revolved continuously. Nevertheless, this improved doctrine, while adequate for the explanation of the phenomenon of days and nights, was also erroneous, and had to be replaced by a still broader interpretation of astronomical facts.

Astronomers of the Ptolemaic school saw no reason to doubt that the earth was stationary, and they were able to predict [294] eclipses and planetary movements in spite of this fundamental misconception. Mysteries and discrepancies remained, however, until students of the heavenly bodies were willing to admit that the sun was the center of the system and that the earth revolved like her sister planets.

If adaptations were the only evolutionary phenomena in need of explanation, the doctrine of environmental causes might serve scientific purposes for as many centuries as the Ptolemaic astronomy, but it has become very apparent that many organic changes are going on which have no connection with adaptation, and which would not be explained by selection, even if everything claimed for it were to be admitted.

To think of species as normally in motion will be found very difficult, no doubt, by those who have been so long accustomed to take it for granted that they are normally at rest. The difficulties of readjustment are still further increased by the fact that the available technical language and customary forms of expression have been elaborated for the exposition of the static doctrine of environmental causation, and lend themselves only with difficulty to the presentation of the opposite doctrine, that species are normally in motion.¹ Many distinctions formerly considered of value now appear to have little significance. Many things are readily explainable which seemed utterly mysterious before, and many new problems can be approached which have hitherto appeared quite inaccessible.

Since the time of Darwin a long and varied series of amendments and supplements have been proposed for the doctrine of natural selection, and no end of diversity of individual opinion has existed among biologists regarding the adequacy and relative significance of the various factors and forms of selection. The kinetic theory enables us to look beyond all this cloud of discussion and to perceive that selection is not merely inadequate as the cause of evolution; it is not an evolutionary cause at all, in the concrete physiological sense; it does not set evolution in motion, nor keep it going.

[295] The difficulties which attend the presentation of the kinetic theory arise, no doubt, largely from this fact, that it breaks with the Darwinian traditions and recants the whole doctrine of selection as the actuating cause or principle of evolution. It seeks for the laws and causes of evolution, not in the environment, nor in a "hereditary mechanism" of the organisms themselves, but in the association of organisms into specific groups of interbreeding individuals, which are the units of evolutionary motion. The reader is therefore duly warned that, unlike most of the suggestions made since the time of Darwin, kinetic evolution does not come as an amendment to natural selection.

Those who may wish to experiment with the new method of biological locomotion had best unload beforehand all their prepossessions regarding natural selection as an evolutionary cause. This does not mean that selection is to be permanently abandoned, but it can be taken up later, and put to a much more useful purpose than before. Indeed, the material analogy may be carried a step further by saying that the supposed evolutionary properties of selection have been due to an unsuspected admixture of kinetic implications, the selection idea in itself being quite inert, and incapable of actuating even a logical conception of evolutionary motion.

Theories which located the causes of evolution in natural selection or other forms of environmental reactions have considered the species normally stationary until acted upon by the external forces. Theories which located the causes inside the organisms have thought of evolutionary motion as proceeding in definite directions without regard to environmental influences, except as they might work the extermination of types poorly fitted to the conditions they happened to encounter. The kinetic theory, in appreciating the fact that the evolutionary change goes forward in a network of descent woven by the free interbreeding of the individual members of the specific group, reaches the conception of a highly composite, indeterminate motion carried along without any environmental causation, but at the same time capable of being deflected through selective influence into channels of adaptation.

[296] The most feasible way of presenting the kinetic interpretation and of comparing it with other alternative views has seemed to be that of canvassing further this question of the nature of the motion by which evolution is supposed to be accomplished in accord with the different doctrines. It may be that by so doing the issue can be made more direct and that there will be less risk of wandering into the unprofitable side‑paths of aimless discussion. The fact already referred to, that the vocabulary of evolution has been constructed so largely for the explanation of static doctrines, makes it necessary to review briefly some of the primary terms and distinctions.

PHILOSOPHICAL USES OF EVOLUTIONARY MATERIALS.

Circles can be described through any three points, and new systems of philosophy can be elaborated out of a few primary distinctions. As geometry and other speculative sciences of number and space relations have been called upon to assist in the measuring of land, the building of machines, the navigation of the sea, and the exploration of the heavenly bodies, so have the methods of philosophy been applied to evolution. This is not only because philosophers have become interested in evolution, but because philosophical systems are the most available form of mental machinery for dealing with complex miscellaneous, hypermathematical problems, like evolution.

It has been the ambition of philosophers to frame general descriptions of the universe of thought in terms of logical consistency. Indeed, the tendency in philosophy has been to place by far the greater emphasis upon the logical consistency, each philosopher assuming the right to choose his own particular universe for descriptive purposes. Unfortunately for evolutionary philosophers, their systems are confronted, sooner or later, with the concrete facts of plant and animal life, and then no amount of logical consistency can atone for a biological oversight. Theories may be perfectly logical and yet be utterly inadequate. But even though not correct or final, philosophical theories of biology may still amply justify themselves by aiding in the discovery of relations which might have remained unsuspected and hence uninvestigated. The ungrateful facts may [297] refuse to support the theory which has led to their discovery, but this does not render the facts of less value for practical purposes, nor even for use in other and better theories. It is as idle to condemn theories as to worship them; it is the old counsel of using and not abusing.

Theories of evolution have been made thus far from the facts of variation, the differences which exist among the members of the same species. In each of the different systems it has been assumed that a certain kind or group of variations represented steps in the evolutionary journey. The philosophical circles of doctrine have been described in different planes in accordance with the selection of particular lines of samples from the multitudinous facts of variation.

The theory of natural selection is supported by the facts of adaptation and geographical distribution. The theory of direct adaptation was based on variations of accommodation, on the fact that organisms are often able to adjust themselves to a considerable range of environmental conditions. Nãgeli's determinant theory was based on the fact that the plants most carefully studied by him showed tendencies of variation in definite directions. The theory of mutation rests on facts of abrupt modifications in the form and structure.

The kinetic interpretation claims the consideration of believers in the other doctrines because it affords a larger outlook upon the facts of nature. Adaptation and mutation no longer appear as unconnected or contradictory phenomena, but are completely reconciled under one simple inference.

The kinetic theory differs from its predecessors not merely nor principally in dependence upon a different series of facts of variation, but also in the method of combining them. It is not merely a circle cut in one plane or described on one crosssection of data, but considers all three dimensions of space. It permits us to understand that variations are not all of the same character or of the same evolutionary significance. It also recognizes that as species are networks of descent and not mere aggregates of similar organisms, so evolution is not merely a summary or integration of variations, but is accomplished only through the normal extension of the specific reticulum. [298]

In pre‑evolutionary days there was no need to make special studies of variation, since it was freely admitted by the scientific public that the differences of varieties and even of species arose from environmental influences upon normally stationary types. The supposition was that genera had been created, rather than species, though Linnaeus interfered with this view by combining many of the groups recognized by his predecessors as genera and by holding then that species also were specially created.

The significance of this history is that the two ideas, first, that of normally uniform and stationary species, and second, that of the environmental causation of variations, were inherited from the pre‑evolutionary period and have continued to be used without scientifically critical warrant.

Moreover, the first quest for evolutionary causes was not made in the direction of more thorough study of the constitution of species, but was concerned rather with the exploration of the boundaries and the gaps between species. The issue raised by Darwin, and more especially by Huxley and other controversial biologists, was that of proving to the theological public that new species could be produced by evolution, instead of definitely investigating the means by which the evolutionary progress of species is accomplished. The chief interest was directed, not to evolution itself, but to the two results of evolution, speciation and adaptation, the generally admitted pre‑Darwinian doctrine of environmental causation of variations serving all the immediate needs of the discussion.

TYPES OF EVOLUTIONARY THEORIES.

Static Theories.—According to the theory to which the name Darwinism is generally, though unjustly, limited, evolution is brought about by the influence of environment, which causes organisms to vary, preserves advantageous modifications, diminishes or eliminates the relatively unfit, and thus transforms or subdivides species.¹ Such theories may be called static because they assume that species are normally in a state of rest or stable [299] equilibrium, so that evolutionary motion appears as the result of forces external to the organism. Differences among the individuals of a species are ascribed to environmental causes; without such disturbing influences the species is thought to remain stationary and uniform. Darwin and many others have believed in spontaneous variations, but it has been argued that such must be 'swamped' in the general average by intercrossing, so that without the external influence of selection there could be no progressive change.

Darwin himself admitted that in the domestic animals 'man does not cause variability and cannot even prevent it,' but on the same page he made the contradictory statement that 'the initial variation is caused by slight changes in the conditions of life,' and this has served as the cardinal principle of those who have claimed to be Darwinists, while rejecting the wider perception cited above. Again in the same work (p. 79) Darwin is ready to admit that 'a somewhat complex, though apparently useless, structure may be suddenly developed without the result of selection.'¹

Saltatory Theories.—That variations can be preserved by selection, and are frequently so preserved among domesticated animals and plants, cannot, of course, be doubted, but the difficulty of believing that natural conditions would provide the necessary selection or segregation at the right junctures has led many biologists to look with favor upon the idea that new species have not arisen by imperceptibly gradual changes, as Darwin supposed, but by a succession of leaps, as it were. This view is defended by reference to the so‑called 'sports' or very pronounced variations occurring among domestic plants and animals.

Mr. Francis Galton has compared the organism to a polygonal body which comes to rest at a point considerably in advance of its former position when its equilibrium has been sufficiently disturbed. Professor De Vries has adopted the saltatory view, as a result of his studies of what he calls mutations, or pronounced and readily transmissible variations of domestic plants. [300] Instead of slow or gradual changes of the characters of species there are supposed to occur at remote intervals in the life of a species relatively brief periods of mutation in which violently abrupt variations are given off in an explosive manner. Each of these discontinuous variations is considered as representing the production of a new species, there being no gradations between it and the parental type. Unfortunately, the wide application of this analogy is prevented by the fact that in many natural groups descent from a single individual is impossible. Moreover, the new types or sports studied by Professor De Vries are, like other closely inbred plants and animals, much less fertile than their wild progenitors, thus increasing the probability that the inbreeding or segregation necessary to secure and preserve these abnormalities would give them a fatal handicap in the struggle for existence. Finally, the wide distribution, among both plants and animals, of sexual differentiation and other expedients for securing cross‑fertilization, seems a sufficient warrant for distrusting any theory which disregards this important group of evolutionary phenomena.

Determinant Theories.—The noninheritance of acquired characters led Nägeli and Weismann to formulate what may be termed determinant theories, under which the motion of species is not thought of as caused or directly influenced by the environment, but as the function of internal "mechanisms of descent." Nägeli believed that species did not vary in all directions indiscriminately, as Darwin had held, but that they kept, without selective influences, a definite direction. He therefore concluded that the organization of living matter contained what he called a "Vervollkornmungsprinzi," or principle of perfection, which carried them ever upward along the road from simplicity to complexity.

Weismann sought in his doctrine of determinants to render this conception more concrete regarding the nature of the internal mechanism, and to provide a means of selective influence. Determinants may be described as biological atoms, resident in reproductive cells and able to determine in advance the character of the new organism, independent of its environmental relations. The environment also has no effect on the next generation, [301] selection pertaining not to the characters themselves, but to the determinants which might repeat the characters in the next generation. Further elaboration of the doctrine of determinants has been made in the belief that the external conditions, while unable to act through the body of the organisms, might act directly upon the reproductive cells. Others assume conflicts or struggles between determinants (germinal selection) as possible factors in evolutionary motion.

As a suggestion that evolution might be the result of external influences, and as a means whereby characters imposed by the environment could be transmitted, Darwin invented the theory of pangenesis, to the effect that the germinal material carrying reproductive influences was assembled from all parts of the body of the parent organism. Direct evidence for this supposition has never been found; indeed, the contrary proposition, that acquired characters are not and cannot be inherited, has commanded the belief of Professor Weismann and his numerous followers. Having cut loose, as it were, from environment, which had been the chief resource of static theories, they have sought the explanation of the evolutionary problem in a so‑called "hereditary mechanism," by which the characters of successive generations are held to be predetermined in the reproductive cells. The structure of the living cell has accordingly received the attention of many earnest investigators and a new science of cytology has been rapidly built up. But, as in the pursuit of her somewhat older sister, embryology, no general uniformity of structure or processes has been discovered. Biology has been enriched by the addition of a vast number of interesting facts, but the minute structure and internal organs of plants and animals, including the structure and organs of the component cells themselves, have been found to share the general diversity of nature, and to be as much in need of evolutionary explanation as the external characteristics of the various natural groups.

With an infinity of biological facts to draw upon, no theory need remain without support, real or apparent. An evolutionary inference warranted in one group may be quite false as a general law, and in this sense an inadequate theory may be more misleading than one which is actually erroneous. Thus [302] each of these types of evolutionary theories may be said to rest upon certain groups of evolutionary facts which are more or less completely ignored by the others. The niceties of many adaptations to environment have led Darwin and his followers to almost exclusive reliance upon that factor. Saltatory theories provide larger variations, but require even more effective isolation. Determinant theories deny the influence of environment and must ascribe adaptations to accident or to pre‑established harmony. All three theories antagonize the obvious fact that a very general tendency of organic development has been toward the increase of facilities for cross‑fertilization. These have been interpreted as inimical to evolution because they interfere with the preservation of the abnormally close‑bred variations which have been mistaken for true steps in the progress of organic series.

KINETIC OR SYMBASIC EVOLUTION.

Somewhat between the doctrines of selection and of determination, but distinct from both, is another conception of evolutionary motion, that it is caused neither by external environments nor by internal mechanisms, but goes forward as a necessary result of the normal specific constitution of living matter. It is observed that organisms normally exist and make evolutionary progress only in large groups of interbreeding individuals. Evolution is, in a word, symbasic; that is, organisms must travel together along the evolutionary pathway, and must be connected with each other by an intricate network of descent in the weaving of which the diversities of the members of a species have a definite physiological value. Without diversity of descent the cellular organization deteriorates. This being the case, it is easy to understand that new variations are prepotent, and that species make more rapid evolutionary progress in proportion to their numerical size. The larger and more widely distributed the species, the greater the opportunities of variation and of evolutionary progress.

Kinetic evolution is thus the reverse of many current theories, in that it recognizes a normal and necessary movement of change not caused by environment. It is the reverse of the selective [303] theory of Darwin in holding evolution to be independent of natural selection. It reverses the panmixia doctrine of Professor Weismann, in that it treats the interbreeding of the numerous and diverse individuals of species as conducive of biological motion, instead of as hindering it. It is the reverse of the mutation theory of Professor De Vries, in that evolution is held to go forward normally in entire species, and not merely in individuals or in narrow lines of descent.

One of the chief weaknesses of all the static doctrines, both saltatory and selective, lay in the apparent necessity that new variations be isolated from their relatives in order to preserve their new characters and make evolutionary advance possible, for the fundamental concepts of the static doctrine are the normally stationary average and the swamping effects of intercrossing.

The kinetic theory differs fundamentally from all its predecessors in recognizing the fact that evolution is not a process of segregation, but of synthesis and integration. The transformation of species in nature is brought about by the sharing of individual variations through interbreeding. Conjugation and cross‑fertilization do not hinder evolution, but are essential to the gradual building up of the intricate coordinations of characters through which adaptations and other desirable changes go forward. Selection, inbreeding, isolation and other forms of segregation, reduce the number of accessible variations, narrow the basis of the vital structure, and result in organic weakness, sterility and extinction. Selective isolation accentuates particular variations and has been utilized in the diversification of domestic varieties of plants and animals useful to man, but abnormal and weak from the evolutionary standpoint, and affording no complete analogy with the natural development of organic types. The sterility of many hybrids and the tendency of inbred varieties to produce relatively infertile sports may prove to be explainable by the same fact of inadequate fertilization. For want of better words it may be said that the vital tension of inbreeding is too little, while that of hybridity is too great; the normal course of biological evolution lies, obviously, between the two extremes. Evolution, or biological motion, [304] appears to be necessary as well as universal. Free interbreeding between the members of large organic groups, or species, is the condition under which biological evolution is going forward in nature, and we have no reason to seek its cause in any aberration or specialization of structure or function.

The fundamental and truly dynamic causes of evolution still lie hidden in the equally unknown causes of genetic variation, but the evolutionary history of a group of organisms is a process which a kinetic theory adequately explains by supplying physiological reasons and methods.

The ultimate theory or stage of evolutionary explanation must await far more complete knowledge of the nature of the phenomena to which we commonly refer under such abstract terms as matter and force, expressions which we can neither describe nor define, except in a purely formal manner. Much is gained, however, by the recognition of the fact of normal evolutionary motion, by perceiving that organic development is a kinetic phenomenon, for the species no less than for the individual. Individuals and species are conditioned, but not caused, by their environments; they descend from other species and from other individuals in continuous series of ever‑changing forms. There is an inside as well as an outside physiology of evolution, and it is idle to ignore either the one or the other.

To advance from the static to the kinetic point of view gives us ready and practical solutions for many problems which on the static basis bid fair to have required long periods of time and large expenditure of money. It brings also, as does every advance of science, a host of new questions which the static evolutionist could never have asked, such as the rapidity of evolutionary motion and the means of accelerating, retarding or deflecting it.

A kinetic theory of evolution does not need to explain variation any more than it needs to explain symbasis and environment; it accepts these three groups of biological facts, and correlates them as evolutionary factors. Conversely, a theory of variation is not necessarily a theory of evolution; the two questions may be viewed as quite distinct. The recognition of evolution as a kinetic process does not conflict with a dynamic [305] explanation of variation, but contributes to such an achievement by rendering the problem more definite. It affords another conception of how evolution may be accomplished, but a conception more comprehensive than those which have gone before; one which does not depend upon any theoretical or doubtful relation, but upon the well ascertained and universal fact that organisms exist everywhere in species‑groups of diverse individuals freely interbreeding to form a complex network or fabric of descent.

To some there may appear to be no practical distinction between the static and the kinetic views. Not a few naturalists have entertained truly kinetic conceptions of the facts of organic nature, even while continuing to misrepresent them by the use of the static terminology. For descriptive purposes, such as the tracing of phylogenies, the differences are less important, but fundamental divergence is obvious in approaching the physiological questions of methods and causes. The probable truth of a theory does not depend merely upon the number of facts which can be assembled under it, but also upon the coherence and practical consistency of the relations alleged. Of two theories otherwise equal the more simple and direct should receive the greater confidence. The kinetic theory is not compelled to ascribe utility to all characters, and can explain useful and useless characters by reference to the same facts of organic diversity and association in species.

SUMMARY OF EVOLUTION THEORIES.

Static theories view the species as normally stationary, and ascribe evolutionary motion to environmental causes of adaptation. The static theory commonly called Darwinism (though avoided by Darwin himself) treats adaptations as caused indirectly through natural selection, by the survival of the fittest of the individual variations. The static theory of Lamarckism treats adaptations as direct results or responses to environmental influences.

Saltatory theories view the species as normally stationary except for rare intervals of sudden transformation or "mutation" caused either by the environment or by internal "forces" [306] of unknown character. Selection can determine the survival of mutations adapted to environmental conditions, but exerts no direct adaptive influence.

Determinant theories view species as moving gradually in definite directions in obedience to internal "principles of perfection" or "mechanisms of descent." Adaptation depends on the coincidence between evolution and environment; selection exerts no direct influence.

Kinetic theories view species as normally in motion, but not in a single or definite direction, and not as a result of environmental causes. The normal evolutionary motion of the species may be restricted and deflected by the selective action of the environment, resulting in adaptation.

The adjacent tables may assist in showing the relations between these different types of evolutionary theories. Table I indicates the methods by which the various doctrines answer some of the principal questions regarding evolutionary motion. Table II brings these questions into relation with the conclusions reached in previous chapters. Discrepancies between different evolutionary doctrines are often explainable by the fact that some of them are in reality theories of adaptation or of speciation, rather than of evolution. Thus, as the table shows, interbreeding is a strongly negative factor in the multiplication of species (speciation), but at the same time it is a strongly positive factor in evolution. The chief factors in adaptation and speciation have only negative or restrictive effects upon evolution.

NORMAL CONDITION OF SPECIES.

The most fundamental diversity of opinion regarding the nature of evolutionary motion is that of the normal condition of species. Two assumptions are possible and have equal warrant for scientific consideration. Under theories of environmental and selective causation, it has been taken for granted that species are normally stationary and uniform unless acted upon by some disturbing external influence. The question of causes, on this assumption, is a simple one. The difficult problem is to explain how the external influences produce the organic results which have been ascribed to them. Fifty years of study have been [308] expended on this phase of the problem, but with no direct results. For this reason, if for no other, the careful consideration of the alternative possibility would be justified.

[307] TABLE I.
OUTLINE COMPARISON OF TYPES OF EVOLUTION THEORIES.

TABLE II.
EVOLUTIONARY CONDITIONS, FACTORS AND RESULTS.

The kinetic theory is not dependent, however, upon merely abstract or inferential justification, but is supported by the evidence of all observations and experiments which have a bearing upon the question. That groups of organic individuals become different whenever they have been isolated for any considerable periods of time, may be taken as proof that evolutionary change is a general and normal condition of the existence of species. It can be asserted, of course, that divergences between groups of common origin are due to differences of environment, but the inadequacy of this explanation is conclusively shown by the many instances where groups have preserved great similarity of habits and environmental conditions, but have attained, nevertheless, to a great diversity of form and structure, as in the conspicuous instance of the animals of the class Diplopoda, and of various classes of the lower plants, such as the mosses and hepaticae.

Two modifications of the stationary assumption had been formulated, previous to the kinetic theory. Under the mutation theory of Professor De Vries, the normal condition of uniformity is supposed to give place at rare intervals to periods of mutation or sudden appearance of new species. In the determinant theory of Nägeli, species were held to be normally in motion, but the motion was supposed to follow a definite direction as the result of internal physical and chemical adjustments.

The changes predicated as normal for species under the kinetic theory are of an indeterminate and composite character. The species is not thought of as changing in one direction merely, but in many characters at once, the required result being a constructive coordination of changes which will increase the vita efficiency of the organism and enlarge its power of utilizing its environmental opportunities.

RAPIDITY OF EVOLUTIONARY MOTION.

Static theories, which have agreed in thinking of species‑as normally stationary, have also taken it for granted that evolutionary [309] changes must be gradual, and some writers have dwelt upon the imperceptible slowness of evolutionary progress. The mutation theory of Professor De Vries adopts the other extreme, in holding that evolutionary motion is abruptly discontinuous, the individual organism leaping, as it were, from one species to another without any steps or gradations. From the kinetic standpoint, mutations like those studied by Professor De Vries are interpreted as abnormal and degenerative phenomena, but the fact is recognized that the individuals of many species in nature have very recognized differences, so that the steps of evolutionary progress may not always be infinitesimally gradual. There are indications that prepotent new characters may often transform a species or variety in a comparatively short period of time.

CONTINUITY OF EVOLUTIONARY MOTION.

Theories which ascribe organic changes to selection or to environmental causes imply that progress is limited to the characters which happen at the time to have environmental significance. In this view evolutionary motion, though gradual, must be described as occasional, rather than as continuous. After a period of selective development a species might cease, for a time, to be affected by selection and remain stationary, or might even retrograde, as claimed by Weismann and others.

In the mutation theory the idea of occasional change is carried still farther, so that evolutionary motion would need to be described as intermittent and occurring only at rare intervals. This is the type of evolutionary theory which comes nearest to the older doctrine of separate creation of species. It represents species as arising from single individuals, and denies gradual or continuous progress. It declares that evolutionary motion is saltatory or discontinuous; that there are sudden changes or jumps from one species into another. Such an evolution could not be described as taking place in species, but between them, the species themselves being essentially stationary except when acted upon by special "forces." Whether the forces are external or internal is a matter of opinion which subdivides saltatory evolutionists into two subordinate schools. [310]

Saltatory evolution consists of a series of abrupt lateral displacements, each species remaining stationary and unchanged from the time of its origin by mutation. No forward progress of the members of interbreeding groups is provided. Motion takes place only in the individuals which give rise to the new groups. Selection would thus have no influence upon evolutionary motion in connection with the mutation theory. Its function would be limited to the determination of the survival of the new species which might prove to be adapted to their environments. Motion is conceived only in simple inflexible lines and not in a network of descent which can bend in adaptive directions when environmental obstacles are encountered.

Saltatory theorists do not deny that diversity exists among the members of species, but they ascribe this to the influence of external conditions or to a general principle of inconstancy or fluctuation, without any special evolutionary significance.

Saltatory theories stand in most direct contrast with those which ascribe continuity to the evolutionary motion of species, which are thought of not as advancing by leaps or sudden transformation of one species into another, but as going forward by gradual steps, larger or smaller. Natural selection by the environment is thought of as changing the average and hence as causing evolutionary motion. The higher groups of plants and animals have so many adaptive characters that evolution by natural selection has been accepted by many biologists as a demonstrated fact.

Determinant and kinetic theories agree in expecting evolution to be continuous, the one because the internal mechanisms would continue to act, the other because the interbreeding of the ever‑diverse individuals of the species is being continued.

MUTATIONS DISTINGUISHED FROM NATURAL SPECIES.

There is a wide and fundamental difference between the kind of evolutionary motion shown by mutations of inbred domesticated species and that by which the progressive development of natural species has been brought about. The condition of inbreeding under which mutations appear has so far weakened the organism that the newly modified form is recessive, that is, [311] it tends to disappear when crossed with unrelated groups. Such variations could not spread or propagate themselves in a normally symbasic species; each would need to be carefully isolated in order to be preserved. In the second place, very few, if any, of the thousands of mutations which have come under the eyes of planters and experimenters have proved to be more fertile, in the true reproductive sense, than the parental types. Nearly all of them are conspicuously deficient in this respect, and would thus struggle under a fatal selective handicap in competing with the parent form, if they were not at once wiped out by interbreeding. Mutations have very great agricultural importance, but their practical value will not be enhanced by overlooking this fact of deficient fertility which is fatal to the view that they represent a genuine condition of progressive evolution.

Mutations arise sideways, as Professor Dc Vries explains, but it does not follow that new species are formed in this manner. Mutations are frequent in domesticated plants because varieties in cultivation are separated by inbreeding from the normal forward progress of the whole interbreeding species. Each species when once formed is supposed, under the mutation theory, to remain stationary so that progress can be made only when new varieties become segregated from the mass.

There is, however, another and very different way in which variations can contribute to evolutionary progress. Instead of being recessive mutations, the variations which have practical evolutionary significance are prepotent, and can work one change after another in the gradually advancing group. The true evolutionary significance of mutations is not that species arise by mutation, but that the progressive steps, by which the evolution of species is gradually accomplished, are not imperceptibly small. There may be a very appreciable advance between two successive individuals.

Very acute selection or some other way of separating a new mutation from its unmodified parent stock must be imagined in order to account for its preservation, but plants and animals abound in characters which could scarcely have been perpetuated in this way. With self‑fertilized plants a single individual [312] can start a new race or variety, but with sexually differentiated animals this is much more difficult, since interbreeding is necessary for reproduction. An actual instance will illustrate the point. In all the millipedes of the world‑wide order Merocheta the olfactory cones of the antennae are four in number, arranged in a square, with the single exception of a series of closely related East African genera of the family Gomphodesmidae,¹ which are unique in the possession of ten olfactory cones arranged in a circle. That the four cones in a square is the ancestral condition, is certain, because it is shared also by all the other orders of the very ancient class. Diplopoda, many members of which are known from the carboniferous period. That the number is invariable in the order Merocheta can not be claimed, since, obviously, it must have varied at least once, when the circle of ten cones came into existence. No variation has been recorded, however, either in the four‑coned or the tenconed genera, on the many thousands of specimens which have been examined.

Nor are there any indications that the ten‑coned condition is an advantage which has gained any favors from natural or other forms of selection. The ten‑coned genera as a group show no other conspicuous peculiarity and have contributed, apparently, only n average share to the evolutionary diversification and geographical distribution of the family. Moreover, the habits and environmental relations of the whole class Diplopoda are such as to reduce the influence of natural selection to a minimum.²

Under such circumstances the sidewise origination and preservation of a ten‑coned new species as a mutation seems highly improbable, but there is, on the other hand, no reason why a genetic variation to ten cones should not spread through a species and be carried forward into the other species and genera into which the ten‑coned group might afterward subdivide. If there had ever been millipedes with the intervening number of [313] cones we have every reason to expect that indications of them would remain, either in species with such numbers or in occasional individual variations. The facts of mutation may help us to be reconciled to the probability that millipedes with five, six, seven, eight or nine cones may never have existed, but they do not warrant the general inference that evolution goes forward by the origination of species sideways by mutation.

The difficulty is not that the mutations of domesticated plants and animals are not as different and as readily to be described and distinguished from each other as natural species. Nor is it impossible that some of the species named and described in formal botanical and zoological classifications represent mutative variations from narrowly segregated wild types. The differences are not formal or theoretical, but physiological and practical. The conditions under which the mutations of cultivated plants and animals arise are not those under which the constructive evolution of nature has gone forward, and the mutations are deficient in the primary requirements of vigor and fertility.

That discontinuous variations may contribute to the evolutionary progress of species in nature is no part of the mutation theory of De Vries, which definitely rejects and denies any gradual evolution, any continuous change and accumulation of characters. Species once formed by mutation are just as stationary and immutable, according to De Vries, as Linnaeus said they were. All the evidences of gradual evolutionary divergence of organic groups accumulated by Darwin and his successors are ignored in the mutation theory, because no evolutionary changes were detected in the original OEnotheras which Professor Dc Vries kept in his garden for eighteen years.

The kinetic theory is not thus at odds with the facts of science. It provides an evolution of species by a thoroughly gradual, continuous process, more broadly continuous, indeed, than any suggested before. It recognizes that new variations are prepotent, and are able to accumulate and to transform the species in which they appear. Species are normally in motion and do not depend upon the intermittent interference of selection, nor upon mutation, for the development of new characters. Instead of finding the motive power or [314] active principle of evolution in natural selection or in mutation, the kinetic theory finds evolutionary causes in normal diversity and free interbreeding in specific networks of descent.

Both the selection theory and the mutation theory imply that new characters and new types have to be preserved by isolation. Under the kinetic theory it is clearly perceived that isolation explains only the multiplication of species, but is not an evolutionary factor, or even a necessary condition of evolution. The kinetic theory provides for the first time a consistent outline of a method of gradual and continuous evolution in normally extensive, freely interbreeding specific groups, the condition in which organisms everywhere exist in nature.

PRINCIPAL AGENT OF EVOLUTIONARY CHANGE.

At this point the various theories show, perhaps, their most obvious divergencies. The doctrine of pure selection, or Darwinism, holds that selection is the actual cause or principle of evolutionary advance, supporting this by various other assumptions, such as an environmental causation of variations or a correlation between useful and useless variations.¹

The isolation theory of Gulick appreciates the inadequacy of selection and seeks for special conditions or behavior which can explain the evolutionary progress of groups of individuals which have merely been isolated from the parent species without having been placed in appreciably different environments. The Lamarckian doctrine of direct adaptation finds its greatest advantage here, in that the environment itself is supposed to cause the changes directly. Professor De Vries argues, in some of his writings, that mutations are due to environmental causes, though frankly admitting that the connection of events is unknown. [315]

It is commonly taken for granted by the advocates of the selection hypothesis that a certain constant of variation will be maintained by the species, so that the cutting off of the extremes on one side will cause a still greater development on the other, and thus actually move the species along.

This idea may never have been very definitely formulated, but it is obvious that many writers on selection have relied upon the unexpressed assumption as affording the means by which selection could produce evolutionary change in a normally stationary group of organisms.

The Darwinian doctrine of variation grafted upon the older idea of stationary species resulted in the conception of a species composed of variable individuals, but with a stationary specific average. Experiments with domesticated varieties had shown that selection could change the center of gravity or characteraverage of a group, and this idea applied to nature at large gave the hypothesis of evolution through selection.

In arguing the inadequacy of selection, Mivart, Dc Vries and others have taken the ground that selection could not carry the specific average beyond the boundary or limit of range of variation for the original group, and this is the logically correct inference, unless the idea of a constant of variability be included as a factor of the problem. But even this is inadequate to account for the general evolutionary results, for unless the further notion of a normal tendency to progressive change be added, the presumption would be that the selectively reduced species would attempt merely to reproduce its lost members, to regain its original size and cover again the field from which it has been excluded by selection.

It may be held, therefore, that both in logic and in fact the explanation of the ascertained and generally admitted data of selection depends upon the recognition of a normal and spontaneous tendency of species to evolutionary change. It is this tendency, this specific kinesis or law of motion, which carries species into close selective contacts with their environments. The species are travelling by their own motion, in spite of selective obstacles, and not because environmental selection is carrying them along. [316]

The determinant theory of Nägeli, as already indicated, ascribed changes to an internal "principle of perfection" of heredity, which conducted the evolution of a species in a definite direction. There was no need, in this view, of showing any direct connection with the environment. Selection was applied to a species as a whole, to preserve or to eliminate, but it was not thought of as actuating evolution or as conducting it in adaptive directions.

The determinant theories of Weismann and his followers may be described as hybrids between the doctrines of Nägeli and those of Darwin and Lamarck. They predicated a cellular mechanism of heredity for conducting the process of evolution, but supposed that this mechanism could be actuated or affected by environmental influences and compelled in this way to carry the species in directions of adaptation.

Darwin, in his theory of pangenesis, assumed that all parts of the body of the parent contribute materials to the germ‑cells and hoped thus to explain how characters acquired from the environment might be passed on to succeeding generations. Weismann denied the inheritance of acquired characters, but he nevertheless repeated Darwin's attempt at providing for the inheritance of environmental influences, because it appeared impossible without this to construct a theory of environmental causation and explain the facts of selection and adaptation.

Weismann was well aware that his theory of determinants was so complex as to appear improbable, but he defended it with persistence on the ground that it was the only way in which heredity could be understood. Unfortunately, the vast complexity of ideas does not explain the facts of organic descent, but only adds to them an even more mysterious hypothetical field. Moreover, the data of environmental relations do not accord any better with the Weismannian than with the Darwinian hypothesis. Experiments have not shown that there is any close, constant or definite relations between environment and heredity. The most that can be claimed is that the environment, in some manner still quite unexplained, may sometimes induce an instability, or tendency to stumble and fall from the normal hereditary pathway of the type. [317]

The theory of determinants afforded, at most, a method of thinking about the process of organic succession, but it does not appear that this way of thinking is either correct or necessary. It assumes a complete diversity of nature between germinal and somatic cells, which the facts do not warrant, especially among plants, and it assumes further that there are definite mechanical directive relations between the germ‑cells and the resulting organisms, which the facts also refuse to indicate. Of the real nature of heredity we know, as yet, absolutely nothing, any more than of analogous phenomena, instinct and memory. Speculations, even of purely hypothetical character, may sometimes be of service in the treatment of scientific problems, but no speculation should be cherished which hides or even casts a shadow over facts.

Under kinetic evolution the symbasic interbreeding of the diverse individuals of the species is held to be the principal agent of evolutionary change, since it is in this manner that the prepotent variations which appear among the component individuals are transmitted and combined into the complex organic result. Interbreeding is held to effect an integration of individual variations inside the species, instead of each variation being considered a new species, as in the mutation theory.

Symbasis is one of the general conditions of organic exist ence, but under static theories its evolutionary significance was so completely overlooked that no term was provided by which it could be directly and definitely symbolized. The word interbreeding, if used alone, would generally be misunderstood in one of two opposite and equally unfortunate senses. Some writers use interbreeding as synonymous with inbreeding or close‑breeding, and some for wide cross‑breeding, which are exactly the conditions to be avoided in the discussion of normal specific relations. Another term being indispensable, symbasis was introduced, in allusion to the fact that the individual members of species are normally associated in groups. The expression also lends itself most conveniently to the description of kinetic interpretations, in view of the fact that the association of organisms into symbasic groups is looked upon as one of the principal agencies of evolutionary progress. [318]

The introduction of a new term is always to be deprecated, and may help very little, after all, in the explanation of a new distinction. The word has to be explained, as well as the idea. Nevertheless, there are occasions like the present, where progress in expression is likely to be permanently hampered unless we can be permitted to place definite labels upon our phenomena and refer to them by unequivocal word‑symbols.

Symbasis, more properly than any other ascertained fact, can be called a cause of evolution. It may not cause variation, but it does enable variations to be combined into a general evolutionary change of type.

UTILITY OF NEW CHARACTERS.

New characters, as mere fortuitous variations, might or might not be useful, but if selection were the only cause of evolution, progress would be limited to characters of definite utility. Every character, therefore, which has attained to any considerable degree of expression would have a definite use, or would have had use at some former time in the evolution of the species. This logical necessity of predicating the utility of all characters is the most obvious weakness of the theory of selection, for there are large numbers of character differences between species which are not only obviously useless at present but which were probably equally useless in the past.

Gulick's isolation theory does not insist on the utility of specific differences, nor do the mutations of De Vries or the determinate changes of Nägeli and Weismann follow, of necessity, the course of environmental utility. Selection would explain the disappearance of types too far lacking in fitness, but adaptation would remain a mere coincidence, depending on whether adaptive variations happen to appear.

Under the kinetic theory it is possible to admit that useful and useless characters have equal possibilities of appearing and evolving, as long as they do not become actually detrimental, but at the same time selection is admitted to have a definite and practical evolutionary function, since the rejection of harmful tendencies has the power of enforcing more rapid specialization in useful directions. Selection is, indeed, more effective for [319] inducing adaptation under the kinetic theory than under the purely selective doctrine of Darwinism, because in kinetic evolution a much wider range of characters can be expected to reach a sufficient development to render them of selective importance. Under a logical static theory, only those characters could be developed which have selective value from their first inception.

METHODS OF PRESERVING NEW CHARACTERS.

The great weight given to the various forms of selection, isolation, and environmental influence as factors of evolution have been determined largely by the belief that new characters or variations could not be preserved unless they were in some way separated from the unmodified parental type. This opinion has been supported largely by the fact that many of the variations which have been taken for examples of normal evolutionary motion have been in reality more or less abnormal results of the condition of inbreeding common in our domesticated varieties of plants and animals. The prepotency of the unselected wild type has been insisted upon, as well as the swamping effects of intercrossing, when the characters of the carefully selected variety fade away into those of the unspecialized parental form without leaving any apparent result. Nevertheless, the fact seems to be that new characters are prepotent, not of necessity over the whole taxonomic species to which the individual may belong, but at least in the particular variety or group and in the particular stage of interbreeding in which the variation appears. The recognition of the prepotency of new variations makes it obvious that the preservation and continued evolution of new characters does not involve the necessity of isolating the new form or the extinction of the old, after a period of struggle for existence.

Mechanical theories of evolution have centered largely about this question of acquiring characters, but it is still more important to know how characters are preserved after having been acquired. Organisms appear to acquire some characters from the environment, but it does not follow that the characters are also preserved by the environment, or even that the characters [320] acquired from the environment are those which contribute in a definite manner to evolution. The kinetic interpretation enables us to understand the probability that a character is preserved for the same reason for which it appears in the first place.

The name Darwinism is commonly, though rather unjustly, limited to the gradual or selective theory under which variations gained genetic significance only when they were favored by partial or complete isolation, brought about either by the elimination of the less efficient parental form during the struggle for existence, or through geographical or other accidents preventing the swamping effects of intercrossing. This meant that variations did not tend to be preserved, that they tended only to continue their fluctuations around the stationary specific average. This conception was based, as already indicated, on the choice of the fluctuating variations or unspecialized heterism and artism as representing the variations on which evolution proceeds.

Under the assumption that organisms are normally stationary it was natural to ascribe variations to new conditions. It may be found, however, that the facts can be accommodated as well or better by supposing that new conditions of nutrition and growth afford more facilities for variation. Variations, once produced, tend to repeat themselves; not, it may be, in all of the offspring, but at least in some of them. The object of variations, the value of variations for the species, lies not so much in giving them new characters as in giving them a diversity of characters. Variations which appear in a part of the offspring, but not in all, serve most efficiently the purposes of increasing and maintaining heterism, and of insuring diversity of descent, after the manner of the many secondary sexual characters which appear to be quite useless except for this physiological purpose.

The kinetic theory differs from all its predecessors in recognizing physiological reasons for holding that new characters are prepotent. From the fact that they afford opportunity for organic readjustment, they enjoy an advantage over the unmodified type both in accentuation of characters and in vitality and fecundity of offspring. The evolutionary possibilities of a [321] new character may depend as much or more upon its fitting into and supplementing the complex of existing characters as upon any direct utility from the environmental standpoint. Evolution, in other words, may be viewed as an aspect of the physiological process of interbreeding by which the vitality of organisms is sustained.

NATURAL SELECTION AS AN EVOLUTIONARY FACTOR.

The preponderance attained by the selection theory has probably been due, in large measure, to its logical simplicity and consistency in holding that selection is the positive, efficient factor or actuating principle of evolution. The unbiological public has accepted this interpretation of the causes of evolutionary motion with practical unanimity, but among biologists themselves there has always been a wide appreciation that the facts did not warrant the definite generalization which Darwin himself carefully avoided, but which his friends made for him and christened with his name.

All other suggestions of methods of evolution are the result of more or less definite perceptions of the inadequacy of natural selection as an evolutionary cause. No amendment of natural selection has the logical consistency of the original, nor has any gained a comparable popularity in the scientific world. The mistake has been made, if the present diagnosis is correct, in attempting to modify or repair the hypothesis of selection as an evolutionary cause.

Under the kinetic theory selection appears as a negative factor only; its power is to inhibit motion, not to cause it. It is not improbable that selection, by closing other avenues of change, can induce more rapid progress in a particular direction, but such an effect of acceleration would not prove that selection can cause evolutionary motion; it would indicate that a certain amount of change necessarily takes place as the result of causes inherent in the species. A variation eliminated by selection does not help to maintain the needful diversity of descent, and this may make the surviving variations the more effective for inducing adaptive specializations. Selection, by thus restricting the field of change, may be able to focus the evolution upon one [322] variation, but a condenser is not to be reckoned as a source of light.

The kinetic theory therefore definitely abandons selection as a cause or positive factor, and perceives that the influence of selection, powerful though it be in many cases, is of a negative and restrictive character ‑‑ an influence which could not be exerted if the species were not already in motion.

The kinetic theory, though departing radically from the doctrine of selection as an evolutionary cause, is, in a practical sense, much nearer to Darwinism than are many other suggestions which, though intended to supplement the selection hypothesis, would in reality completely nullify it, by denying to selection any true power to influence the course of evolutionary progress. The kinetic theory, though denying that selection is in any proper sense an evolutionary cause, ascribes to it a definite evolutionary function. The environment does not carry the species into adaptive specialization, it only deflects the normal specific motion. The evolution is in the species, the power of deflection in the environment.

Professor De Vries clearly recognizes that the function of selection is regulative and not active, though he still refers to it as a cause of evolution.

"Notwithstanding all these apparently unsurmountable difficulties, Darwin discovered the great principle which rules the evolution of organisms. It is the principle of natural selection. It is the sifting out of all organisms of minor worth through the struggle for life. It is only a sieve, and not a force of nature, no direct cause of improvement, as many of Darwin's adversaries, and unfortunately many of his followers also, have so often asserted. It is only a sieve, which decides which is to live, and what is to die. But evolutionary lines are of great length, and the evolution of a flower, or of an insectivorous plant is a way with many sidepaths. It is the sieve that keeps evolution on the main line, killing all, or nearly all that try to go in other directions. By this means natural selection is the one directing cause of the broad lines of evolution."

"Of course, with the single steps of evolution it has nothing to do. Only after the step has been taken, the sieve acts, eliminating [323] the unfit. The problem, as to how the individual steps are brought about, is quite another side of the question." ¹

This is in notable contrast with the previously quoted dictum of Professor Lankester, regarding an "ever‑watchful natural selection" by which characters are "seized upon" and "raised to a high pitch of growth and function."

INTERBREEDING AS AN EVOLUTIONARY FACTOR.

In full accord with the idea that evolutionary change or motion is caused by selection or environmental influence, are the opinions, already emphasized, that isolation is necessary to preserve new characters, and that the sexual phenomena of interbreeding stand in the way of evolutionary progress by hindering the perpetuation of new characters. These corollaries of the selection hypothesis find no place in the kinetic theory. Interbreeding and other phenomena of sexuality have been reckoned in the present discussion as positive factors in evolutionary motion.

Evolution, in the kinetic interpretation, represents the workings of no special force, principle or mechanism; it is carried forward by the symbasic interbreeding of the diverse individuals of which species are composed. The final and ultimate explanation of evolution must await an understanding of the constitution of living matter. We must learn why the prepotent genetic variations occur, and why the interbreeding is necessary. But having once appreciated the variations and the interbreeding as ever‑present facts, evolution is no longer mysterious; it follows as a natural and obvious consequence.

THE KINETIC FIGURE OF EVOLUTIONARY MOTION.

It will be apparent from the preceding chapters that the evolutionary motion predicated under the kinetic theory differs from that of previous doctrines in important respects. In the first place, it is a highly complex or compound motion instead of a simple one, not to be typified by a push from the environment, by a pull by natural selection, by an occasional mutative leap, nor even by the onward transportation of a determining [324] "hereditary mechanism." The figure of developmental progress under the kinetic theory is that of the advance of a huge and intricate network or trestle, built and supported by the intergrafting of the lines of descent throughout the species. Environmental obstacles can compel the progressive advance of this specific structure to be accomplished by many lateral bendings, but these deviations and displacements need no longer be mistaken for examples of normal evolutionary motion. That individual organisms can step aside, or even fall out of the ranks, proves, at the most, only that such transverse motions are possible; it does not show that they represent the method or the conditions by which the constructive evolutions of natural species go forward. The environmental reactions and mutations are made suddenly and can be readily demonstrated to our impatient eyes, but the coherent advance of the whole specific network has to be inferred from the relations of species as we find them in nature.

Some are inclined to distrust the results of the cosmic laboratory and to prefer to explain evolution by the lateral diversions which can be demonstrated in their own experimental cages and gardens. After keeping Lamarck's evening primroses in his garden for eighteen years without detecting any change, Professor De Vries has concluded that the species is constant and stationary and that further evolution is accomplished only by mutative variations, like those which appeared during this interval.

"There is neither a gradual modification nor a common change of all the individuals. On the contrary, the main group remains wholly unaffected by the production of new species. After eighteen years it is absolutely the same as at the beginning, and even the same as is found elsewhere in localities where no mutability has been observed. It neither disappears nor dies out, nor is it ever diminished or changed in the slightest degree.

. . . "My evening primrose, however, produces in the same locality, and at the same time, from the same group of plants, quite a number of new forms, diverging from their prototype in different directions. [325]

"Thence we must conclude that new species are produced sideways by other forms, and that this change only affects the product, and not the producer. The same original form can in this way give birth to numerous others, and this single fact at once gives an explanation of all those cases in which species comprise numbers of subspecies, or genera large series of nearly allied forms."¹

These inferences were made, of course, without reference to the kinetic conception of evolutionary motion as a specific structure or network of descent. Nor is the possibility considered that a small group of individuals isolated and inbred in a foreign land might behave in an abnormal manner, or at least in a manner that would afford small indication of the normal mode of evolutionary motion. Other parallel cases observed in coffee, cotton, capsicum, tea and other plants, indicate that mutative variations like those of the evening primrose are the regular results of the treatment to which the plants have been subjected in domestication. Instead of illustrating the method by which evolutionary advance is accomplished, mutations appear to represent a stage in the degeneration of organisms which have been removed from the vital fabric of specific descent; they do not show how the evolutionary network is woven, but how the strands can be unraveled. Conditions of uniformity like those of inbred domesticated varieties are to be found in nature only exceptionally, in the relatively few degenerating types which have become regularly addicted to self‑fertilization or to vegetative propagation. Nor do we find under normal evolutionary conditions of symbasic interbreeding and individual diversity these violent mutative departures from the parental types. There is a vastly greater range and flexibility of characters and character‑combinations. Nevertheless, it is very doubtful whether a species as a whole would make an appreciable evolutionary advance in eighteen years. In any event, the fact could hardly be determined from a few specimens in a foreign garden.

All kinds of variations can be described as having been produced [326] sideways. The doctrine of selection, like that of mutation, looks upon lateral or transverse displacements as the steps by which evolution is accomplished. From the kinetic standpoint it appears obvious that only those lateral movements really contribute to the evolution of the species which make a lasting addition to the internal diversity of the species and broaden and strengthen the structural network of descent. Mutations which arise under conditions of inbreeding do not serve this purpose. They are loose loups or free ends of the fabric of descent, torn out by the disarrangement of the tensions of the specific machinery of development. They do not affect the species, of course, if they remain isolated from it. On the other hand, mutations which are allowed to interbreed freely with the wild type, or even with each other, loose their distinctive peculiarities and are merged back toward the ancestral form, and toward the more normal condition of promiscuous individual diversity.

As evolutionary phenomena the mutations described by Professor De Vries have not less of interest and significance than the facts of adaptation and environmental adjustment which served as the basis of earlier theories of evolution. And like the data of the earlier theories, the facts of mutation are capable of being interpreted in a very different relation to the evolutionary motion of specific groups of organisms. Since constructive evolution is accomplished, as far as we know, only in these large groups of freely interbreeding individuals, we may well be cautious in the acceptance of any doctrines which do not take into account the normal constitution of species, and the nature of the motion by which their evolutionary progress is accomplished.

A species is not a merely arbitrary collection or aggregate of organisms; it is itself an organization by which organic existence is maintained and organic evolution is accomplished. It is customary to think of the higher types of organisms as having been made possible by the association of greater and greater numbers of cells, but this association and specialization of cells into tissues and organs has not been accomplished without the meeting of another evolutionary requirement, the association of the organisms into large interbreeding groups, or species. [327]

Organic energy is primarily an integration of cellular energy, and the energy of cellular development has to be readjusted and renewed by conjugations between cells of diverse descent. The answer to the question why this is so must come from a new department of science, a general cellular biology which shall study the problems of cellular organization and association. It is here, if anywhere, that we must learn why organisms are normally diverse, why interbreeding is necessary and why evolution follows as a universal consequence. A species, viewed as a protoplasmic fabric of interwoven lines of descent, is different from any other object in nature, but its properties and potentialities are no less peculiar than its structure and its modes of motion.