Smithsonian Institution. Annual report (1905) 397-412
THE EVOLUTIONARY SIGNIFICANCE OF SPECIES.a
By O. F. Cook.

a) Consists principally of a revision and combination of two articles, "Evolution Not the Origin of Species" (Popular Science Monthly, March, 1904) and "Natural Selection in Kinetic Evolution" (Science, April 1, 1904).

The theory that species came into existence as results of separate creative acts has given place to the belief that the diversity of organic nature has been attained by gradual transformation, but the unconscious influence of the older view remains very strong. Even scientific students of evolution still take it for granted that there should be species, and hope to solve the problem of evolution by finding special conditions or agencies which can make new species out of old ones.

Science is by no means exempt from the very human tendency to place a fictitious value upon rare and exceptional phenomena, and to overlook the significance of familiar facts. Between Linnaeus and Darwin a century of careful descriptive study of plants and animals intervened before the everyday incident of variation was appreciated, and it came to be perceived that species might arise through progressive change, or evolution. Having once assumed, however, that evolution could explain the origin of species, we have taken to seeking for evolutionary causes in the out-of-the-way places of biology, forgetting that the very existence and constitution of species affords evidence of the most fundamental character regarding the nature of the evolutionary process.

Why are organisms grouped into species, instead of manifesting unlimited and indefinite diversity? After reviewing the multiplicity of organic types and the innumerable modifications possible for each, it seems superfluous to doubt that the possibilities of diversity are endless. And yet all higher organisms are associated in species, and such associations have existed, apparently, throughout the biological history of the earth. Each investigator in turn has persevered in the hope that in learning the origin of species he would unravel the secret of evolution, but from every mechanical or environmental standpoint species remain as mysterious as ever.

A species is a species, not through the workings of any hidden cause of evolution, but because the component individuals breed together, and thus remain in interconnected, coherent whole. This is the familiar and obvious fact which has escaped the appreciation of specialists. The individuals of a species are alike, not because of any fixed law or mechanism of heredity, but because they are traveling together along the evolutionary pathway. When a species encounters an environmental obstacle, and is divided into two groups, the parts can then evolve separately and become different. Evolution is not shown in the becoming separate, but in the becoming different, and the process of gradual change would have continued if the original group had not been subdivided.

The policy of taking species for granted has been carried by some to the point of declaring that no satisfactory definition of species could be made, but this was only because they were attempting to combine two incongruous ideas, and failed to distinguish the species as the evolutionary unit of interbreeding individuals from the pre-evolutionary species of analytical classification, "founded on identity of form and structure." In nature, however, there is no identity of form and structure, and no such species as those into which the narrowly formal systematist subdivides his genera.

It is not possible to make a definition which will enable us to recognize species without studying them but, on the other hand, it is entirely practicable to tell what species are, or how they are constituted. A species is nothing more nor less than a connected or coherent group of interbreeding organisms. Interbreeding may enable them to maintain a general similarity throughout the specific series, or there may be sexual and other diversification which subdivides the species into two or more distinct sexes or castes without intermediates.

Adherence to the notion that species means "identity of form and structure" brings hopeless confusion of evolutionary ideas. The fact that characters which have arisen among inbred domesticated plants and animals disappear in crosses with the prepotent wild stock has been taken to prove "the swamping effects of intercrossing" in general. The segregation of new variations is now commonly held to be essential for their preservation, in complete disregard of the fact that the inbreeding unavoidable in such segregation would weaken the organism and give it a fatal handicap in the struggle for existence. The original assumption of this theory is as erroneous as the final deduction. Instead of having, or tending to have, "identity of form and structure," it is a physiological advantage for the members of the species to become more and more diverse, not in order to subdivide into more numerous species, but because diversity of descent. increases the vigor of the individual organisms of which the species is composed.

a) Weismann, A., 1893, The Germ Plasm, 463.

Sexual and other diversification inside the species has continued to become more and more accentuated in hundreds of independent groups of plants and animals, and is everywhere recognized as a mark of greater organic perfection. Professor Weismann held that under constant external influences variation would not occur,a but the practical fact is that in the same species, and under the same general environments, variations not only occur, but are preserved, accumulated, and integrated into sexual differences, not by isolating a part of the species from the rest, but under conditions of free and continuous interbreeding. The differences between the sexes are commonly greater than those which separate the species and genera, or even than those which characterize the families and orders, showing most conclusively that such differences can arise and become established, even inside the species, and quite without segregation. But instead of having been appreciated as the most important agency and the most significant illustration of evolution, sex and symbasic interbreeding have continued to be regarded as obstacles, because they interfere with the fancied necessity of segregation.

Evolutionists, too intent on a practical explanation of the diversity of species, magnified the idea that organisms become adapted to environment, and disregarded the more fundamental fact that species are not by nature stationary, but have an independent motion of their own. This oversight brought us the impossible task of explaining how external conditions produce evolutionary changes, and prevented the perception that adaptations are due to external causes only as environment may influence the direction of the normal and necessary movement of the species.

That evolution is thus an active, universal, and truly physiological process is not considered in current theories, largely because thought and language have continued to follow the bias of the original Darwinian controversy in seeking in evolution an explanation of the origin of species, and in expecting, conversely, that an explanation of the origin of species would also explain evolution. Such a history greatly increases the difficulty of presenting this alternative view, that the multiplication of species is in no proper sense a result of evolution, but is due to entirely distinct causes more often antagonistic than favorable to evolutionary progress.

EVOLUTION VERSUS TAXONOMY.

The early evolutionists were all systematists deeply impressed by the vast complexity of organic nature, a sentiment which we may still indulge, since two centuries of labor have but made a beginning in the task of describing and assigning names to the millions of groups of organisms. Nevertheless, it is to be regretted that biological evolution was viewed and expounded so exclusively from the standpoint of the taxonomist. His interests are greatly at variance with those of the evolutionist, and the confusion of the two distinct lines of investigation has done much to perpetuate the erroneous identification of the origin of species with the process of evolution.

The evolution best appreciated by the taxonomist is one which produces species separable by definite and easily definable characters. He finds such species on islands and in other circumscribed regions, and infers that isolation is an important evolutionary factor, failing to perceive that the "constancy" of insular species is merely a uniformity made possible by the limited area of distribution, and hence usually absent in species of more extensive range.

The systematist is prone to believe that there has been more evolution in a genus of ten readily definable species than in another occupying the same geographical region, but consisting of only one species. For the evolutionist, however, the segregation of the numerous species means that the conditions are less uniformly favorable for the subdivided genus than for the other. Among fossil organisms, also, the more generalized the types the wider was the distribution, the separation of local genera and species following with less favorable circumstances or greater competition. Segregation multiplies species by separating groups of organic individuals, just as the ocean might form many islands from a partially submerged continent. Species are biological islands, but we do not go further in biology than in geography by the discovery that islands must be isolated. Isolation permits evolutionary progress to be made on separate lines, until the differences become of diagnostic utility to the systematist; but that isolation is responsible for the changes which bring about the divergence of characters is a deduction no more logical than that the differences of islands are due to the waters which separate them.

Too narrow zeal in the descriptive task has led many systematists to act on the assumption that the same amount of difference should everywhere receive the same taxonomic recognition, a method sometimes defended on the ground that all variations of form or structure indicate incipient species budding out from the parent stock, and sure to become separate groups like other now segregated types, a supposition quite unsupported by evidence. Far more rational and more secure would be the progress of systematic biology if recognition as species were limited to groups of individuals separate in nature, regard being given to the completeness of segregation rather than to the amount of difference.

It is to be admitted, of course, that when specimens from a new locality offer tangible differences from any previously known, the working systematist must describe and name them as representing new species. To crowd them into an old species by "emending the description," or by calling them a "variety," is to guess at an integration in advance of knowledge; while to refuse to unite "species," which have been shown to belong to a continuous series in nature, is to prefer technical fiction to biological reality. A coherent group of interbreeding individuals is the unit of evolutionary biology to which the term species finds its most proper application. The tendency of some systematists to refer also to intergrading unsegregated subdivisions of such groups as "species," shows how easily conventional taxonomic methods may obscure evolutionary distinctions.

CRITERIA OF SPECIFIC DISTINCTNESS.

Species differ, of course, in the variability of their characters, but, other things being equal, the uniformity of the individuals of a species might be expressed by a ratio between the range and the facilities for interbreeding. A widespread species of sedentary animals or plants will become locally diversified; more frequent intercommunication permits more uniform progress. A single species may have as great a variety of characters as a dozen related groups which have been segregated. Two species may be quite distinct and yet differ much less than the connected extremes of another. That a species differs in different parts of its range does not necessarily mean that a subdivision will take place; it means merely that. characters are originating more rapidly than they spread over the whole species. The integrity of a species is not destroyed by "inconstancy" of characters, but because geographical or other barriers make a gap in the series.

The failure of the extremes of a widely distributed species to breed when brought together does not prove the attainment of specific distinctness nor the approach of it, since internal diversity does not weaken the species, but is an evolutionary advantage, and both extremes may continue to cross freely with the connecting forms which constitute the bulk of the species. Neither does the power to form fertile hybrids prove that two species occupying distinct ranges are one. Faith in such criteria is simply a remnant of the pre-evolutionary theory of the separate creation of species. The only way to ascertain that two groups of organisms are separate species is to find the gap between them. Whether they will breed together or not, and whether the hybrids are fertile and vigorous, or weak, sterile, and aberrant, may indicate the period and degree of divergence of the types crossed, but affords absolutely no evidence as to whether the series to which they belong in nature are continuous or interrupted. Specific distinctness is a question much more geographical than evolutionary. Evolution continues whether the species is divided or not; the divergence of the parts is rendered possible by the cessation of the interbreeding, which would otherwise maintain the coherence and relative uniformity of the undivided group.

SEGREGATION AND VITAL MOTION.

The systematist "separates" species because they are "different," but the evolutionary significance of species does not appear from formal descriptions of these biological islands; it lies in the fact that isolated groups of organic individuals universally acquire diagnostic differences. Isolation has furnished millions of these tests of the universality of biological motion, but it does not cause the motion. Evolution is independent of isolation, and without it has often brought about in the same species great diversity of sexes, castes, dimorphic and alternating generations of many species of plants and animals. Without evolutionary progress there would have been no species as we now know them, but the causes of the segregation of species are not causes of evolution; segregation merely permits this universal tendency to become more manifest. If it should be found that evolutionary divergences sometimes assist natural selection or physical barriers in the work of subdividing species, this would mean that evolution sometimes results in segregation, not that segregation causes evolution.

Evolution is a process of change in species; it is the journey of which individual variations are steps. Evolution changes the characters of species, but it does not originate species; it makes species different when segregation affords the opportunity.

Natural selection may assist geographical and other influences tending to the division of species, but it is not on that account a cause of evolution; it represents the determining aspect of the environment—the factors which influence the direction of the vital motion, but not those which induce the motion. Natural selection may explain differences between two species, but. not the becoming different. It is an external incident or influence and not an active principle or agency of organic evolution. Adaptation is possible because there is a vital motion which can be deflected, not because the environment changes the characters of species. The river of evolution flows through the land of environment; the conformation of the valley determines the course of the stream, but the water descends by its own gravity.

In the course of its progress the species explores the adjacent territory and follows the line of least resistance to the variations it is able to put forth. Changes are necessary to maintain the vitality of the species and also to keep it abreast of its environmental opportunities, and if no adaptive movement can be made it is still unable to remain stationary, but continues to change in characters indifferent to the environment, or even actually detrimental.

The species encounters obstacles and subdivides because it is in motion; the diversity of form arises because variations can no longer spread freely among the individuals of the species, not because the environment introduces new characters.

a) "Even sugar beets, the oldest 'selected' agricultural plants, are far from having freed themselves from the necessity of continuous Improvement. Without this they would not remain constant, but would retrograde with great rapidity." DeVries, H., 1905, Species and Varieties, 109.

That species occupy definite regions of distribution has been taken by some to mean that the individuals are similar because they are molded by similar influences, but that this inference is wrong is shown both by the wide diversity of conditions under which some species exist, and by the even wider diversity of form and structure often found among the members of the same species, in the same environment. Similarity of conditions may permit plants and animals of different origins to develop similar variations, and to share, finally, the same adaptive characters, but identical conditions do not put an end to individual variations, nor to evolutionary progress.a

THE INFLUENCE OF SELECTION.

b) Cook, O. F., 1901. A Kinetic Theory of Evolution, Science, N. S., 13: 960.

In denying that selection has any power to initiate or actuate developmental changes there is no intention to imply that it has not profoundly influenced the course of evolution in many groups. Indeed, it may be claimed that the kinetic theoryb afforded the first concrete explanation of the workings of natural selection. Vital motion not only makes selective influence possible, but it meets the ancient and hitherto fatal objection to the doctrine of adaptation, since it shows how characters may originate and develop to the point of utility or harmfulness, where adaptive selection can take effect.

That there are species, varieties, mutations, or hybrids which differ in one, two, or three characters, as commonly asserted in discussions of Mendel's laws, is quite unwarranted by facts. The mention of a single peculiar character may suffice to designate a species or variety for taxonomic purposes, but in evolutionary studies it is careless to forget that the diversity is general and multifarious, like that of individual apple trees or men. Evolution is a continuous summary or integration of this individual diversity, and is not a simple process, but highly multiplex, as much so, indeed, as the lines of descent in which the life of the species goes forward. A composite general direction is maintained by the species because the multitudinous strands of individual descent are bound together by interbreeding. The variations take place in particular threads, but evolution signifies rather the progressive change of the whole organic network.

The evolution of a new type means changes in many directions and of many kinds in the germ cells and in the various tissues and organs, as well as in the external form of the complex cell colony which we are accustomed to look upon as a single individual.

a) See Cook, O. F., 1904. The Vegetative Vigor of Hybrids and Mutations. Proc. Biological Society of Washington, 17:83.

Each cell, tissue, organ, and feature is undergoing evolution, and for normal and permanent progress these manifold developments must keep together. When single lines or slender strands of descent are separated from the main network the congruence of type is lost. The normal variation and individual diversity of the species having been eliminated, the evolutionary coordination of cells, organs, and functions breaks down, and abrupt changes or aberrations of heredity appear. These degenerative mutations may not differ in their essential nature from normal variations, but the conditions of their appearance are abnormal, and the results often disastrous.a Mutations, like hybrids, are sometimes completely sterile, and they may have at the same time an increased vegetative vigor. The vegetative vigor of many mutative varieties of domesticated plants has doubtless delayed the recognition of their abnormal evolutionary status, though the abnormality of infertile hybrids has long been appreciated. It is paradoxical, indeed, that the increased vigor which accompanies normal variations and crosses should also attend degenerative changes, but there is room for this apparent contradiction in so complex and many-sided a process as evolution.

A domestic variety may be "improved" by the further increase of the one or two characters or qualities which render it valuable, but a new specific or generic type is the compound or resultant. of many variations in many characters. By close selection, which restricts evolutionary progress to a narrow line of descent, a "single character" may push out farther in a decade than the natural multiplex evolution would carry it in a century or a millennium, but such a specialization weakens and imbalances the organisms, and is a process of degeneration rather than a constructive evolution. Selective inbreeding and other forms of isolation accentuate single. characters, but the interbreeding of normally diverse individuals (symbasis) weaves new types out of the variations of many lines of descent.

a) O. F. Cook, 1903, "Stages of Vital Motion," Popular Science Monthly, 63: 16.

All organisms are subject to selective influence in the sense that variations are rejected with a promptness proportional to their harmfulness in the given environment, but generally this leaves a very wide latitude of possible changes in which selection does not interfere. The instances are relatively rare in which existence becomes acutely dependent upon the development of some one characteristic or quality, and such narrow selection does not strengthen the type, but insures and hastens its extinction.a

The neglect of this distinction vitiates much evolutionary literature, both that which treats selection as an actuating "force" and that which rejects selection for "discontinuous variation" or the "mutation theory." It is true that many variations of inbred domesticated plants and animals are very abruptly discontinuous, and that such changes are not caused by selection, but these facts in no way militate against others equally obvious, that the natural evolution of new types is a relatively slow and gradual process, and that selection influences the direction of this continuous vital motion. The older selective hypothesis was only half erroneous. Selection does not set stationary organisms in motion, but it often guides spontaneous change. It does not explain evolution or vital motion in general, but it does explain adaptation, or motion in some particular direction, as when one species differs from its relatives in special characters which enable it to exist in a special environment. That all adaptations are mere coincidences is as improbable as that all characters represent useful adaptations.

Selection is not, as many "Darwinians" have maintained, the true, efficient, cause of evolution. The vital motion of species proceeds whether selection is operative or not. Species do not acquire characters from the environment, but merely in accordance with it. At any point in the evolutionary journey selection may determine whether certain characters shall be acquired or not. It is an obstacle in the environmental road over which the species would travel, instead of being the source of power of the organic automobile. Selection prevents motion in one direction, but permits advance in another. It explains why a particular character becomes accentuated in a particular species, but is no more a cause of the developmental progress of the species than the turns of the road are the motive power of the vehicle.

The hypothesis of selection as the active principle or causal agency of evolution became illogical and useless as soon as the inheritance of acquired characters was discredited. The first idea without the second does not account for adaptations. The "selection" of Nageli, Weismann, and other believers in a "determining principle" or "hereditary mechanism" of evolution is a very weak substitute for the original Darwinian idea, since it is able to eliminate only the hopelessly unfit, but is quite without means of influencing the survivors.

Segregation enables species to attain differential characters, and selection assists their accommodation to environment, but both these possibilities rest on the more fundamental fact that organic evolution goes forward without external causation in groups of diverse, interbreeding individuals. If a species stood still selection could effect nothing except its partial extinction. In the recognition of a continuous and universal evolutionary motion the kinetic theory supplies the long-sought explanation of selective influence. Selection ceases to be a mysterious evolutionary cause, but retains a practical and easily comprehensible evolutionary function.

THE SPECIES A PROTOPLASMIC NETWORK.

a) Cook, O. F., 1899. Four Categories of Species, American Naturalist, 33: 287. The "species" into which paleontologists arbitrarily divide geological series of organisms may be explainable by evolutionary progress alone, but the multiplication of the contemporaneous species of a given horizon is a different question.

The traditional illustration of organic descent by a tree with ever-dividing branches is entirely misleading as a suggestion of the nature of evolutionary processes, because individuals do not follow each other in simple series. Successive generations are connected by endless intergraftings of the lines of descent. A species may be treated systematically or statistically as an aggregation of individuals, and may be described by an averaging of the characters of these, but from an evolutionary point of view it does not exist as a species because of the possession of a certain complex of characters, but because the component individuals breed together; through this alone is the integrity or coherence of the species maintained. For evolutionary purposes we may think of the same species existing thousands of years hence, and with any or all of its characters changed.a It is not necessary even that the individuals of a species remain alike; in many unrelated natural groups extremely diverse sexes, castes, and "forms" remain associated in the same species and travel together on the evolutionary journey, sharing the same environment, but without any tendency to become "exactly alike." Moreover, we know that sexual and other diversities inside the species are not casual or accidental, but normal and advantageous, facts quite overlooked in static theories, which have viewed life from a narrowly systematic standpoint and have argued that interbreeding is a hindrance to evolution in that it prevents the preservation of new characters.

The kinetic theory, on the contrary, ascribes the fact that organisms are everywhere bound up into species to a property of fundamental evolutionary importance, and interprets the multitudinous devices for maintaining the coherence of groups of interbreeding organic individuals and the equally general manifestations of sexual and other diversification inside specific lines, as due to the same requirement of protoplasmatic organization, an interlacing network of descent. Without cross-fertilization species would not cohere, but would split into numberless independent, diverging lines. This takes place with organisms long propagated asexually, whether artificially or in nature. For example, the genus Sphagnum, which very rarely produces spores, offers a multiplicity of varieties nowhere approached among mosses having normal sexual reproduction; but, notwithstanding so many differences in minute details, Sphagnum has remained a very compact, unprogressive group. Cross-fertilization prevents this type of diversification, but it need not on that account be supposed to impede evolutionary progress. Evolution is not merely a progressive diversification; it requires also a progressive synthesis of characters by the interbreeding of the individual members of specific groups.

That sexual reproduction is a substitute or improvement of multiplication by fission is another partial and misleading view which has contributed much toward the concealment of the causes of evolution. The division of cells is the only method of organic increase; conjugation is not multiplication, but serves as a preliminary stimulant to the necessary cell division. What is growth, for example, among the filamentous algae, composed of chains of cells, is reproduction among the unicellular species where divided cells become separate individuals. Only among the simplest organisms, if anywhere, is indefinite reproduction possible without the assistance of conjugation.

The new science of cytology has made us aware that the division of cells is not a passive or a simple process, but is extremely active, complex, and varied. Living protoplasm is in motion, and the discovery that cell walls are not hermetically closed, but are perforated by delicate protoplasmic strands, lends strength to the belief of some biologists that protoplasm circulates, not only inside the individual cells, but through the entire organism. Conjugation may signify that such a circulation extends also throughout the species. Or, to vary the analogy, the net-like structure of protoplasm may be thought of as continuous, not only in the individual, but as binding together the whole species by the intercrossing of the lines of individual descent. As individual organisms will in different degrees endure subdivision and are able to restore or regenerate the lost part, so species may survive a certain amount of segregation; but if too small a group of individuals be cut off, it perishes through the reproductive debility long recognized as inherent in inbred or narrowly segregated organisms. For taxonomy the tree motion of descent was sufficient as means of indicating the history and affinities of species and higher groups, but evolution is a process which must be studied inside the species, and here the diagram of relationship is not dendritic, but reticular.

SYMBASIS A CAUSE OF EVOLUTION.

a) Symbasis signifies etymologically a moving together or in company, and refers to the fact that organisms exist and make normal evolutionary progress in groups, rather than on simple or narrow tines of succession. The word may be used also in a physiological sense, to indicate a normal and advantageous range of interbreeding among the individuals of organic groups. It is to be distinguished on the one side from wide cross-breeding and on the other from narrow Inbreeding, both of which produce inferior offspring and interfere with evolutionary progress. The confusion arising from the very frequent use of interbreeding in the contrary sense of inbreeding would also compel the introduction of a new and unambiguous term.
b) Among the bees fertilization may be omitted for a single male generation, and among the plant-lice for several wingless generations, but such instances are admittedly exceptional and specialized.

If reproduction by means of cell division is reckoned as an essential property of protoplasm, equally fundamental importance can scarcely be denied to the property called symbasis,a which requires this interweaving of numerous lines of descent and this simultaneous movement of organisms in specific groups. As organic complexity increases there is a greater necessity for cross-breeding, as evidenced by the accentuation of sexual diversity and by the decline of asexual propagation and of the power of regenerating lost parts. Organisms which have traveled farthest upon the evolutionary journey are most dependent upon symbasis. Nowhere among the higher animals, including many thousands of species of arthropods and vertebrates, is there known to be a long-continued series of nonsexual individuals.b In comparison with the higher animals, plants are but loose and unspecialized aggregations of cells, and yet among them also sexual differentiation has made great progress, and in some orders contrivances to insure cross-fertilization are highly developed.

a) Under a kinetic theory the existence of sexual reproduction and cross-fertilization In many fungi in which these processes are still unknown may be inferred from the simple fact that the Individuals are grouped into well-defined species.

The extent to which conjugation exists among the lower groups is not yet determined. That it may be omitted for many generations of a simple organism should not be taken to mean that it is entirely absent or has no importance, since among the higher animals, where cross-fertilization is recognized as indispensable, the growth of the body to maturity requires millions of cell divisions, each of which would mean a new generation in a unicellular species. The supposed absence of sexual reproduction in certain parasitic and saprophytic groups is a confirmatory exception in view of the obvious degeneration of such organisms.a

To the many speculations on the purpose of sex and cross-fertilization it can do no harm to add the conjecture that the presence of moderately diverse qualities of protoplasm facilitates cell division. Some have held that the function of sex is to assist evolution by producing variations, and others that it neutralizes variation by maintaining a stable average. From the kinetic point of view it appears that symbasis, as represented by the phenomena of sex and of cross-fertilization, is not an impediment to evolution nor a device to cause variation, but a means of communicating it. Variations appear without sex, and may even be accumulated, as by the adding of one bud variation to another in plants propagated by grafts or by cuttings, like the breadfruit, apple, and banana. Such progress is, however, slow and halting, and is accompanied by a decline in reproductive fertility. Symbasis not only sustains the vitality of organisms already evolved, but it is directly responsible for the upbuilding of the complex structure and vital economy of the higher plants and animals, and it builds the faster when by the differentiation of sexes two sets of variations can be accumulated.

To symbasis is due also the arrangement of organisms in the coherent groups called "species," or what may be termed the specific constitution of life. Conjugation is the means of symbasis, as division is of reproduction. Sexual and other dimorphism, and the numerous specializations, devices, and instincts by which cross-fertilization is secured, are aids to symbasis, just as the spore-sacs, ovaries, and placentae facilitate reproduction. The phenomena of reproduction and those of symbasis are combined, perhaps inextricably, but all attempts at assigning them to a single cause or property have failed.

a) Static theories, under which species are held to be normally stationary, may be subdivided into two groups, those which look upon evolutionary progress as gradual and as actuated or carried along by natural selection, and those which treat the motion as discontinuous or saltatory and due, not to selection, but to abrupt variation or mutation. Selective theories, again, may hold either that the environment causes the desirable variations or "acquired characters," or they may Imply the motion of a somewhat constant range of variability in species, which are thought of as growing out farther on one side because selection keeps them pared off on the other. Movement is thus ascribed variously to the direct action of the environment, to selective isolation, to abrupt transformation or mutation, or to some combination of these.
     Under Haeckel's biogenetic law evolution appears as a resultant of (1) palingenesis, a positive hereditary tendency to repeat the ancestral form, and (2) kenogenesis, a negative, disturbing, adaptive Influence located in the environment. The late Professor Hyatt summarized a similar view by characterizing heredity as gentipetal and environment as gentifugal, the one tending to make all individuals alike, the other causing difference and evolutionary progress.
     The kinetic theory depends upon none of these supposed factors, but interprets vital motion as continuous, gradual, and self-caused, or inherent in the species, though the environment is thought of as Influencing the direction of organic change. Selective Influence Is neglected altogether by still other theories, such as that of Naegeli, in which evolution is explained by an internal "hereditary mechanism," supposed to carry the species along in a definite direction.

Cross-fertilization is commonly misunderstood to be merely an accessory of reproduction, and a negative factor in evolution, because it is supposed to conduce to the permanence. of the specific type by averaging away the new characters which arise as individual variations. There is the amplest experimental evidence that crossbreeding is necessary to maintain the quality and efficiency of the individual, but static theoriesa have led to the belief that evolutionary progress requires conditions unfavorable to the individuals of which species are composed, since under such conditions selection is most effective and abrupt variations are most striking and numerous. The alternative kinetic theory holds that cross-fertilization, as the active agency of symbasis, is a positive and primary factor of evolution, coordinate with variation itself. Symbasis is the multiplier of the evolutionary equation; it brings about the distribution and combination of individual variations into the resultant vital motion of the species. Evolution no longer appears as an abnormal or exceptional phenomenon, and it becomes clear that the conditions under which the species is most prosperous are also those which permit the most rapid evolutionary progress.

THE PREPOTENCY OF VARIATIONS.

a) Mr. William R. Maxon, of the U. S. National Museum, calls my attention to a very early announcement of the mutation theory.
     "The truth is that species, and perhaps genera also, are forming in organized beings by gradual deviations of shapes, forms, and organs, taking place in the lapse of time. There is a tendency to deviations and mutations through plants and animals by gradual steps at remote irregular periods." Rafinesque, 1832. Quoted in Journal of Botany, 30: 310.

The first corollary of the law of symbasis is the prepotency of variations. The combination of variations not only permits the structure of the organism to be strengthened and rendered more efficient, but also gives prepotency, due to the opportunity of vital motion. Variant individuals being thus both vigorous and prepotent, it is easy to understand why diversity, and not uniformity, is the tendency of normally extensive species; changes are necessary and welcome, and the perpetuation of them does not require segregation. Numerous and well authenticated instances of distinctly prepotent variations are known, and such were taken by Mivart and other zoologists to prove that species do not originate by gradual change, but abruptly or by "extraordinary births," a view quite similar to the more recent theories of mutations,a but distinctly more practical, because the "mutations" of plants which are the basis of the inferences of Professor De Vries are not prepotent but "recessive," presumably because they do not represent true genetic variations, but are symptoms of what may be described as an evolutionary debility, due to inbreeding. The disappearance of mutative characters when the new variations are crossed with the parent form or with each other is merely the recovery, as it were, of the health of the species when the abnormal condition of inbreeding has been removed, as shown so conclusively in Darwin's well-known experiments with pigeons, and confirmed by an abundance of similar facts. Though differently interpreted, many other facts supporting this view were collected by Darwin, who summarized the results of his studies of Ipomea, Digitalis, Origanum, Viola, Bartonia, Canna, and the common cabbage and pea, as follows:

b) Darwin, The Effects of Cross and Self Fertilization in the Vegetable Kingdom, p. 27. New York, 1895.

The most important conclusion at which I have arrived is that the mere act of crossing by itself does no good. The good depends on the individuals which are crossed differing slightly in constitution, owing to their progenitors having been subjected during several generations to slightly different conditions, or to what we call in our ignorance "spontaneous variations."b

Differences between the plants of different habitats mean also different lines of descent and attendant variations, and the beneficial results of bringing these together may be explained by reference to symbasis rather than to the "slightly different conditions."

While it may not be insisted that species, as described and named by systematists, are never originated by "extraordinary births" or from "mutations," both suppositions are obviously improbable as general explanations. Mutations are seldom fitted to survive because they are less vigorous and less fertile than the parent type, so that they must be segregated at once in order to be preserved. And even prepotent variations have no necessary connection with the origination of species, since however rapidly the characters of a species might change, it would still be the same species until a subdivision had taken place. The more a species evolves the more different from its relatives it becomes, and the more satisfactory for the purposes of systematic study, but this progressive transformation of the "type" carries with it no necessity for subdivision, nor any indication that evolution is concerned with the origination of species.

SUMMARY.

Evolutionary study and thought have been hindered by the confusion of two unrelated biological phenomena, (1) evolutionary progress or vital motion, and (2) the origination or multiplication of species. The "origin" of a species is not more evolutionary than any other stage in its history. The causes of the subdivision of species are not causes of vital motion; the two processes are quite distinct. The separation of two species is not a focus of the evolutionary problem; it is a mere incident of developmental history.

Segregation is the principle or active cause of the multiplication of species, but the nature and causes of evolutionary progress are not to be ascertained by discovering that species originate by subdivision. Vital motion is continuous, and is neither actuated nor interrupted by the segregation which multiplies species.

Natural selection may assist in the segregation of species, but it is not a factor in evolutionary progress except as it influences the direction of vital motion. Specific groups become diverse when the component individuals no longer share their variations through symbasic interbreeding; not because new characters are induced by external influences. Evolutionary divergence may take place under identical conditions, and in characters which have no relation to the environment and no value to the organism except to permit the necessary vital motion.

A stationary heredity or the continued repetition of an identical structural type exists nowhere in nature; variation is an inherent evolutionary property. Segregation is not necessary for the preservation of variations; genetic variations are prepotent and are more rapidly propagated by crossing with the parent form.

A second evolutionary property of organisms is symbasis, which has built up the complex structure of the higher animals and plants by combining individuals into the interbreeding groups called species. The evolutionary species is not a complex of characters or a mere aggregation of similar plants or animals; it is a protoplasmic network held together by the interbreeding of the component individuals. Symbasis accelerates vital motion, but hinders the multiplication of species.

Species and evolution are different aspects of the same fact; evolution goes forward within specific lines as a manifestation of the same property which necessitates the existence of species; variation and cross-fertilization are not antagonistic phenomena, but are two phases of the same creative process.

Cook bibliography