Proceedings of the Washington Academy of Sciences, 7: 301-323 (March 19, 1906)



MORE fundamental than any other problem relating to organisms is the question of the method by which their development has been accomplished. Any progress in this direction places us nearer to an understanding of the real nature and essential conditions of organic existence. To solve the problem, or even to approach a solution, requires a choice to be made among the infinity of biological data which science has already amassed, to say nothing of the still greater multitudes of unrecorded facts which encounter us on every side. Without a choice of clues or a criterion of evidence, our search is unscientific, hopeless wandering, with every probability of failure and no prospect of success.

The history of evolution has furnished, even in its first half-century, ample evidence of the truth of this fact. The first step toward a causal explanation still remains to be taken; indeed, we have not yet decided which way to face in taking it; whether to seek the causes of evolution in the environment or in the organisms themselves. The present paper gives reasons for believing that the chief agency of evolution is to be found in the association of organisms into interbreeding species, and, not in the external conditions, nor in the isolation of individual variations. A species is not a mere aggregation of similar individuals, but a fabric of interwoven lines of descent, and it is only in such associations that evolutionary progress goes forward, or that the vitality of organisms can be permanently maintained.

1) A Kinetic Theory of Evolution, Science, N. S., 13: 969-978. 1901.

The standpoint from which these biological relations can be perceived was indicated as far back as 1895, but was first formally presented in 1901.1 It differs fundamentally from earlier structures in the same field of thought in its conception of the nature of evolutionary motion. Two alternatives had thus far monopolized the interest of the scientific world, and both had proved to be inadequate to accommodate the facts of organic existence, or to conduct us toward more detailed explanations of them.

2) Evolutionary Inferences from the Diplopoda, Proc. Entomological Society of Washington, 5: 24, March, 1902.

The progressive development of organisms had been conceived as due (1) to environmental causes, and (2) to determinate internal forces or "hereditary mechanisms." The kinetic theory was the result of contact with facts which showed that both these suppositions were wrong. The true actuating causes of evolution do not lie in the environment. Neither is the forward progress or vital motion of species determinate, or restricted to a particular direction; it has great freedom of choice of environmental opportunities.2

The kinetic interpretation accommodates and admits natural and consistent relations between numerous other facts which had appeared to conflict with each other or with the doctrines which had undertaken to explain them. The normal condition of evolutionary progress is found in symbasis, that is, in the traveling together of the members of the specific group. New variations among the individuals of such groups are prepotent and can be preserved, whet"The Law of Adaptive Radiation."her useful or not, without being segregated. The environment does not cause the evolutionary variations, but it can induce adaptations by restricting the progressive development of the species to particular directions or characters. Selection is thus a negative factor, instead of a positive or actuating agency of evolutionary motion.

In line with the previous teaching, that evolution is due to the environment, it has been held that interbreeding hinders or prevents evolution by interfering with the preservation of new variations; sexuality, in other words, has been reckoned as anti-evolutionary. In complete contrast with this is the kinetic interpretation, that the continued interbreeding of the numerous and diverse individuals of the species is essential to sustained organic progress. Evolution becomes, in short, a sexual process. This distinction is not merely a matter of terms and definitions, but is capable of being tested by application to established facts of evolutionary history.

In accordance with the earlier view, that sexuality was anti-evolutionary, it has been assumed that the complex and specialized bodies of the higher plants and animals are asexual structures whose development has been accomplished by the suppression of sexuality in alternating generations of individuals. A more careful inspection of the facts shows that instead of evolution having been accomplished through alternation of generations, or having been accompanied by a greater and greater accentuation of asexual structures, it has remained closely attached to the sexual process of cell-conjugation, and dependent upon it. The bodies of the higher plants and animals are not built up between conjugations or subsequent to the completion of the conjugation of the parental reproductive cells, as often supposed. The reproductive cells divide and build up the new structure while still in the sexually double or conjugating condition.

1) Cook, O. F., and Swingle, W. T., 1905. Evolution of Cellular Structures. Bulletin 81, Bureau of Plant Industry, U.S. Dept. of Agriculture.

This phase of the subject has been treated in a previous publication.1 The present paper undertakes only a brief and informal presentation of some of the general consequences and applications which flow from the recognition of symbasic interbreeding as the normal condition of organic existence, and of evolutionary progress. By emphasizing and applying the fact that organic descent is a continuous network, it seeks to avoid the danger of mistaking the results of violations of the law of symbasis for examples of genuine, constructive evolution. All evolution might be described as organic change or motion, but it is not safe to assume the converse, that any and all organic changes represent evolution. Degeneration is quite as general a phenomenon as evolution, and the two are easily confused.


Many discussions of evolution rest upon abstract terms which have no concrete meaning or definite application. Such expressions as prepotency and reversion are veritable stumblingblocks in the evolutionary theories of those who use the words without taking into account the different relations of the phenomena grouped under them. Having once made the assumption, for example, that mutations are instances of a normal saltatory evolution, it is natural to look upon the prepotency which brings "reversion" as tending to prevent evolutionary progress by "the swamping effects of intercrossing," of which the last decades have heard so much. Segregation appears essential for the preservation of new characters; it becomes, in other words, a primary factor or condition of evolution. This series of deductions leads, however, to a biological absurdity, because extreme segregation or inbreeding not only puts an end to true evolutionary advance, but causes the deterioration of the organisms themselves.

The phenomena which have been interpreted as mutations and reversions can be accommodated under a kinetic theory of evolution without this fatal inconsistency of inference. Instead of affording progressive new characters, or constituting new species, there are reasons for believing that mutations are digressive lapses from normal heredity, induced by inbreeding or too great segregation. The "prepotency of the wild type" which "swamps" these abnormalities is not a backward step along the highway of evolutionary progress. It marks, instead, a return from a too narrow sidepath. The reversion is only formal; it represents a restoration rather than a retrogression.

Evolution has seemed to go backward only because the sidepath has been mistaken for the main thoroughfare. The prepotency which seems to obliterate the mutational "new species" is the same which carries forward the evolutionary progress of the whole specific aggregation of interbreeding individuals. The real and permanent advance is made in the main body of the species, not among the stragglers from the flanks, nor by the distraught captives of our cages, pastures and gardens.

That the plant mutations which "come true to seed" are often extremely uniform or constant, does not make it certain that they are true species, but indicates, rather, the contrary, since prosperous natural species show abundant individual diversity. To give such "sports" formal descriptions and Latin names does not prove that they represent genuine species formed in the normal course of evolution; it simply assumes the identity of two biological conditions essentially distinct.

The possibility that mutations, or even genetic variations, may also be induced by new environmental conditions, as believed by Darwin, is not excluded. But even in such cases the environment would need to be regarded as furnishing the occasion of the change, rather than as being the true, actuating cause. Very diverse mutations, of the coffee plant, for example, have been found to arise under the same environment, and closely similar mutations under very different environments.

The changes by which many organisms are able to accommodate themselves to different conditions appear to be of little or no direct significance for evolutionary purposes, though the diversity manifested under the different conditions may serve the same physiological purposes as other intraspecific differences in connection with symbasic interbreeding. Evolution is an integration of genetic variations, not of environmental influences.

Segregation, or isolation, conduces to the formation of new species by the subdivision of older groups, but it is not on that account to be reckoned as a cause of evolution. Free interbreeding throughout the range of a species tends to keep the characters uniform, but it does not tend to keep them stationary. The characters remain relatively uniform because interbreeding holds the members of the group well together on their evolutionary pathway, not because progress is prevented by interbreeding. Free interbreeding "swamps the incipient lines of variation" only when the change is of a degenerative nature, and not truly symbasic and constructive.

If the two geographical halves of a species become separated they will also become different, but this only shows that evolutionary motion is everywhere taking place; it does not prove that either of the new species has travelled farther than the undivided group would have gone, or that segregation has served as an agency of evolution. Evolution has very little to do with the origination or subdivision of species; this is almost entirely a matter of segregation, geographical or otherwise, and is a mere incident of the process of change. That separated groups of organisms so universally and so promptly become different, affords the strongest possible testimony that evolutionary motion is not determinate or limited to one direction, but it gives no warrant for looking upon isolation as contributing to evolutionary progress.

As general evolutionary factors, natural selection and geographical isolation are negative and restrictive; they influence, but do not actuate, the progress of species.

To say that isolation causes species-formation because it brings the separated groups under different environmental or selective conditions is only to confuse the issue. Segregated groups become different, even in the same environment, and in characters having no relation to environmental differences which may exist. Unsegregated groups can remain relatively uniform in very different environments. No evidence has been found that any action of the environment can produce evolution, either by direct transformation or by the indirect influence of selection and segregation. All nature abounds, on the other hand, with evidence that evolution can take place without environmental differences, without selection, without isolation. Evolution takes place without any external cause or compulsion, and is capable of no explanation which does not recognize the fact that specific groups or organisms, no less than sidereal systems, are in motion.

1) The abnormal amplitude of mutational variations has been likened in another place to the unusual fluctuations of temperature in disease. The abnormality is in the conditions; mutations may be of the same essential nature as normal variations, into which they seem to grade as insensibly on the one side as they do into obvious monstrosities on the other. Professor DeVries has explained that he gave Oenolhera lamarckiana special attention in his search for mutations because it was "rich in monstrosities." DeVries, 1905, A New Conception of the Origin of Species, Harper's Magazine, 110: 212.

The normal individual diversity which has been destroyed by inbreeding is not restored by mutation; possibly it would reappear if the different mutations were propagated in sufficient numbers and allowed to intercross freely; but in domestication they always suffer still further inbreeding.1 Finally, even crossing ceases to be effective for restoring the normal condition of intergraded individual diversity. Hybrids of inbred mutations often follow closely the parental lines, and soon separate again into the distinct types, as discovered by Mendel. It is hoped by some to recombine these fag-ends of undone creation into "new species," but this is to see Persian rugs in rag carpets, or oil paintings in three-color prints.

A general misconception of the nature of evolutionary motion has arisen because attention has been directed so largely to domesticated species, in which descent has been limited to single or very narrow lines. Phenomena of degeneration induced by inbreeding have been interpreted very often as results of changed environmental conditions. The mistake has been made of supposing that evolutionary progress is a mere resultant of external influences, whereas it is in reality a highly composite motion carried forward in the intricate network of descent of the normally interbreeding species. Natural selection forbids the weaving of patterns discordant with the environment, but no external influence actuates the loom. Nor need we allege any other and more hypothetical force or agency as conducting the change, the necessity of which is inherent, not in the individual organisms as such, but in the association of diverse individuals in interbreeding groups or species. If the physical basis of this law of symbasis were understood the general fact of evolution would also be comprehended as a natural and necessary consequence. Instead of preventing evolution by "swamping effects" symbasic interbreeding is the true method or principle by which evolution has been accomplished.

Normal descent does not go forward in simple series of uniform individuals; it is a broad network of closely interwoven diversity. Once frayed by inbreeding into narrow, "unit-character" shreds, the vital fabric is hopelessly weakened, and the hereditary pattern distorted. The higher the organisms the more acute the requirement of symbasic interbreeding, and the more prompt and obvious the damage wrought by abnormal segregation. To insist that mutational aberrations are suddenly originated, genuine species, is the same as to assert that the idiot offspring of cousins afford true examples of the steps by which the perfection of the human race has been attained.

1) Stages of Vital Motion, Popular Science Monthly, 63: 14. May, 1903.

Through long-continued selective inbreeding, cultivated plants have been broken up into numerous local varieties of mutative origin. These are frequently quite as distinct from each other, in the purely descriptive, taxonomic sense, as wild species in nature, but their evolutionary status is very different. Wild species in the truly normal and progressive (prostholytic1) evolutionary condition have a multifarious, intergraded individual diversity, not to be found in mutative varieties. Species which have not been domesticated too long show the intermediate (hemilytic) condition of retarded evolution. Inbreeding has induced an abnormal uniformity in which the degenerative mutations begin to appear.

Thus the coffee shrub has not yet become a mere congeries of local varieties, but has an astonishing uniformity of type. Seeds brought from remote regions and sown in the same place produce plants of almost indistinguishable likeness. Of very distinct, true-to-seed mutations of coffee, however, there is no longer any lack, but very few of them have been preserved and cultivated, because of their inferior powers of seed production — a very practical proof of their degenerative nature. That adverse conditions or abnormally restricted distribution may bring about in nature evolutionary conditions analogous to those of our domesticated plants, is, of course, to be expected, but very rare and local species are correctly looked upon as remnants verging toward extinction rather than as ascendant newborn types.

Some have thought to reconcile the idea of a progressive evolution with the older notion of constancy of characters among the members of a species by supposing that evolutionary changes proceed by imperceptibly gradual, infinitesimal steps, and must therefore have required millions on millions of years. As a matter of fact, however, differences between the individual members of species in nature are commonly quite perceptible, and often strikingly obvious.

1) The Evolutionary Significance of Species, Smithsonian Report for 1904, p. 397.

It has been attempted, also, to distinguish between what are called continuous, or gradual, and discontinuous, or saltatory, variations, the former to be found within specific lines, the latter initiating new species. This distinction is artificial and misleading; variations may be discontinuous but they do not disconnect the species. No reason is apparent why a species might not be completely transformed within a few years, decades or centuries through the acceptance, by all of its members, of a new character or characters. Instances where such changes appear to be going on have been adduced by several naturalists. Evolutionary progress can be accomplished in this way much more rapidly than if it were necessary to replace the older form of the species with the progeny of a mutation, which needs to be kept isolated from the older species lest it be swamped by intercrossing. Prepotency, the power to transform the species, instead of being swamped, is the practical difference between genetic variations and mutations.1

The kinetic theory sets no limits to the length of the steps, nor to the rapidity with which they may be taken. It implies, however, that the evolutionary progress of the species goes forward as a network of descent, broken neither by sudden transformations nor by periods of stationary constancy. As far as our present perceptions carry us, variations may appear fortuitous. Evolution, however, is not accidental nor casual, but necessary and universal. Neither is it passive nor intermittent, but persistently and continuously conservative and constructive.


1) The Heredity of "Angora" Coat in Mammals, Science, N. S., 18: 760, 1903.

That inbreeding induces many of the evolutionary aberrations of domestic plants and animals is shown by the fact that such characters commonly disappear in crosses with the symbasic, or freely-interbred, wild type. Darwin's classical experiments with pigeons have been repeated and supplemented by many observers in Europe and America, and additional testimony of the same kind has been published recently by Professor Castle.1 The "Angora coat" and other similar abnormalities of inbred animals are found to be recessive, in the Mendelian sense; that is, the long hair disappears when crossed with the short.

Whether such characters are "recessive" or "dominant," or whether they appear at all, may depend on the relative degrees of inbreeding, rather than upon any special strength or weakness of characters as such. Like normal genetic variations, mutations are prepotent with their own equally inbred relatives, but abnormalities induced by inbreeding can be corrected when more remote lines of descent are brought together. Professor Castle maintains that to preserve such mutations as the longhaired guinea-pigs and horses they must be bred with others of like kinds, but in accordance with the present interpretation it will be found more effective to continue inbreeding with their own immediate, unmutated relatives. The fact that these longhaired mutations arise in the first place from short-haired parents, should not be overlooked.

Plant mutations which can be propagated asexually or by self-fertilization are often remarkably constant. With animals the experiment is more difficult because some crossing, at least of individuals, is necessary to reproduction. The remote chance that mutations sometimes initiate new species would be still further attenuated if it were necessary that two of the same kind arise at the same time and place in order to make possible the preservation of the new type.

2) A Kinetic Theory of Evolution, Science, N. S., 13: 969, June 21, 1901.
Under the kinetic theory2 no fundamental importance is ascribed to mutations. The fact that one member of a group of inbred individuals has mutated, is accepted as an excellent reason for believing that others are ready for the same step, thus explaining at once the relative prepotency of a mutation under continued inbreeding, and its "reversion" in the presence of the wild stock or of a more symbasic breed. On the other hand, the crossing of two mutations of distinct ancestry, even though of closely similar form, constitutes a decrease of inbreeding, and carries with it a possibility of restoration to the normal type. Darwin found that crosses of unrelated white pigeons "reverted" to the blue plumage of the wild type, but he did not hold that such precarious, pathological variations are factors in the evolution of species in nature. Symptoms of disease have often helped, however, to understandings of healthy functions.

Mutations are abnormal manifestations of the normal phenomenon of variation or diversity inside the species. The prepotency of mutations when bred with their own inbred relatives corresponds to the prepotency of normal variations. The "reversion" or negative prepotency of a mutation in the presence of a more widely symbasic stock does not prove that new species originate in nature by the segregation of mutations; it simply increases the improbability of a general theory of evolution built on the narrow basis of the mutations of domesticated plants and animals.

The rejection of the hypothesis of the origin of species through mutation does not make it necessary to disregard any of the facts which have been collected to support it. The objection is not to the data, but to the generalization, and to the use of a standpoint which can be maintained only while other equally pertinent facts are disregarded.

1) Science, N. S., 19: 110, January 15, 1904.

In his report of experiments on "Color Inheritance in Mice"1 Professor Davenport notes that albino mice of mixed parentage were found to be more prepotent, or less completely recessive than those of pure descent. Instead of more gray progeny as an inheritance from the gray parent, they gave a larger proportion of white offspring, a result as directly in accord with the kinetic theory as it is at variance with the current mechanical explanations of Mendel's laws. White mice are now an inbred domesticated variety while the gray mice with which they are compared have had much more recent opportunities of interbreeding. Recessive gray mice can doubtless be secured by inbreeding, and dominant white mice by interbreeding.

That mutations like those which "Mendelize" as "pure recessives" should be able to "revert" after many generations, to a parental type by crossing with each other, would also seem to show that the whole question is one of ancestry and methods of descent, rather than of pure germ cells, chromosomes, or character units. Such explanations of Mendelism can only show in higher relief the abnormality of the phenomenon, instead of justifying themselves as general "principles of heredity."

In the higher plants and animals the conjugation of the parental nuclear elements is not completed until the fusion of chromatin, or mitapsis, has occurred, before the so-called "reducing-division" which precedes the formation of the germ-cells for the next generation. Inability to form normal germ-cells may explain why the line of descent is broken at the stage of sexual reproduction, in sterile mutations and hybrids, though in other cases equally fatal derangements may appear, either before or after the reproductive period. The failure of the chromosomes of sterile hybrids to behave normally is no proof of the existence of a predetermining "hereditary mechanism"; it is but one of the many related phenomena which show that the evolutionary mischances of hybrids and mutations are not confined to the external form, but may affect any part of the organism, and even the cells of which the body is composed.


Evolutionary debility and derangement through inbreeding are old and well-known facts, but, notwithstanding the frequent use of the term, it has yet to be shown that there is any such phenomenon in nature as reversion, in the strict sense — any actual doubling back upon the evolutionary road. There is sometimes an arrest of development; accidents or unfavorable circumstances may keep a plant or animal from attaining the normal stature or form of its species, and thus leave it with a suggestion of a more primitive or ancestral type. A variety narrowly selected in one country to secure the accentuation of its peculiar characters, may deteriorate, or fail to reach the same degree of specialization when the cultural conditions of growth are changed. Through degeneration, or loss of complexity, a species may appear more primitive or less evolved than it really is. To reversion is also ascribed the occasional cropping-out in the individual of some ancestral peculiarity (atavism), but these minor fluctuations of form minister to the healthful diversity of the species, and are far from proving that evolution has turned backward. The transformation of pistils and stamens into petals, as in the formation of double flowers and similar mutative changes, is not, as sometimes supposed, a reversal of evolutionary processes, but is in the direction of developmental history--an over-shooting of the mark, as it were. Reversion would change petals back to stamens; this seldom happens, and when it does we recognize it as a recovery of normal form and function. It is now coming to be appreciated that the evolutionary history of the higher plants has involved a progressive sterilization and vegetative specialization of parts which were once devoted to reproductive purposes. Even the cells of which the bodies of the higher organisms are composed are sexual in their origin and represent a condition of prolonged conjugation.

The final inconsistency in terms is reached by those who have suggested reversion as the cause of the same phenomena which it is held to obliterate, that is, the mutations themselves. This is to use the one word reversion in two directly opposite senses.

1) The "Maragogipe" mutation of Coffea arabica, for example, has a superficial resemblance to Coffea liberica, and has been held by some to be a cross between the two. Other mutations of coffee originating in Central America share features of several of the wild African species.
    Mr. Luther Burbank has found that hybrids also are sometimes more obviously similar to other members of the genus than to their own parents. Thus the Wickson plum, a hybrid between Japanese varieties of Prunus triflora, was believed by Professor L. H. Bailey to be descended from P. simoni, a Chinese species which had not been introduced into the United States at the time when the cross was made.
    The same phenomenon occurs among human hybrids. Mulattos are sometimes very black, and sometimes white. Wallace observed in the Portuguese settlements of the Malay Archipelago that the mixed population has "become darker in color than either of the parent stocks," and in Brazil that crosses between Portuguese and Indians are "not infrequently lighter than either parent.' (The Malay Archipelago, p. 257.)
    "Another clear fact is the rapid loss of resemblance of the offspring to the Indian parent, the white element always predominating; the aboriginal seems to be merged into the Spanish in two generations." (Orton, The Andes and Amazon, 3d Edition, 465.)
Mutations often suggest other species of the genus, as in Coffea, and have been termed "reversions" to an ancestral character;1 but just such "reversions" are said, also, to "revert" to the normal type of their own species, through intercrossing. It may be admitted, perhaps, that a mutation is as near as anything to the original idea of reversion; it is at least a diversion, an evolutionary aberration, or wandering aside. But in this sense reversion becomes synonymous with mutation, and is thus a superfluous term, as well as inappropriate. It is equally at variance with the current meaning of the word to refer to the recovery of the normal form of the species as reversion, since this process is conservative and reconstructive rather than degenerative or retrogressive, however much an "improved" breed may appear to "deteriorate" when crossed with its wild or less inbred relatives. If this be reversion the word should he relieved of all sinister implications, at least in evolutionary usage.

Better than the substitution of a new term for "reversion" would be the transfer of emphasis from this negative concept to the kinetic view of prepotency, not in the Mendelian sense of an arbitrary and inexplicable "dominance" of one character over another, but mindful of the law of proportion between symbasis and prepotency, without which the facts of descent are a hopeless tangle of apparent contradictions. The prepotency of a variation depends upon the extent of the normal interbreeding under which it arises. The law of mutation is the biological converse: As the lines of descent are narrowed the amplitude of variations increases and reproductive fertility declines.


The abnormality of mutations is scarcely to be appreciated without a recognition of the normal diversity (heterism) of the members of the same species. Mutation is a reaction from the abnormal uniformity which is the first effect of selective inbreeding. Not only do the same or closely similar mutations occur repeatedly in the same species, but different species and genera may mutate in the same way, just as the same disease may call forth similar symptoms in different plants or animals. But even in this respect mutations may be looked upon as furnishing indications of the behavior of normal variations. Species, like other bodies, can move only from where they are; each "new character" is, after all, only a modification of parts already existing. The novelty is very largely that of the language in which it is described. Genetic variation is not completely indeterminate, fortuitous or in all directions at random; nor is it narrowly determinate or limited to one character, or two characters, or to any small number of characters, as we well know from the excellent example of individual diversity afforded by the members of our own species. Variation does of necessity have reference to characters already existing, and must be consistent with these if the change is to be advantageous. Some varietal or racial characters are also prepotent over others, and with sufficient opportunity of interbreeding will continue to spread, and to become more and more accentuated.

1) Instances of parallel development have been reviewed recently by Professor Osborn as affording "evidence of a predisposition to similar evolution." (Science, N. S., 21: 28, January 6, 1905.)

It is therefore in accordance with the most obvious probabilities of kinetic evolution that nature should abound in instances of parallel development.1 The same or similar variations are likely to arise more than once and to have a similar welcome or rejection by characters already existing. Tendencies of variation once begun in a species are continued, even after the species subdivides. Each natural group, of whatever rank, was once a single interbreeding species, and every such group represents, in evolutionary history, the subdivision of an original species. Each character or tendency can continue its development, though in the company of different later variations in each of the groups, as they have successively segregated. The static theories, which ascribed evolution to environment, might appear to explain parallel variation under parallel conditions, but the recognition of the kinetic principle enables us to understand parallel variation even under different conditions.

Inside specific lines descent is a completely connected fabric, but superspecific descent, the phylogeny of genera, families, and orders, is not reticular at all. For lack of adequate evidence we may be unable to decide which is the nearest relative of a given group, but when we represent our groups as having complex interrelationships we are merely making graphic representations of alternative solutions of unsolved problems. The common possession of an ancestral character affords, in itself, no assurance of closer relationship, nor do the separate acquisitions of similar characters. Each character must be placed, as it were, in its true chronological position before its phylogenetic significance can be appreciated. Without careful regard for sequences, phylogeny becomes as hopeless as history without dates.

1) American Naturalist, 36: 353, 1902.

If the parallelism of variation be accentuated by selective influences there occur wonderful approximations in the characters of different and unrelated organisms living under diverse natural conditions in remote and isolated regions. The facts have been effectively summarized by Professor Osborn and made the basis of what is called "The Law of Adaptive Radiation."1 In each continental area and geological period there have arisen among the mammals specialized groups adapted by their teeth to all the different kinds of food available. There are always some with slender skeletons and long legs adapted to escape by running, and others stout-footed and heavy-limbed, able, in all probability, to protect themselves by sheer strength and ferocity or by defensive armor.

Adaptive radiation is inconsistent with both of the current ideas, that evolution is caused by the environment or by a predetermining hereditary mechanism. The conditions are too diverse to cause such similarity of results, but at the same time the results are too diverse to warrant the inference of predetermination. The truth lies, obviously, between the two extremes. The environment does not cause evolution, but neither is evolution independent of the environment. Evolution must produce characters which the environment can admit, and with unspecialized mammalian types as a beginning, the requirements become similar, even though the regions be different.

With mammals the selective factors are at the very highest, and by competing with and preying upon each other they make by far the most effective part of their own environment. The struggle for existence is a stern reality, and the issue rests, very often, on a narrow margin of speed, strength, armament, or endurance. It need not surprise us, then, that the numerous geological and geographical experiments enumerated by Professor Oshorn have turned out so much the same. Kinetic evolution explains the power of radiation, and the selective conditions explain the adaptive results, the extent of adaptation being proportional to the thoroughness of the selection, providing of course, that the group be not narrowed to the point of degeneration. The most specialized types have ever been the most liable to extinction.


The prepotency of symbasic wild types and the "reversion" of domesticated varieties when selective inbreeding is relaxed, are manifestations of the biological laws of which mutation and Mendelism represent the violations. The problems are historical rather than mechanical; to interpret the facts in terms of descent rather than in those of crudely inadequate and wholly hypothetical "hereditary mechanisms." The organism may be described, for some purposes, as a machine, but it is no mere corn-sheller or steam engine, and there is no assurance that we have, as yet, even a basis of conjecture regarding the principles on which it is constructed, or the ultimate nature of the materials of which it is made. What the mechanism does, however, is a very practical and pressing question which need not be postponed on account of any lack of agreement in general theories, if, indeed, the workings of the device do not afford the best clue to an understanding of its structure.

The formal recognition of gravitation and other natural laws or properties has proved useful, although mechanical explanations are still lacking. The principles of evolution are being sought in rare and exceptional phenomena while the apples continue to fall unregarded. Many evolutionary experiments have been proposed which would require extensive and costly facilities to be maintained for very long periods of time. Such suggestions may not be carried out, but they have a present interest as showing that current theories of descent do not apply in nature at large, where the evolutionary possibilities of organisms have been tested continuously for millions of years, and the results are open freely for our inspection.

The question turns on general biological interpretations and standpoints far more than on formal proofs and demonstrations, either syllogistic or statistical. The history of biology shows what diverse and contradictory theories can be proved, or at least rendered plausible, if their authors are allowed to select the facts to go with them.

A general law of organic succession must accommodate all the pertinent facts. Each biologist can test it with the data of his own experience if he have imagination enough to assume, for the time being, the required standpoint. Indeed, one might formulate procedure in such matters by saying that the more general the law the less susceptible it is of being established by reference to any small group of facts. Such reasoning from circumscribed data has always to be bolstered up by the argument, expressed or implied, that the facts must mean what is alleged because they cannot mean anything else, a formula which transmutes our ignorance into knowledge, by sheer intellectual alchemy. We unconsciously admit the author's unconscious assumption that his standpoint is correct and final, and instead of testing it by our own facts we accept his at their face value, though every one of them may beg the question it is supposed to answer.

As long as experiments are limited to conditions of inbreeding by which the desired phenomena can be induced, there will be no lack of evidence for mutations and Mendelism. But even if all the animals and plants were successively domesticated, inbred and conventionalized into "character units," we would still be as far as ever from having ascertained that these are the means by which the constructive evolution of nature has gone forward.

There can be no certainty that any particular species may not, at some remote period and place, be crowded into a narrow corner of the environment, and made to yield degenerative mutations, but this possibility should not cause us‑to forget that the broad fabrics of continuous, diverse, and gradually changing descent are being woven in the living looms of all the widespread species in nature. To say with Professor Haeckel and others that the abnormal is the important for evolution, is not merely to frame a paradox, it is to confess what in theological language would be termed a most pernicious heresy. For do not isolation and inbreeding represent the very principle and essence of biological evil, the ever-present danger of deterioration, which nature is taking such infinite pains to escape, by all the devices of sex and symbasis? All new characters must, indeed, be classed as abnormal if we think of species as normally constant and stationary, but to base evolution on the degenerate abnormalities of inbreeding darkens counsel indeed.

1) Evolution and Physics, Science, N. S., 20: 87, July 15, 1904.

The acceptance of the laws of planetary motion was impeded by mediaeval theology, but thought is now clouded by the opposite tendency, an equally unscientific fear to admit the reality of phenomena not immediately explainable in current terms of physics and chemistry.1 The facts of vital motion are obscured by mechanical dogmas, vastly complicated, and yet wholly incompetent. In terms of physics and chemistry, we do not know why cousins may not marry, why inbreeding is destructive, or why symbasis is necessary to maintain organic strength and evolutionary progress; but we may be certain that evolutionary doctrines which disregard such primary facts of descent are fatally defective.

Current theories require that new characters be saved by segregation, but organisms are not like chemical compounds, to be preserved by keeping them from contact with others. Protoplasmic compounds are noted, it is true, for their extreme lability or tendency to decompose as soon as life is extinct, but this fact, instead of proving that vital processes are due to the mechanical "forces" hitherto recognized, only shows in higher relief their hyperphysical stability.

1) Professor DeVries also explains in a preceding paragraph: "Chemical substances are changed into others by definite and measurable steps, and hence it seems to me that this rule might prevail for the minutest material particles which determine the hereditary qualities of organisms." (Harper's Magazine, 110: 210, January, 1905.)

The atomic theory of matter has led Professor DeVries and others to assume that morphological alterations are "incumbent on slight chemical changes of the representative particles of the hereditary qualities."1 We know, however, that organic structures and vital processes persist through a wide range of physical conditions, and in spite of changes of the material particles, or even of the chemical substances of which the living tissues are composed. There is a stability of motion as well as of rest; new characters can be preserved by prepotency better than by segregation.

The higher we go in the scale of organic existence the more obvious these facts become. To keep alive the bodies of the higher animals there must be a constant supply of new materials, and a removal of the fatigue-products of the high-grade vital activity. The animals were able to out-strip the plants largely because they developed superior facilities for secretion and excretion. They are able to make use of a much greater variety and complexity of compounds and can also rid themselves of waste products with more freedom. Plants are able, for the most part, to excrete only gaseous compounds; other rejectamenta have to be accommodated inside the cells or laid down in the cell walls.

The idea that the cellular bodies of plants and animals are built up on simple principles of "developmental mechanics" sees no significance in the wonderful series of gradually superposed complexities which have attended the advance of organisms to their present stages of perfection. To build up our bodies, cells have become associated in immense numbers and highly specialized inform, structure and function. The number and complexity of chemical substances has likewise increased from the simple inorganic compounds used by the soil bacteria to those supplied by the mixed diet of civilized man.

1) The Food Plants of Ancient America, Smithsonian Report, 1903, 481-497.

Reproductive cytological processes have advanced from the brief fusions and prompt redivisions of simple and equal cells to intricate combinations which may not require renewal for decades and centuries. The Sequoias of California and Dracaenas of the Canary Islands live as individual trees for thousands of years, and some of our cultivated plants have been grown from cuttings since the earliest dawn of primitive agriculture, behind all human history and tradition.1

Still other avenues of vital motion and achievement are to be seen in the complexity of individuals, sexes and polymorphic forms which the higher plants and animals often maintain inside the same species, and in the multitudinous reproductive devices and instincts for weaving this diversity into the still more intricate fabric of descent; a social evolution, in short, which is at once the basis and the prophecy of the still higher intellectual and personal development of man himself.


The causes of evolution are still unknown, but we have arrived at the perception that evolution has a very practical physiological function which explains the general fact of progressive change. Organisms are under the necessity of motion; it is the only way that they can maintain their stability and continue to exist. Instead of being moved by environmental causes from a condition of normal constancy of characters, they are, by their very constitution, wheeled against the environment, seeking new avenues along which motion can be made. Nor are their impulses toward diversity and evolutionary progress limited to the environmental side. Species of common origin and inhabiting the same region are found, very often, to have become different in many ways, internal as well as external, which can have no direct reference to the environment.

Instead of having been built upon any general rules or principles of nutrition or tissue-formation, we find in different natural groups the utmost diversity in the solutions of the same bionomic problems, each a testimony of the protean constructive powers of life and of the futility of physiological generalizations based on single species or a few related types.

The more specialized groups abound in characters which instead of being explainable as called forth by natural selection, and hence as useful, appear to have been pushed to worse than useless extremes. It is as though species were impelled from within by an essential kinesis or property of motion to make trial of every feasible degree of expression of every attainable character. Kinesis is not a mysterious force or mechanism to be sought in reproductive cells; it is a general property of organisms, as gravitation is of matter. And of kinesis we know more than of gravitation. Two factors and two results are already obvious. The factors are heterism, or intraspecific diversity, and symbasis, or interbreeding in a specific network of descent. The results are the sustained variety of the interbreeding organisms, and the continuous progressive modification of the specific groups.

The normal evolutionary progress or vital motion of organisms is symbasic; they advance in large groups of interbreeding individuals, commonly called species. Separate mechanical explanations of each example of this law are as superfluous as the mediaeval angels who pushed the planets round and hurled the meteors. Nobody doubted that the meteors and planets moved, but special causes continued to be conjectured until it was discovered that the earth itself was also in motion. If species were normally stationary, the environment must needs have impelled them. They have, however, motions of their own.

1) Natural Selection in Kinetic Evolution, Science, N. S., 19: 594.

Natural selection neither originates species nor actuates their further development; progressive change would go on whether selection were active or not, and whether the environment were uniform or not. Nevertheless, selection conduces to adaptation, since by permitting changes in some directions and forbidding them in others, it deflects the specific motion. The workings of natural selection are adequately explained only under the kinetic theory, which recognizes the physiological value of organic changes as such, and which thus supplies the materials on which selection can act.1

The organic structure is held together and supported by the symbasic interweaving of different lines of descent. When the vital fabric is weakened by narrow segregation or selective inbreeding, mutative degenerations and Mendelian disjunctions appear. Variations thus induced afford examples of evolutionary motion, but in its aberrant and destructive form. Symbasic evolution is a process of constructive integration; it proceeds the better when the diverse individuals of a species remain together, not when they are kept apart. Variations of positive evolutionary significance are prepotent; they strengthen the organism, and are shared and preserved by the vigorous, interbreeding members of the species. The conditions under which a species enjoys its greatest numerical prosperity are also the most favorable for its evolutionary progress.