Science Monthly, 63: 14-24 (May 1903)
STAGES OF VITAL MOTION
O. F. COOK,
U. S. DEPARTMENT OF AGRICULTURE.
THAT the organic universe moves, all evolutionists believe; but the opinion is still prevalent that species change only as the result of external influences, and that evolution is thus a merely passive process, a biological malleability or plasticity. What have been termed static theories of evolution are based on this bald assumption that species are normally in a state of rest or constancy, a notion contradicted by every pertinent fact. Motion in the biological field is, indeed, more obvious than in astronomy, since every separate group of organisms becomes different from its relatives, quite independent of external conditions, except as these may influence the direction of progressive change.
Adaptation and Environment.
|*Reactions to environment are often termed 'adaptations,' but the word in this sense is without evolutionary significance because it has not been shown that any non-congenital variation is hereditary.|
No direct and causal connection between environment and genetic variation has been demonstrated, in spite of many assertions and theories. It, is axiomatic that evolving organisms must vary from where they are, or in characters they already possess; and as continued existence presupposes adaptation to environment, variations often strengthen adaptations, especially since characters favoring the geographical and numerical increase of the species are likewise best fitted for distribution inside the species. In this way it is possible to understand adaptations without the inheritance of 'acquired' characters impressed upon the organism by the environment. Under the static assumption that species normally maintain a stable average each specific difference needs a separate explanation as the result of an external influence, and the preservation of each new variation must be supposed to require the segregation of a new species. To make place for the modified progeny and protect it against admixture it was thought necessary that the parental type be eliminated, a method gratuitously sanguinary and wasteful, since the new character can be much more rapidly propagated by grafting it into the old species than by founding a new species with a single ancestor—a suggestion often quite impracticable. In contrast with the infinite complexity of this theory is the general explanation afforded by the recognition of biological motion, through which species achieve adaptation because they are able to put forth variations in the necessary directions; not because environment causes the variations, nor because the variants are isolated from their unimproved relatives. Variation is not a consequence of adaptation; adaptation is a result of variation.*
Heredity and variation are not two opposing forces, the one tending to preserve and the other to destroy the specific type; they are two closely adjacent aspects of the single process of organic succession. The permanence of types is not secured by stable or unchanging characters, but by individual diversity or inconstancy, and the consequent power to move in advantageous directions. Organisms are so constituted that the persistent repetition of the same form or character complex is not possible; the supposition of a non-progressive heredity comes from the pre-evolutionary period. Heredity does not oppose variation; evolution is the inheritance of variations, facilitated by cross-fertilization. The causes of variations are also the causes of the accumulation of variations, and of the resulting diversity of species. Variation and cross-fertilization are the means, while selection and isolation are the incidents, of a continuous organic motion. Species are not normally at rest, nor are their motions predetermined by external forces or by internal mechanisms; they are not compelled in one direction, but must move in some direction, as variation and environment permit.
The Accumulation of Variations.
Kinetic Theory of Evolution,' Science, N. S., XIII., 969, June 21, 1901;
'Kinetic Evolution in Man,' Science, N. S., XV., 927, June 13, 1902.
‡Symbasis signifies etymologically a moving or standing with or together. The similarity of the word to symbiosis is perhaps objectionable, but may assist in the appreciation of the distinction between static and kinetic views. Symbiosis means the living together of different species of organisms on terms of mutual advantage. Symbasis refers to the fact that organisms exist and make normal evolutionary progress together or in groups commonly called species rather than in simple or narrow lines of succession.
Static theories are further inadequate because they neglect the fact that change or biological motion is necessary to maintain the vigor and efficiency of the organism. A kinetic theory,† on the other hand, recognizes such motion as normal, and as facilitated by cross-breeding, instead of being hindered. In whatever environment and however propagated, organisms of all types and all categories of complexity are changing or evolving, though with unequal rapidity. Organisms multiplied asexually and thus connected only in simple or linear series make slow progress in comparison with groups in which variations can be distributed through cross-fertilization. The more complex the organic structure the greater the necessity that it be supported, as it were, by many diverse, intergrafting lines of descent. The reasons for this have not been explained, but for purposes of expression it may be ascribed to a special property or requirement called symbasis,‡ served at once by variation and by cross-fertilization. To what peculiarities of substance or structure symbasis is due we have as yet no intimation, but the same might have been said of gravitation and many other properties of matter for which names have proved useful, as well as of growth, irritability, and similarly unexplained attributes of protoplasm.
Professor Baldwin's most recent and plausible improvement of the static theory
the preservation of new characters seems still to be ascribed solely to natural
selection. ('Development and Evolution,' p. 156, New York, 1902.)
† 'The Origin and Significance of Spines: A Study in Evolution,' by Charles Emerson Beecher, Am. Jour. Sci., VI., 1-120, 125-136, 249-268, 329-359, 1898. I am indebted to Mr. Charles Schuchert, of the U. S. National Museum, for bringing this able paper to my attention.
Variations do not appear and are not selected or accumulated merely because of their usefulness or desirability with reference to environment, but useless or even injurious characters may be adopted as a means of evolutionary movement.* Specialization in the sense of extreme accentuation of characters is often harmful and therefore not to be ascribed to adaptation. The influence of natural selection increases with the nicety of adjustment already attained, or as the range of permissible variation is narrowed. Adaptive specializations also commonly imply a narrow dependence on external conditions, and thus give no assurance of permanence for the type; they are more common on the side-twigs of life than on the main branches. Evolution is both accelerated and retarded by narrow selection or segregation; accelerated if the motion be estimated on the basis of a single character; retarded if the organism be viewed as a whole. Normal evolutionary progress does not go forward on the line of a single character, but requires the accumulation of many variations to maintain the structural coordination and functional cooperation of parts. External modifications require less coordination than internal, and are often exaggerated far beyond the requirements of use, and beyond the limits of developmental welfare.†
|* As explained later on, a result of extreme segregation or narrow inbreeding is to accentuate variation or produce abrupt changes or mutations. It is as though the closing of all except one of the avenues of change compelled abnormal speed in that direction.|
Organic change and diversity inside the species are necessary and universal, but species and higher organic groups decline and become extinct if their variations become limited to non functional parts and do not provide, as it were, the facilities by which adjustment to changing environment may be maintained. Nevertheless, fitness for the environment is only one aspect of the evolutionary problem; adaptation is an incident and not a cause of evolutionary progress. Results commonly ascribed to selection are due to the normal motion of organic groups. Environment, including natural selection, segregation, isolation and other aspects, is a negative and not a positive factor in evolution. Instead of causing biological motion, environment is able only to influence its direction by presenting obstacles to some tendencies of variation while permitting others to go forward.*
|† Even under static theories it has been found advisable to distinguish between 'physiological' or 'direct,' non-hereditary variations due to environment, and 'congenital,' 'direct' or 'fortuitous' variations notably hereditary, though doubtfully connected with environment.|
This separation of evolution from environment is not lessened by the fact that environment frequently determines the existence or degree of expression of characters. The absence of a substance necessary to the formation of a certain color or pigment prevents its formation, as may also the absence of the heat or sunlight necessary for its elaboration. To expect that external conditions should not influence organisms would be to ignore the fact that they grow by what they take in from the outside, and can not build without materials. By being placed under different conditions two individuals can be rendered far more different than they otherwise would have been, but to call these differences 'variations' and then to generalize that variations are caused by environment is simply the old-fashioned fallacy of the undistributed middle.† There is not the slightest probability that the causes which make related organisms different under different conditions are those which make organisms of common origin different under the same conditions. In his paper on 'Nutrition and Selection' Professor De Vries shows that one of the variations of the poppy, depends for the degree of its manifestation upon the abundance of food, or is correlated with vegetative vigor. This does not justify, however, Professor De Vries' inference that all characters are so correlated; and that the dependence was not absolute, even in the instance described, was shown when a reversal of cultural methods did not eradicate the character. The same reasoning applied to the human species would discover that some characters appear only among well-fed people, and that such characters are hereditary and persistent, but we are not compelled on this account to infer that all the differences now existing among us have arisen through over-eating.
Unsuspected differences or powers of variation sometimes appear under new environments, but it has not been shown that such potential or latent characters are less congenital, or otherwise less normal in any evolutionary sense. The wonder is not that organisms build differently with different materials, but that they are able to build with the same materials such infinite diversity of form and structure.
Conditions Favoring Evolutionary Progress.
That with adequate materials supplied by abundant food a species would be able to exhibit a larger range of variation, is undoubtedly true, and offers no difficulties in a kinetic theory. The more favorable the conditions or the more successful the adaptation, the more numerous the individuals; also the more extensive would be the manifestations of the variational possibilities of the species, and the more rapid the resulting evolutionary progress. If static theories of evolution were correct numerical increase would not favor evolutionary change because it would diminish the chances of the segregation on which the preservation of variations has been thought to depend.
The most advanced organic types—those which have traveled farthest on the evolutionary journey—are not natives of islands, but of continents. The greatest and most rapid evolutionary progress has not been made among organisms of localized distribution, but among those having facilities for wide dissemination and free interbreeding. Large species move faster than small. Insular species become diverse from their continental relatives mainly because they are left behind by the latter rather than because isolation favors evolution.
|*Mr. F. A. Lucas calls my attention to the interesting fact that a similarly accelerated development occurred among the pterodactyls, a second winged group of reptilian ancestry. In the Jurassic and Cretaceous periods pterodactyls attained a rapid and extensive differentiation of genera and families. Likewise the early Eocene mammal types appeared very abruptly and had a very wide distribution.|
Segregation did not denote evolution either in the remote or in the more recent past. As the geological record is followed backward the more generalized types are found to have more generalized distribution, and if in former ages evolutionary changes were more rapid than at present in any particular group this may well be correlated with a period of very favorable conditions permitting the simultaneous existence of vast numbers of individuals in species continuous over large areas. The later subdivision of these generalized types betokens less favorable circumstances which reduced the numbers or otherwise localized the distribution, and thus segregated the new groups. The birds outnumber the reptiles,* the insects the myriapods, the composites the palms. The better the facilities for distribution the more rapid the evolution.
|* Degeneration and extinction as the result of inbreeding has not been sufficiently considered as an explanation of the dying-out of insular animals protected from competition and other dangers of continental forms. There are, for example, human remains on many Pacific islands uninhabited at the time of their discovery by Europeans.|
On the other hand the greater the localization and the fewer the individuals the slower the evolutionary progress of a species, and the more uniform the characters. Their supposed constancy leads systematists to ascribe specific rank to insular forms differing in details utterly inadequate for the diagnosis of widely distributed continental types. Multiplicity of species does not signify that the land-snails of the isolated valleys of the Hawaiian Islands are in a state of more rapid evolution than other mollusca, but that the characters of these segregated groups are so uniform that systematists can readily define and distinguish them. Many very small species are known, but they are extremely few in comparison with those of larger distribution, and with suggestive frequency they present indications of approaching extinction.*
The Significance of Mutations.
The uniformity of such narrowly segregated groups is the same as that of many of our varieties of domesticated plants and animals, the history of which is also brief. We have, moreover, with these the opportunity of observing the further symptoms of the process of decline.
As though to compensate for the want of access to the normal number of variations, those which occur become more and more striking, and may even be more different from the parent form than the wild species of the same genus are from each other. They have been said, in other words, to 'answer the definition of species.' Professor De Vries has courageously accepted the results of this reasoning and has equipped his new Oenotheras with specific names and introduced them to the scientific world as new members of the vegetable kingdom in regular standing, while the description of many other 'De Vriesian species,' is threatened by some of our too-progressive naturalists.
The inadequacy of natural or other forms of selection as an explanation of evolution has become more and more appreciated, and has decreased confidence in the Darwinian idea that species originate by imperceptible gradations, impelled by natural selection. Professor De Vries and his followers argue accordingly that species must originate by definite and abrupt changes, and have set out to search the biological field for instances to support this theory. But if the present interpretation of evolutionary facts and factors be correct the forms described as 'mutations' are not true evolutionary species, either actual or potential. Mutations more fertile than the parent type have not been reported. They do not arise through normal evolution, but are symptoms of debility due to the absence of evolutionary opportunities; they are not parts of an ascending series, but are obviously declining toward extinction. This difference of interpretation well shows the antithesis of static and kinetic theories of evolution. Under the former mutations have been accepted as genuine examples of the methods by which species are formed in nature, while under the latter they appear as but the dying spasms of small groups of organisms suffering a fatal separation from the life of their species.
Mr. A. F. Woods has kindly brought to my attention an important confirmation of this association of mutation with reproductive debility, namely, that cultural methods calculated to encourage vegetative growth at the expense of reproductive vigor or fertility are also distinctly favorable to the appearance of mutations and of physiological abnormalities such as variegation of foliage. Professor De Vries made Oenothera the special object of his study because the frequency of fasciation and other monstrosities seemed to indicate a high degree of structural instability. The abnormality of this class of evolutionary phenomena was not considered. It was inferred instead that the condition of 'mutation' is a somewhat rare and temporary state through which organisms pass at the period of formation of new species, and the failure to find equal 'mutability' in other plants did not prevent the drawing of general conclusions.
Definitions of Evolutionary Stagers.
As a summary of the above discussion three evolutionary conditions may be formally distinguished:
1. Prostholytic or Progressive Stage.—The prostholytic or progressive stage of evolution is found in large species of wide distribution containing abundant individuals with free intercrossing of numerous lines of descent. There is unlimited diversity or inconstancy of individual characters, and variation is indefinite and continuous in the sense that endless fluctuations and intergradatious are present. The requirements of symbasis are fully met; interbreeding is normal and reproductive fertility is high.
2. Hemilytic or Retarded Stage.—The hemilytic or retarded stage of evolution is reached in species or subordinate groups of restricted distribution containing a limited number of individuals with few and closely interrelated lines of descent. Characters are nearly uniform and variation slight. The requirements of symbasis are not fully met, but the deficiency has not yet resulted in reproductive debility.
3. Catalytic or Declining Stage.—The catalytic or declining stage of evolution appears in closely segregated groups of relatively few individuals propagated by inbreeding or on single lines of descent. Variations are few, pronounced, and abrupt or discontinuous, also relatively constant and with little or no intergradation. The catalytic stage implies a violation of the law of symbasis, or inadequate cross-fertilization, together with the resulting deficiency of fertility.
Effects of Inbreeding.
These stages or states of evolution are distinguished and named in the belief that they will afford a useful addition to our evolutionary vocabulary. They are, however, parts of a connected series of events with no lines of separation between them. All organisms which too close segregation has brought to the catalytic stage have passed through the hemilytic. For example, the recently domesticated pecan tree of our southwestern states is still in the first or normal stage of evolution, and only a small proportion of the seedlings produce nuts like those of the parent tree. Selective inbreeding for a few generations would first produce uniformity, or 'fix the type,' as the expression is, by inducing the hemilytic or retarded stage of evolution, while a too narrow and persistent selection or the segregation of a single line of descent would hasten the decline and eventual destruction of the very type it might be designed to perpetuate. Coffee has not been domesticated for much more than a thousand years, and although selection has not been practised, very pronounced and constant variations are now appearing in considerable numbers, but all less fertile than the parent stock. That inbreeding tends to 'fixity' of characters is true only for a time; organisms in the catalytic stage are rendered less uniform as well as less fertile by continued inbreeding. Uniformity and vigor can be restored, as breeders already know, only by the repetition of the process of selective segregation after cross-breeding with another stock.
The catalytic stage is attained more slowly by asexual propagation, and the variability is far less pronounced, but partial or complete sterility has appeared in a considerable series of unrelated tropical plants long propagated only by cuttings, such as the banana, pine-apple, sugarcane, sweet-potato, Irish potato, taro and yam.
Parthenogenesis may also be viewed as a form of asexual propagation, and habitual self-fertilization is another stage of sexual and evolutionary decline. Self-fertilization is supposed to be normal in several of the cereal grasses and in many other plants, though it is obviously unsafe to infer that cross-fertilization is entirely superfluous because frequently absent. With the cereals and other plants of similar history self-fertilization may prove to be a result of cultivation in northern latitudes where the weather is often unfavorable for pollination by the wind or by insects, so that selection would encourage variations least dependent upon cross-pollination. I learn from Mr. Jesse B. Norton that the more primitive, hardy, and disease-resistant oat varieties of South Europe open their glumes widely and thus invite cross-fertilization, while in most of the varieties bred in the colder and more rainy climate of Northern Europe the glumes separate much less, and do not expose the stigmas, thus showing that cross-fertilization has been abandoned. Darwin proved that there is no benefit in the crossing of closely related individuals, as distinguished from fertilization by the pollen of the same flower, and since domestication implies inbreeding the habit of self-fertilization would involve no additional injury, but would have an important practical advantage in greatly increasing the chances of pollination and seed-production.
Mutations and Hybrids.
The recognition of symbasis, or the necessity of a broad foundation to sustain the organic structure, permits the inference that some hybrids are sterile and variable for the same reason that closely inbred plants and animals decline in fertility and produce mutations or deviations from the normal type. A hybrid is a mixture or cross between individuals which would not be expected to mix in nature. Among domesticated plants hybridization signifies the reverse of selection, the crossing of varieties which the breeder commonly strives to keep separate. Generalizations to the effect that hybrids as a whole are sterile, variable, weak or vigorous are fallacious, since the results of the crossing depend upon the evolutionary status of the parents. By segregation or inbreeding normal or progressive variation gradually gives place to uniformity and then to mutation, but hybrids between distant types pass at once from the progressive stage to the catalytic. On the other hand, crosses between inbred or closely segregated stocks may show increased vigor and stability, and thus reverse the process of decline. Hybrids, therefore, may be either prostholytic or catalytic as they tend upward or downward in the evolutionary series.
*The dialytic or divergent stage might be described as the reverse of the hemilytic; it is characterized by the facts discovered by Mendel, Spillman and others, which may be taken to signify that the characters upon which close-bred varieties have diverged do not combine into an average in the hybrid offspring, but remain antagonistic and follow one or the other of the parental lines.
Cross-breeding and close-breeding have the same limits of sterility; and between each and the mean of normal evolution there is, as shown by the experiments of Mendel, Garton, De Vries and others, a region of the relatively infertile abrupt variations variously termed sports, mutations and hybrids. The weakness and sterility of too distant crosses and of too closely isolated or inbred plants or animals may be due alike to a deficiency of normal fertilization, and may be accepted as evidence that the true course of evolution lies along neither of these extremes but follows the natural mean between them.
|* Criminologists have found in the human species the same tendency of abnormal individuals to fall into recognizable types. Inbreeding is also recognized as a frequent cause of aberrations from the mental and physical average of the race, just as sterility and emotional abnormality are among the most frequent phenomena of criminality.|
Catalytic variations have not the indefinite number and diversity of the progressive stage; like the symptoms of other disorders of plants and animals the same or closely similar mutations recur in somewhat definite proportions, and are not peculiar to single species, but many members of a genus or family may be similarly affected. It is therefore not necessary to interpret the independent repetition of the same symptom of evolutionary debility as an evidence of the inheritance of definite character complexes or units.* The truly admirable but often misinterpreted experiments of Mendel did not result in the discovery of 'principles of heredity' so much as they revealed limits of hybridization, in that hybrid plants were found which inherited the characters of only one parent. The failure of strongly divergent or antagonistic characters to combine into a permanent average in hybrids gives, however, no basis for denying that normal evolution proceeds by the synthesis or accumulation of acceptable variations, nor is abrupt or discontinuous variation in individuals in any way incompatible with the probability that in nature evolution goes forward only through the gradual transformation and subdivision of species. The emphasis placed by Bateson, De Vries and others upon abrupt variations is warranted by no general pertinence of the facts, and is but a consequence of the failure to perceive that the origination or multiplication of species is an incident rather than an instance of evolution.
Cross-fertilization Accelerates Evolution.
Organic succession will not persist on too narrow lines of descent, does not normally leap aside from its course, and will not bridge over too broad a chasm of evolutionary divergence. Amount of difference in the external characters of two groups affords little indication regarding the behavior of their hybrids. Some groups treated by the systematists as closely related species will not even hybridize, while in other instances plants assigned to different genera are mutually fertile. Such discrepancies are doubtless due partly to inadequate classification and partly to the fact that organic evolution is attended also by a cytological or cellular evolution the progress of which may not be consistently uniform. That the cytological or cellular evolution is commonly slower than that which affects external characters seems probable because domesticated plants and animals more different than mutually sterile wild species are still completely fertile. That all the types produced wider domestication from the same wild species hybridize freely, and thus do not have the stability and isolation of natural species, was frankly admitted by Darwin and Huxley as 'one of the greatest obstacles to the general acceptance and progress of the great principle of evolution,' and it is no less an obstacle to the acceptance of the complicated and self-contradictory static theories formulated as alleged improvements of the views of these evolutionary pioneers. If, however, evolution be recognized as a kinetic process this fundamental difficulty completely disappears, since the cross-fertilization which hinders the segregation of species is not on this account an obstacle to evolution, but is, on the contrary, the most important agency for the acceleration of vital motion. By overlooking this fact builders of evolutionary theories have continued, as it were, to stumble over the corner-stone of the biological structure.