Popular Science Monthly/Volume 64/March 1904/Evolution Not the Origin of Species
|EVOLUTION NOT THE ORIGIN OF SPECIES.|
U. S. DEPARTMENT OF AGRICULTURE.
IT is a misfortune frequently lamented that new truth, the most precious attainment of each generation, is also the most unwelcome. We do not hasten to sweep out our stock of laboriously collected ideas, even after the worthlessness of the assortment has been declared. This conservatism of vested intellectual interests not only postpones the utilization of the results of scientific inquiry, but it has an even worse effect when it impedes further investigation and warps our perception of facts.
The great obstacle in the popularization of the fundamental and obvious biological fact of evolution was the theological dogma of the separate creation of species, and toward the overthrow of this the arguments of Darwin and his immediate followers were, of necessity, directed. After four decades the debate upon the general question may be said to have closed. The thoughtful public believes that species were not made like cakes by the baker, but that the diversity of organic nature has been attained by gradual changes and transformations, a process commonly thought of as 'evolution.' As soon, however, as we pass into the field of biology proper, and seek to know the nature and causes of this process, all unanimity of opinion vanishes. The fact of evolution is no longer doubted, but biologists are still writing thousands of pages annually in support of the most diverse and contradictory interpretations of evolutionary phenomena.
In spite of the external simplicity of the idea, evolution affords extremely complex and elusive problems, and in addition to the inherent difficulty of these, discussion is still cumbered with the original terminology. Vast quantities of argumentation wasted in attempting to convince theologians that they did not know how species were made are already forgotten, but a more troublesome legacy remains, in that the words and ideas upon which attention became focused during that struggle still darken our views of biological problems.
Evolutionists, too intent on a practical explanation of the diversity of species, seized upon 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 ception 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 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. It is, therefore, a matter of literary as well as of scientific difficulty to present an 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, since 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 has produced species separable by definite and easily definable characters. He finds such species on islands and in other circumscribed regions and thereupon decides that isolation is an important evolutionary factor, failing at the same time to perceive that the 'constancy' which he ascribes to insular species is merely a uniformity made possible by the limited area of distribution, and hence often absent in species of more extensive range. Small segregations of individuals acquire uniformity of characters more promptly than if they were larger, and the sooner afford diagnostic differences of use to the systematist. This does not, however, decrease the probability that evolutionary change is slower in small groups than in those which the systematist abominates—large assemblages of individuals, offering great variety of characters, but without uniformity of combinations.
The systematist is prone to believe that there has been more evolution in a genus of ten 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 farther 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 between 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 systematic 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 great diversity of form and structure, as witness the dissimilar 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 results in 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.
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 all 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 division takes place when variations can no longer spread freely among the individuals of the species, not because the environment introduces new characters.
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 or to evolutionary progress.
The Function of Selection.
By denying that selection has any power to initiate or actuate developmental changes it is not intended to imply that it has not profoundly influenced the course of evolution in many organic groups. Indeed, it may be claimed that the kinetic theory of evolution affords 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 theory of selection, since it shows how characters may originate and develop to the point of utility or harmfulness, where selection can take effect.
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 abandoned. The first idea without the second does not account for adaptations. The 'selection' of Nägeli, Weismann and other believers in a 'determining principle' or 'hereditary mechanism' of evolution is a very weak substitute for the Darwinian idea, able only to eliminate the hopelessly unfit, but quite without means of influencing the survivors. The recognition of a continuous and necessary vital motion permits us to understand that the rejection by the environment of a harmful variation encourages adaptation by accelerating the development of any more adaptive variation which may appear.
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 even hastens its extinction.
The Significance of Species.
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 its characters changed. 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 prevents the preservation of new characters and is thus a hindrance to evolution.
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 protoplasmic 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.
The Species a Protoplasmic Network.
That sexual reproduction is a substitute or improvement of multiplication by fission is another partial and misleading view which has contributed much towards 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 algæ 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 notion of descent was sufficient as a 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 dentritic, but reticular.
Symbasis a Cause of Evolution.
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 which requires this interweaving of numerous lines of descent and this simultaneous movement of organisms in specific groups. As organic complexity increases there is 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, 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.
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.
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 placentæ 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.
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 cross-breeding is necessary to maintain the quality and efficiency of the individual, but static theories require us to believe 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, as it were, the multiplier of the evolutionary equation, because it compels 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.
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 recently published 'Theory of Mutations,' 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:
"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."
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.
Evolutionary study and thought have been hindered by the confusion of two unrelated biological phenomena, (1) evolutionary progress or vital motion, and (3) 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 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 two phases of the same creative process.
- 'Stages of Vital Motion,' Popular Science Monthly, LXIII., 16, May,. 1903.
- 'Four Categories of Species,' American Naturalist, 33:287, April, 1899. 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.
- Popular Science Monthly, May, 1903. Symbasis may be defined further as the property of which sexual diversity and cross-fertilization furnish the phenomena. The word may also be used physiologically to signify a normal and advantageous range of interbreeding among the individuals of organic groups. It is to be distinguished on the one side from wide crossbreeding and on the other from narrow inbreeding, both of which produce inferior offspring and interfere with evolutionary progress.
- 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.
- There is also the possibility that they secure from their hosts protoplasmic compounds of high complexity which serve as a partial substitute for conjugation. It is further to be observed that 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.
- 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 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 notion 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. The kinetic theory rejects all these supposed factors and interprets vital motion as continuous, gradual and self-caused, or inherent in the species, but 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.
- Darwin, 'The Effects of Cross and Self Fertilization in the Vegetable Kingdom,' p. 27. New York, 1895.