Factors Of Evolution Other Than Selection
( Originally Published 1909 )
THE most important question raised in biological science by Darwin's work is whether natural selection has been the sole, or but the main, cause of the descent of species, or organic evolution. Darwin's own answer to this question was quite distinct. "He stoutly resisted," says Romanes, "the doctrine that natural selection was to be regarded as the only cause of organic evolution."
In many parts of his works Darwin showed that he believed in the possibility of the inheritance of the effects of the use and disuse of organs, the Lamarckian factor. This view, and also the admission of still other factors, is clearly set forth in the first paragraph of the conclusion to the 'Origin of Species.' It has been said that a more strongly worded passage cannot be found in Darwin's writings, and that the last sentences present the only note of bitterness in all the thousands of pages Darwin wrote.
"I have now recapitulated," he says, "the facts and considerations which have thoroly convinced me that species have been modified during a long course of descent. This has been effected chiefly through the natural selection of numerous successive, slight, favorable variations, aided in an important manner by the inherited effects of the use and disuse of parts, and in an unimportant manner, that is, in relation to adaptive structures, whether past or present, by the direct action of external conditions. and by variations which seem to us, in our ignorance, to arise spontaneously. It appears that I formerly underrated the frequency and value of these latter forms of variation, as leading to permanent modifications of structure independently of natural selection.
"But as my conclusions have lately been much misrepresented, and it has been stated that I attribute the modification of species exclusively to natural selection, I may be permitted to remark that in the first edition of this work, and subsequently, I placed in a most conspicuous. position namely, at the close of the Introduction the following words: 'I am convinced that natural selection has been the main, but not the exclusive, means of modification.' This has been of no avail. Great is the power of steady misrepresentation; but the history of science shows that, fortunately, this power does not long endure."
While Darwin thus admitted the possibility of other factors of evolution, Alfred Russell Wallace, the co-originator of the natural selection theory, has believed natural selection to be the all-sufficient factor. Romanes has thus contrasted the views of Darwin and Wallace:.
According to Darwin, Natural Selection has been the main means of modification, not excepting the case of Man. (a) Therefore it is a question of evidence whether the Lamarckian factors have cooperated. (b) Neither all species, nor, a fortiori, all specific characters, have been due to natural selection. (c) Thus the principle of Utility is not of universal application, even where species are concerned. (d) Thus, also, the suggestion as to Sexual Selection, or any other supplementary cause of modification, may be entertained; and, as in the case of the Lamarckian factors, it is a question of evidence whether, or how far, they have cooperated. (e) No detriment arises to the theory of natural selection as a theory of the origin of species by entertaining the possibility, or the probability, of supplementary factors. (f) Cross-sterility in species cannot possibly be due to natural selection.
According to Wallace, Natural Selection has been the sole means of modification, excepting in the case of Man. (a) Therefore it is antecedently impossible that the Lamarckian factors can have cooperated. (b) Not only all species, but all specific characters, must necessarily have been due to natural selection. (c) Thus the principle of Utility must necessarily be of universal application, where species are concerned. (d) Thus, also, the suggestion as to Sexual Selection, or of any other supplementary cause of modification, must be ruled out; and, as in the case of the Lamarckian factors, their cooperation deemed impossible. (e) The possibility and, a fortiori, the probability of any supplementary factors cannot be entertained without serious detriment to the theory of natural selection, as a theory of the origin of species. (f) Cross-sterility in species is probably due to natural selection.
This comparison makes it evident that the Darwinism of Darwin is natural selection plus other factors, while the Darwinism of Wallace is natural selection alone as the cause of evolution. In late years this view of Wallace's has been highly developed by Weismann and his followers, who have argued for the all sufficiency of natural selection, and have especially opposed other factors which Darwin admitted might have aided natural selection. This view of Wallace and Weismann should be called 'Neo Darwinism' or 'Ultra Darwinism.' In discussing the criticisms of natural selection it has been shown that there are serious difficulties in the way of universal application of neo Darwinism. For this reason even Weismann has been forced to modify his views.
Most prominent of the theories of the factors of evolution which have rivaled natural selection is that put forth, before Darwin's work, by Lamarck (1744-1829). Lamarck's theory did not attract much attention during his lifetime, but since Darwin's time Lamarckism has become well known. The Lamarckian theory is commonly referred to as the theory of use and disuse and the direct action of the environment in modifying organs. Moreover, it holds that characteristics acquired during the lifetime of an individual are transmissible by heredity.
The Lamarckian view, as given by Osborn, is formulated in the four well known propositions following:
(1) Life, by its internal forces, tends continually to increase the volume of every body that possesses it, as well as to increase the size of all the parts of the body up to a limit which it brings about.
(2) The production of a new organ or part results from a new need or want, which continues to be felt, and from the new movement which this need initiates and causes to continue. (This is the psychical factor in his theory, which Cope later has termed Archesthetism.)
(3) The development of organs, and their force or power of action, are always in direct relation to the employment of these organs. (At another point he expands this into two sub-laws : "In every animal which has not passed the term of its development, the more frequent and sustained employment of each organ strengthens little by little this organ, develops it, increases it in size, and gives it a power proportioned to the length of its employment; whereas the constant lack of use of the same organ insensibly weakens it, deteriorates it, progressively diminishes its powers, and ends by causing it to disappear." This is now known as the Law of Use and Disuse, or Kinetogenesis.)
(4.) All that has been acquired or altered in the organization of individuals during their life is preserved by generation and transmitted to new individuals which proceed from those which have undergone these changes.
The greatest weakness in the Lamarckian theory is the assumption of the inheritance of acquired characters; this Lamarck took for granted, and did not try to demonstrate. As Kellogg has well said:
"That an animal in its lifetime, and especially during its immature life, can effect very considerable changes in some of its body-parts by special use or disuse of these parts, or that certain parts may be modified through the influence of external stimuli, is familiar knowledge. The heart and lungs can be enlarged by special use; in short, almost any of the organs of the body which are actively used can be modified either by unusual or extra use, or by unusual lack of use. Now this use is, in Nature, almost always of the character of a better aiding in successful living; that is, it is adaptive use. If such betterment of organs and their functions acquired by individuals could be inherited by their young, it is obvious that general adaptations of this sort could be rapidly developed in the course of generations, and new species, new, that is, because of the adaptive changes thus effected, be formed. This is the essential thought in Lamarck's theory of the method of adaptation and species forming.
"The essential principle of Lamarckism is an orthogenetic evolutionary progress toward better and finer adaptation and adjustment resulting from the inherited effects of actual use, disuse, and functional stimulation of parts. It is a great thought and a clear one, and only needs the proof of the actuality of the inheritance of individually acquired characters to make it one of the principal causal explanations of adaptation and species change.
"However, it is exactly this proof that is wanting. At any rate, proof of the character and extent necessary to convince all or even a majority of biologists is wanting. The examples or cases brought forward by Lamarckians of the alleged inheritance of mutilations, of the results of disease, and of use and disuse, are not convincing. It is one of Weismann's positive contributions to biology to have analyzed case after case of alleged inheritance of acquired characters and shown its falseness, or at least uncertainty. Many of these cases he has been able to explain as a result of selection; others remain inexplicable; a few only are insisted on by the Lamarckian champions as indisputable examples of such inheritance. But this very paucity of so-called proved cases, where there should be thousands of obvious examples if the principle were really sound, is argument against Lamarck-ism.
"Our knowledge, too, of the mechanism of heredity makes strongly against the theory of the inheritance of acquired characters. Another of Weismann's positive contributions to biology is his generally sound distinction between the germ-plasm and the soma-plasm and parts of the many-celled body. At maturity the animal body is composed of a small mass of germ-plasm (germ-cells), situated in the ovaries or testes, and a great mass of somatic tissues and organs, all the rest of the body, in fact. Now, what is the condition that exists in the body after a somatic part is modified by use or disuse or by other functional stimulus, as when a muscle is enlarged by exercise, the sole of the foot calloused by going bare-foot, an ear more finely attuned by training? We have a definite physical change in a definite organ, but is the germ-plasm in any way changed or affected by this superficial or specific somatic modification, or, if changed, is it changed so that it will produce in its future development a similar change in the same organ of the future new individual? What possible mechanism have we in the body to produce or insure such an effect on the germ-plasm? The answer is obvious and flat; we certainly know of no such mechanism; in fact, what we do know of the relation of the germ-cells to the rest of the body makes any satisfactory conception of such a mechanism as yet impossible.
"But even were the inheritance of acquired characters now an established. fact, or if it should come to be one, it must be kept in mind that Lamarckism could be substituted only partly for Darwinism. There are many adaptations and much species-forming that Lamarckism might explain, but also there are hosts of adaptations that Lamarckism cannot explain."
A number of American biologists have added to the principles of Lamarck that of natural selection. Without denying to natural selection a more or less important part in the process of organic evolution, members of this school believe that much greater importance ought to be assigned to the inherited effects of use and disuse than was as signed to these agencies by Darwin. It is obvious that neo Lamarckism has to face the 'problem of the heredity of acquired characters this is the fundamental and as yet unproved proposition of the theory.
The Darwinian theory is based upon variations which occur in all directions, unfavorable as well as favorable, and hence are known as indeterminate variations. Definite lines of development are produced from these chance variations by the elimination in the struggle for existence of all other lines; that is, natural selection permits only certain kinds of variation to persist and to accumulate. According to this theory the persistence of the useful is explained, but there are certain phenomena which cannot so easily be explained. Among these may be mentioned development along definite lines which are not advantageous and over-development of parts to a harmful degree. Moreover, there is difficulty in explaining the beginnings of advantageous modifications from fluctuating individual variations. It is to explain these phenomena that the theory of 'Orthogenesis' has been developed.
According to the theory of orthogenesis variations are predetermined and hence development is fixed along definite lines. There are various dews as to the origin of these predetermined variations. The Lamarckians would base them on the perpetuation and accumulation of the effects of use and disuse, etc. ; Roux would explain them on the battle of the parts theory and Weismann on the germinal selection theory, referred to later in this chapter.
Many phases of the theory of orthogenesis have been advanced by Nageli, Eimer, Cope and others. Some of these go so far as to say that orthogenesis takes the glace of natural selection. Nageli belongs to this school, and he "believes that animals and plants would have developed about as they have even had no struggle for existence taken place and the climate and geologic conditions and changes been quite different from what they actually have been," says Kellogg.
Others hold that orthogenesis is an adjunct of natural selection. Prominent among the latter is Professor Whitman, of Chicago, who sees no fundamental contradictions between the theories and who believes that orthogenesis and natural selection are factors of evolution working together at times; in other words, determinate orthogenetic variations are preserved by natural selection a conclusion which appeals to many biologists as most reasonable.
Associated with the name of de Vries, the Amsterdam botanist, is the Mutation Theory, or Heterogenesis. But this general conception of species-forming on a basis of the occurrence of occasional, sudden, fixed and often considerable changes or variations in the offspring of a plant or animal is a conception not of course new with de Vries, but one variously expressed by numerous biologists from Darwin's time on, especially by von Kölliker, Galton, Dail, Bateson, Emery, Scott and Korschinsky. It is, however, chiefly due to the patient, persistent, well-planned and extensive experiments and observations of de Vries that this theory of species forming by heterogenesis, or as called by de Vries, by mutations, has recently received so much renewed attention.
"The meaning of heterogenesis," says Kellogg, "in connection with species-forming and descent is essentially this : Whereas by the Darwinian theory species are trans-formed slowly and by slight changes in at first one or two or a few and only later in more parts, and all new species are derived from the old ones (which usually disappear as the new ones appear) by the gradual selection of the advantageous ones among the regular slight, fluctuating, individual variations (known commonly as Darwinian variations and which mostly occur according to the law of error), by the theory of heterogenesis new species appear suddenly, not by a selective choosing among the slight, fluctuating Darwinian variations, but independently of selection and largely independently of the so-called Darwinian variations by the appearance in fixed definite form ofseveral to many slight to considerable variations, which give the new species definite characteristics differentiating it often in many particulars from the old species, which differentiating characteristics are fully and faithfully transmitted to the succeeding generations of individuals derived from this suddenly born new species."
Extensive observations and experimentation to test the mutation theory are now in progress, and it is too early to form a final opinion as to its bearing on the theory of natural selection. It has been warmly welcomed, but even its friends admit that it needs the support of more experiments and more facts. T. H. Morgan sums up the advantages of the theory as follows : "Since the mutations appear fully formed from the beginning, there is no difficulty in accounting for the incipient stages in the development of an organ, and since the organ may persist, even when it has no value to the race, it may become further developed by later mutations and may come to have finally an important relation to the life of the individual.
"The new mutations may appear in large numbers, and of the different kinds, those will persist that can get a foot-hold. On account of the large number of times, that the same mutations appear, the danger of becoming swamped through crossing with the original form will be lessened in proportion to the number of new individuals that arise.
"If the time of reaching maturity in the new form is different from that in the parent forms, then the new species will be kept from crossing with the parent form, and since this new character will be present from the beginning, the new form will have much better chances of surviving than if a difference in time of reaching maturity had to be gradually acquired.
"The new species that appear may be in some cases already adapted to live in a different environment from that occupied by the parent form, and if so, it will be isolated from the beginning, which will be an advantage in avoiding the bad effects of intercrossing.
"It is well known that the differences between related species consist largely in differences of unimportant organs, and this is in harmony with the mutation theory, but one of the real difficulties of the selection theory.
"Useless or even slightly injurious characters may appear as mutations, and if they do not seriously affect the perpetuation of the race, they may persist."
Still another causal factor of the descent of species is to be found in the isolation theories differently expressed by various authors.
Altho to many biologists isolation alone is sufficient to account for the origin of species, most evolutionists consider it to be a very widespread and effective auxiliary theory to natural selection. Selection needs help from just such a factor. Just what is meant by this theory in its different phases Kellogg describes as follows:
"If, in a species, a number of individuals show a certain congenital variation, this variation will probably be lost by cross-breeding with individuals not having it, unless the individuals having it are in the majority or unless they become in some way isolated from the others and segregated so that they will breed among themselves. By such isolation and such in-and-in breeding the newly appearing congenital variations might soon become established, and if advantageous be so considerably developed as soon to distinguish as a variety or incipient species the members of the isolated colony. With time a distinct new species might result. Are there means to produce such isolation of groups of individuals belonging to a common species?
"The answer to this is certainly an affirmative one. There seem to be, indeed, several means of producing isolation, and the isolation may be variously named accordingly. Undoubtedly the most important of these kinds of isolation, at least in the light of our present knowledge, is that known as geographical or topographical isolation. Isolation produced in other ways may be called biologic or physiologic or sexual isolation. In the case of geographic or topographic isolation the isolated group or groups of individuals are actually in another region or locality from the rest of the species, this being the result of migration, voluntary or involuntary.
"In biologic isolation the individuals of the species all inhabit the same territory, but become separated into groups by structural or physiological characters which prevent miscellaneous inter-breeding. The real founder and most insistent upholder of the theory of species-forming by isolation (geographic and topographic isolation) was Moritz Wagner (1813-1887), a traveler and naturalist, whose wanderings and observations brought to him the conviction that while natural selection might modify species and even produce continuous evolution it could never differentiate species, that is, produce new species."
In support of isolation theories, it is argued by biologists that isolation is very important as one factor in forming species. It is, however, obvious that isolation in itself cannot be the basic and all sufficient cause for the production of specific differentiation, any more than any selective factor can. The prerequisite in both cases is the occurrence of variation. What are the variations and how are they produced? These are the fundamental questions in species-forming. Isolation is a tremendously favoring condition, but not a primary cause of species-forming. It tends to help along, to hurry up species disintegration, not to initiate it.
The Germinal Selection Theory advanced by Weismann attempts to explain the origin of variations. According to Weismann's neo Darwinism, only congenital variations, those present at birth, are transmissible by heredity. In brief, the theory holds that the germ-plasm may be influenced by conditions under which an organism lives and may 'acquire' variations in a determinate or favorable direction. Knowing this factor, "we remove, it seems to me," writes Weismann, "the patent contradiction of the assumption that the general fitness of organisms or the adaptations necessary to their existence are produced by accidental variations a contradiction which formed a serious stumbling block to the theory of selection. Tho still assuming that primary variations are 'accidental,' I yet hope to have demonstrated that an interior mechanism exists which compels them to go on increasing in a definite direction the moment selection intervenes. Definitely directed variation exists, but not predestined variation running on independently of the life conditions of the organ-ism as Nageli, to mention the position that the most extreme advocate of this doctrine has assumed ; on the contrary, the variation is such as is elicited and controlled by those conditions themselves, tho indirectly."
There are numerous minor theories proposed to explain difficulties in the more general theories. For these the reader is referred to special books like Kellogg's 'Darwin-ism Today,' Jordan and Kellogg's 'Evolution and Animal Life' and Morgan's 'Evolution and Adaptation.'
Most biologists at present seem inclined to look for truth in a combination of the several theories. Thus Whitman says: "Natural selection, orthogenesis and mutation appear to present fundamental contradictions, but I believe that each stands for truth, and reconciliation is not distant. The so-called mutations of 'Oenothera' are indubitable facts, but two leading questions remain to be answered.
First, are these mutations now appearing, as is agreed, in-dependently of variation, nevertheless a production of variations that took place at an earlier period in the history of these plants? Secondly, if species can spring into existence at a single leap, without the assistance of cumulative variations, may they not also originate with such assistance? That variation does issue a new species, and that natural selection is a factor, tho not the only factor, in determining results, is, in my opinion, as certain as that grass grows altho we cannot see it grow. Furthermore, I believe I have found indubitable evidence of species forming variation advancing in a definite direction (orthogenesis) and likewise of variations in various directions (amphigenesis). If I am not mistaken in this, the reconciliation for natural selection and orthogenesis is at hand."
Others, like H. F. Osborn, think that there are still some unknown factors of evolution. In a lecture entitled 'The Unknown Factors of Evolution' Osborn says: "The general conclusion we reach from a survey of the whole field is, that for Buffon's and Lamarck's factors we have no theory of heredity, while the original Darwin factor, or neo Darwinism, offers an inadequate explanation of evolution. If acquired variations are transmitted, there must be, therefore, some unknown principle in heredity; if they are not transmitted, there must be some unknown factor in evolution."
The present plight seems to be that species-forming and evolution are not susceptible of explanation without the help of some Lamarckian factor; and, on the other hand, the actuality of any such factor in the light of our present knowledge of heredity cannot be assumed. The discovery of the 'unknown factors of evolution' is therefore one of the most important quests of present-day biologic research.