Popular Science Monthly/Volume 34/February 1889/On the Causes of Variation I
|ON THE CAUSES OF VARIATION.|||
WHATEVER influence we may attach to environment and external conditions, it is self-evident that they alone have not been sufficient to induce the wonderful variety of life existing upon the globe to-day. Indeed, so far as natural selection implies necessary utility, necessary adaptation to surroundings, it is, as I have said, defective. We know very well that introduced species from one continent to another, or from one country to another, have proved better adapted to the changed conditions than the indigenes or endemic forms. This is readily comprehended on two grounds: First, that species which liave, in the course of time, experienced a greater struggle among themselves in large areas, have an advantage over those in more limited areas in which the struggle has been less intense; secondly, that species which have accommodated themselves to the changes in life conditions which civilized man induces, have a great advantage when, following man's migrations, they are brought into competition with species which have not yet been subjected to such conditions. Again, no valid reason can be urged why, within a given area, one species predominates over another in so far as mere adaptation is concerned. The influences of environment alone would tend to unify the fauna and flora of a given region. Theoretically, so far as climate and physical conditions are concerned, there is no reason, through regions where these are uniform, why a single animal should not prevail to the exclusion of all others, providing it was vegetarian, or that the particular plant which furnished food to such an animal should not prevail to the exclusion of all others. The hickory and the blade of grass must be considered equally adapted to the environment with the oak, and so on all through the multifarious forms of both vegetal and animal life: so that this diversity of form can best be explained by some principle like natural selection, and by the interrelation and interaction of organisms and the struggle between them for existence. This is illustrated in many directions. To take a striking example: no one doubts that if the larger carnivora of Europe and Asia were introduced into Australia, the marsupials would soon have to give way, and could survive only by the acquisition of special functional modifications and larger intelligence such as we find in our opossum. Yet it would be folly to conclude that the marsupials are less well fitted to the physical conditions which obtain in Australia than their introduced exterminators.
From what has preceded, we are, I think, justified in rejecting the interpretations of both extremists as to the scope and meaning of natural selection. It can not be debased to the mere expression of the universally observed fact of variability; yet it must be restricted, because it not only implies something to be selected, but its promulgator limits its scope to the selection of something that is useful. As a philosophy it considers only processes, and leaves remote origin and cause untouched. The following limitations are probably justified to-day, and will help to more exact use of the term:
1. It deals only with individual variation from whatever cause, and should not be applied to simultaneous variation in masses.
2. It deals only with variations useful to the organism in its struggle for existence, and can exert no power in fixing the endless number of what, from present knowledge, we are obliged to consider fortuitous characters. It can not perpetuate useless organs; nor those of a vestigiary or obsolescent character.
Even with these restrictions, the principle is far-reaching and profoundly important; but it quite fails to account for many of the most interesting manifestations of life that are obviously not necessary or life-preserving, of which many will occur to every one, such as, among lower organisms, many superficial details of structure; or, as among higher organisms, odd habits and customs, playful instincts, ethical traits, etc. Its limitations must be narrowed. in proportion as we come to understand the other laws of modification and the causes of variation in masses. Let us briefly consider some of these causes.
We soon find that they admit of a certain amount of classification, the minor divisions of which, as in all systems of classification, more or less fully interlock or blend. They fall, however, into two chief categories, viz., (1) external conditions or environment, which are, at bottom, physical, and (2) internal tendencies or promptings, which are, at bottom, psychical. We shall also realize more fully that there is good reason for the varying importance which has been placed on natural selection because it represents a broad principle, based on the outcome of both these categories, but particularly of the latter. Its value is not a fixed one, and must needs change with the increase of exact knowledge of the other factors, and did in fact change in the mind of its originator. We shall further find that there are laws of evolution which permit of formulation and expression, and which have influenced or controlled the mode of variation, but which must not be confounded with or included among the causes of the variation proper, though here, again, the line between the two kinds of factors is not always easily defined.
The conditions of organic modification may, therefore, roughly be classed as (A) external and (B) internal, and these may be almost indefinitely subdivided. The former class includes (]) physical and (2) chemical forces, and in a broad way may be said to induce modification independently of natural selection, however much this may act with them as a secondary cause. Certain prominent features of the physical forces are worthy of mention: as light, temperature, water (stagnant, or in motion), climate (under which term may be included meteorologic phenomena, as electricity, atmospheric pressure, etc.), mechanics (gravitation, wind, stress, friction, etc.) and geographies (migration, isolation, etc.). The chemical forces may be considered under the subdivisions aquatic, atmospheric, food, and soil. In class A may also be included (3) vital or organic force in so far as this is concerned with the interaction of organisms, and it is seen thus to link the two great classes. The second class (B) includes (1) physiological and (2) psychical forces. Prominent among the former, as causes of modification, are worthy of mention those connected with genesis itself: as heredity, physiological selection, sexual selection, hybridity, primogenital selection, and what I would call sexual differentiation, and philoprogeneity. Among the latter may be included use and disuse, individual effort, etc.; and last, but not least, the emotions.
Now, with the limited definition given to natural selection, all the forces in class A act independently of it, while the rest are more or less fully aids to its action. Time will not permit of much detailed consideration of the physical and chemical forces. Nor is such consideration necessary; for their influence, as Darwin well remarked, is obvious. Fundamentally, they must needs limit and control all manifestations of life, of which indeed, on evolutional grounds, they are the material basis. Change of physical environment may affect function first and chiefly, but this involves change of form and structure which are integrated by heredity. The surface of the earth and the waters upon it and the atmosphere above it have necessarily conditioned the chief modes of animal locomotion, as swimming, flying, crawling, and walking, while the five great classes of vertebrates find the explanation of their structure, as J. B. Steere pointed out at the Ann Arbor meeting, in the conditions of life in water, in shallows, in the air, on land, and on trees and rocks.
External Conditions.—By external conditions, or environment, we include all influences on organisms which act from without, and in carefully considering them we shall find it difficult to draw the line between those which are really external and independent of any motive or inherent tendency in the organism, and those which are not. Hence, the general term "external conditions" is resolvable into various minor factors. Considering the influences as a whole, we find that in the 1844 essay, or sketch, Darwin gave more weight to them as producing variations, and as modifying habit, than he did in the "Origin"; yet we all know that he felt convinced, when this work was first issued, that natural selection was the main, though not the exclusive, means of modification. Before his death, he was again led to attach greater importance to them. As late as March, 1877, he wrote to Neumayr, of Vienna, that "there can not be any doubt that species can be modified through the direct action of the environment. I have some cause for not having more strongly insisted on this head in my 'Origin of Species,' as most of the best facts have been observed since its publication." He was led to this modification of his views by Neumayr's essay on "Die Congerien," and by Hyatt's work in showing that similar forms may be derived from distinct lines of descent. In his correspondence with Huxley, Darwin remarks that one point has greatly troubled him. If, as he believed, accidental conditions produced little direct effect, "What the devil determined each particular variation? What makes the tuft of feathers come on the cock's head, or moss on the moss-rose?"
It is quite plain, indeed, that subsequent to the publication of the "Origin," and especially in 1862, in his correspondence with Lyell, Darwin was inclined to give more power to physical conditions, and, in fact, was wavering in his mind as to the force of the different influences at work. In his letters to Hooker, in 1862, the same tendency may be noted, and the preparation of the "Variation of Animals and Plants under Domestication" led him to believe rather more in the direct action of physical conditions, though he seemed to regret it because it lessened the glory of natural selection, and, to use his own language, "is so confoundedly doubtful." One can plainly trace from the correspondence how, prior to the publication of the "Origin," he more and more, as his facts accumulated, and as the theory of natural selection grew upon him, relegated to an inferior place the influence of environment; while, subsequent to the publication of that work, and up to the time of his death, the tendency seemed to be in the opposite direction.
Many eminent workers have differed greatly from Darwin in the influence allowed to these external conditions, and this is particularly the case with our American writers. Indeed, no one can well study organic life, especially in its lower manifestations, without being impressed with the great power of the environment. Joseph Le Conte speaks of the organic kingdom lying, as it were, "passive and plastic in the molding hands of the environment." Leidy, Wyman, Clark, Packard, etc., have insisted on the influence of physical conditions. Baird and Ridgway on geographical distribution. Whitman on concrescence, Hyatt on gravitation, Cope and Ryder on mechanical stress, have all published valuable corroborative evidence; while many other writers have added their views and testimony, which have been admirably condensed by Prof. Morse in two addresses before this association. Allen demonstrates plainly the influence of climate and temperature in directly inducing specific changes. Weismann, in his remarkable "Studien der Descendenz-Theorie," concludes that differences of specific value can originate only through the direct action of external conditions, and that allied species and genera, and even entire families, are modified in the same direction by similar external inducing causes. In Semper's "Animal Life" (1877) we have the best systematized effort to bring together the direct causes of variation, and no one who has read through its pages can doubt the direct modifying influences of nutrition, light, temperature, water at rest and in motion, atmosphere still or in motion, etc., or question his conclusion that no power which is able to act only as a selective and not as a transforming influence can ever be exclusively put forth as a causa efficiens of the phenomena. Kölliker, in 1873, wrote: "Manifold external conditions, when they operate on eggs undergoing their normal development, on larvæ or other early stages of animals, and on the adult forms, have produced in them partly progressive and partly regressive transformations"; and recognized as most important forces, nutrition, light, and heat. Indeed, the direct action of environment must have been, as Spencer puts it, "the primordial factor of organic evolution."
In so far as it offers evidence, entomology confirms the conclusions of the writers in other departments of natural history, above referred to, and offers a host of most conclusive proofs of the direct action of the physical and chemical factors which I have enumerated. Justice, however, could not be done to the facts within the limits of an address of this kind, and I pass on to some of the other factors.
It is among what I have called the vital or organic conditions of variation that natural selection has fullest sway, and, as they have been so ably expounded by Darwin and others, they may be dealt with in few words.
Interaction of Organisms.—The productions, as a whole, of greater areas will, whenever they get an opportunity, conquer those of lesser areas, and in this broad sense the interaction of organisms may be said to have had no special modifying power, however great its influence may have been, and is yet, in inducing the survival of the fittest, or in bringing about the present geographical distribution of species. The consequences of enforced migration and of isolation are best considered when dealing with the physical conditions, because they must influence modification of masses rather than of individuals, and either substitute one type for another or remove competing or differentiating influences. But, in the more restricted sense, i.e., the interaction of organisms occupying the same ground—the struggle for existence, in other words, between direct competing organisms—is a prime Darwinian factor of modification, and a whole volume of illustrations may be drawn from entomology; for in no class is the contest more severe, whether with plants, or with other animals, or with one another, than in insects. In no other field of biology, for instance, have the physical conditions resulted in such infinite diversity of form and habit fitted, whether for earth, air, or water, and often for all in the same individual; so, also, in no other field is parasitism carried to such a degree, or are the purely adaptive structures due to this interaction so varied or so remarkable. The entomologist who goes beyond the "dry bones" of his science is inevitably a Darwinian.
In this category must also be included that interrelation between insects and plants which has eventuated in the so-called carnivorous plants, and that still more wonderful interaction between flowers and insects by which each has modified the other, and the facts of which have been so. untiringly observed and so well set forth by a number of writers from Sprengel's day to this, and by none more successfully than by Darwin himself. These are plainly inexplicable on external conditions acting on masses alike, and are meaningless enigmas except on the theory of natural selection, or some supra-natural and dogmatic gospel.
We are thus led, through this last, from the external to the internal factors in evolution, or those of a physiological and psychical nature. In these, natural selection is the key which, so far, best unlocks their meaning, and shows how they have acted in the formation of species and the less fundamental of the great groups. In considering them it is hardly necessary to discuss their relative importance as compared with the external conditions, though it may be remarked that they are the factors which have induced the great variety of adaptive forms and minor differentiations, while the external conditions have governed the formation of the great and more comprehensive types of structure.
Darwin was led to give more importance toward the end than he had originally done to some of these internal factors, and especially to functionally produced modifications. In the "Descent of Man" he says that he did not sufficiently consider variations "which so far as we can at present judge are neither of benefit nor injurious; and this I believe to be one of the greatest oversights I have yet detected in my work." And in the sixth edition of the "Origin" he frankly admits that he had omitted in other editions to consider properly the frequency and importance of modifications due to spontaneous variability. He further refers to morphologic differences, which may have become constant through the nature of the organism and the surrounding conditions rather than through natural selection, since they do not affect the welfare of the species. In short, Darwin's views kept pace with the investigations of his day, and tended in the direction of restricting rather than widening the influence of natural selection. But, as Romanes, and especially Spencer, in his "Factors of Organic Evolution," have fully shown Darwin's position on this subject, I may pass over the detail.
Internal Conditions.—Physiological.—Genesis itself is the first and most fundamental of all causes of variation. The philosophy of sex may, indeed, be sought in this differentiation, as the accumulated qualities in separate entities when suddenly conjoined or commingled inevitably lead to aggregation and heterogeneity—in other words, to plasticity or capacity to vary. Genesis, as a fundamental factor in evolution, may be more intelligently considered under some of its subordinate phases, as heredity, physiological selection, sexual selection, primogenital selection, sexual differentiation, including philoprogeneity, hybridity, etc.
Heredity, as expounded by the ablest biologists, and as exemplified in life, is a puissant factor in evolution, and, though essentially conservative, must, through the marvelous power of atavism, tend to increase individual variability. The subject has been too well considered by Darwin and his followers to justify further discussion of it here. As a cause of variation, heredity must, however, have less and less influence as we go back in the scale of organized beings; for it can not well come into play in agamic or fissiparous reproduction, a fact which has given the abiogenisists one of their strongest arguments, since it is difficult to understand how, for instance, the monera of to-day could have descended without change from the primordial form.
Physiological Selection.—Physiological selection, as suggested by Mr. Catchpool and as expounded by Romanes, is undoubtedly a veritable factor in evolution, and, while giving us another link in the chain of evidence as to the causes of differentiation, lessens in but very slight degree the overwhelming force of the argument for natural selection. It adds, rather, an important element in the evidence therefor, and may be classed as a subordinate cause of differentiation. Romanes's theory is based upon the argument that differences, such as constitute varieties and species in their commencement, would not be preserved by natural selection unless useful, but would be lost again by cross-breeding with forms like the parent, and which had not varied, except upon some hypothesis like that of physiological selection. This could not be prevented except by migration. This difficulty is a general one, was argued by Darwin himself, and has been felt by all Darwinians. The reproductive organs are extremely variable, and sterility may occur not only between species, but between races and varieties, and often between individuals. Physiological selection tends to form varieties by peculiarities in the reproductive system of individuals, which render them unfit for perfect union, or cause them to remain more or less sterile, with other individuals which have not the same peculiarities.
The exact reasons are recondite, and the whole subject difficult of demonstration except from the results, since changes in the reproductive organs are not easily observable. Romanes believes this sterility to be incidental to variation, and hence one of the chief causes of the accumulation of such variation. Wherever there has been modification of the reproductive organs introducing incompatibility between two individuals, even where there has been no other change or variation, we have a valid cause of differentiation which in its consequences must be important. Compatibility or fertility between individuals is of the very essence of selection. Natural selection implies that this sexual divergence is subsequent to or coincident with divergences in other directions; physiological selection, that it antecedes them. To put the case of Romanes more fully, we will suppose that among the natural variations there occasionally occurs something to affect the reproductive organs in such wise as to produce incompatibility—i.e., incapacity of one individual with another of the parent type to unite, or sterility of such union, while it remains fertile with the variation of its own kind. This theory, of course, implies variation in the reproductive organs, or departure from the parental type, in at least two individuals of opposite sex simultaneously, and with this admission, for which we are justified in facts, physiological selection will preserve many peculiarities which need have no necessary connection with the exigencies of life.
The change may be in the organs of reproduction, introducing sexual incompatibility, or it may be due to other causes, as, for instance, the time of flowering in plants, or the season of heat in animals. Even the element of scent becomes important here, as my friend J. Jenner Weir has suggested, since it may influence sexual relationship, so that the very excretions of the body, which vary with individuals, must be allowed their part. Francis Galton has indicated a modification of Romanes's views, viz., that the primary characteristic of a variety resides in the fact that the individuals who compose it do not care to mate with those outside their pale. Incipient varieties are thus thrown off from the parent stock by means of peculiarities of sexual instinct which prompt what anthropologists call endogamy, and check exogamy or marriage without the tribe or caste. This is a very good anthropological illustration of how physiological selection may begin.
Natural selection preserves the individuals best adapted to life-conditions by destroying the less fit. Physiological selection may be said to preserve differences which have no necessary connection with the necessities of life. Neither touches the origin of the variation, but both express laws thereof or methods by which it is accumulated. The inherent tendency to vary, whether in external or adaptive structure, or internal or reproductive character, is simply an observed fact, the causes of which we are endeavoring to analyze.
Physiological selection is remarkably exemplified in insects, and probably in no other class are the modifications which may be attributed to it more easily studied; for in no other class are the genitalia of the male so variable or so complex. There has so far been no attempt to homologize the different parts in the different orders of insects, so that they have received different names according to individual authors. Ordinarily there are two pairs of claspers, themselves very variable, associated with sundry hooks and tufts of hair. There are families, as in the Cecidomyidæ, among the Diptera, in which many species are almost, and others absolutely, indistinguishable except by the differences in the male genitalia. In all other orders there are an immense number of forms which can only be distinguished by a careful study of those organs. Descriptive entomology to-day, which does not take account of these organs, is in fact almost valueless, and we must necessarily assume that, where there is differentiation of structure in these important parts, it implies a corresponding modification on the part of some associated female, even where no other differentiated characters are to be detected, and upon Romanes's law such must be looked upon as physiological varieties, and will be counted good species in proportion as the differentiation involves other observable characters or as their life-habits determine.
Sexual Selection.—The part of sexual selection in inducing variation may next be considered. While it is evidently at the bottom of the diversity in sex so common among many animals, it is difficult to see how it can play any very important part in the differentiation of species, except on the hypothesis that the greater the differentiation between the sexes the greater the tendency to vary in the offspring. In no class of organism is this factor more notable than in insects, and volumes might be written to record the interesting and curious facts in this class alone. As a general rule it may be said that with insects, as with other animals, it acts chiefly in inducing secondary sexual characteristics in the male, and in simplifying the characteristics of the female. Nowhere do we find greater contrasts between the sexes, involving almost every organ, both colorationally and structurally. Where color is affected, the greater brilliancy almost always belongs to the male sex, as in birds. So, where song or sound is employed to attract, the sound-organs are either peculiar to, or most highly developed in, the males. As in higher animals, also, so in insects, we find offensive organs highly developed in the male, and either lacking or but partially developed in the female, wherever the struggle for the possession of the female is by force or strength. It has evolved scent-organs in the various parts of the body, causing modification, especially in the Lepidoptera, of either the membrane of the wing or the scaly covering; it has induced profound modification in the structure of the legs, whether the anterior, middle, or posterior pair, and whether in the whole number or some part of it, or in its covering. The subject has been so fully treated by Darwin, however, that it is not necessary to elaborate it further in this connection. Strictly speaking, it may be said to act in two ways, viz., by conflict of the males for possession of the female, or by attractiveness, the former being most conspicuous among mammals, the latter among birds, and both coming conspicuously into play among insects. It is rather difficult to define the limit of sexual selection as a factor in evolution, but I would not confound it with another factor, not hitherto generally recognized, but which I think must be all-powerful, namely, sexual differentiation.
Sexual Differentiation.—It seems evident that the mere differentiation of sex in itself has been an important element in variation. The principle elaborated by Brooks as a modification of the theory of pangenesis is a good one, and in the main the male may be said to be the more complex and to represent the progressive, and the females the more simple and to represent the conservative element in nature. "When the conditions of life are favorable, the female preponderates, and exercises a conservative influence. When the conditions are unfavorable, the males preponderate, and with their greater tendency to vary induce greater plasticity in the species, and hence greater power of adaptation. Sexual differentiation may, I think, be used to include many other variations and differentiations not otherwise satisfactorily accounted for, and to express the law of the interaction of the sexes upon one another, inducing great differentiation entirely apart from the struggle of the males for the possession of the females, or the struggle for existence. Among insects, particularly, though the same is true among other classes, we find many illustrations of this that can hardly be explained by the other forms of selection.
A few of the more notable in Hexapods may be instanced, as the degraded form of the female in Stylopidæ; in very many Lepidoptera and Coleoptera; in the females of the Coccidæ, in Homoptera, etc. In most of these cases it is the female which has been modified, without any very special modification in the male, though it is a general rule that, in proportion as the female is degradational and stationary, the organs which permit him to find her, or to mate with her, and particularly the antennæ, eyes, and genitalia, are profoundly modified and complex. This is especially noticeable in the Psychidæ, where the female remains in her case, a mere mouthless, eyeless, legless, and wingless grub, and the male has most complex and ramose antennæ and complex genitalia. Another remarkable instance may be cited in the Lampyridæ, where we find every degree of degradation in the female, from partial wings to no wings at all, accompanied with increasing complexity of eyes and antennæ in the male, until at last, in the Phengodini, the female is so larviform that she can hardly be distinguished from the true larva. In all these cases the female has been as profoundly modified as, and often more so than, the male, and in the latter case a phosphorescent power has been evolved so that the attractiveness, as in the human species, is rather on the female side. Again, in the case of Corydalus, in Neuroptera, the profound modification of the jaws in the male into prehensile, sickle-shaped organs is to be explained rather on the interaction, between the sexes, and the facility the modification offers for union, than upon sexual selection in its proper and restricted sense.
In this category must also be included the influence of philoprogeneity, which has modified the female rather than the male either in the primary sexual organs for offense or defense, as in the sting of the aculeate Hymenoptera; or in the secondary sexual characters, as in the anal tufts of hair, secretory glands, etc., of many Lepidoptera; or in modification of various other parts of the body exhibited in various orders of insects to facilitate provision for their young, whether in the preservation of the eggs or the accumulation of food for the future progeny. A notable instance of how far this may be carried is furnished by the female Pronuba, where the ovipositor and the maxillæ are so profoundly modified as to make her unique in her order. Sexual selection can have little to do with these modifications, cases of which might be multiplied indefinitely; nor can they be fully explained by natural selection, in the restricted sense in which we have proposed to use it; nor by physiological selection.
In this category might also be included modification which has resulted in the various forms of females which obtain in the same species, fitted whether for agamic or sexual reproduction, and which are far more readily explained on the theory of sexual differentiation aided by environmental influence, especially food and temperature, than upon any other.
Hybridity.—The subject of hybridity has been fully discussed by many, and by no one more ably than by Darwin himself. It has generally been assumed that the hybrid of any two species is sterile, and, in fact, hybridity has been looked upon as one of the best tests of specific value next to genetic incapacity. The assumption finds its greatest support in genesis among the higher animals, and the most thoroughly differentiated species; but the whole subject becomes complicated as we descend in the organic scale, and hybrids between what naturalists generally separate as good species are far more frequently fertile among plants and lower animals than was formerly supposed; while physiological selection, as we have just seen, may render genesis impossible, or at least prevent it, between varieties and incipient species. In this light, hybridity becomes an important factor in the modification of species. Unnecessary importance has been given, in my judgment, to the fact that domestic and wild species differ in the fertility of their crosses. It is assumed, for instance, that all the known breeds of domestic dogs would be fertile inter se and produce fertile crosses. It seems to me, on the very face, a preposterous proposition, and that many of the breeds of domestic dogs are as distinct specifically, and even generically, so far as this test is concerned, as they are in structure and other characteristics. Who, for instance, has ever known or heard of a cross between a bull-dog and a lap-dog, or between a Newfoundland and a black-and-tan? The difference in size alone would seem to render such a cross, if not a physiological or a physical, at least a practical, impossibility; so that hybridity among domestic animals tends to essentially the same result as among wild animals, and confirms its importance as a differentiating factor.
[To be concluded.]
- From the address of the Vice-President of Section F of the American Association for the Advancement of Science, delivered at the Cleveland meeting, August, 1888.
- In the literature of evolution, those are usually termed rudimentary, but, strictly speaking, this term should be applied only to nascent or incipient structures.
- I am well aware that this term is much tabooed among a certain class of the more materialistic evolutionists, but I use it here for want of a better, and because, as an expression of one form of manifestation of force, it has as much a classificatory value as physical or psychical.