His explanation is that, "as the neutral territory between two representative species is generally narrow in comparison with the territory proper to each, ... and as varieties do not essentially differ from species, the same rule will probably apply to both; and, therefore, if we take a varying species inhabiting a very large area, we shall have to adapt two varieties to two large areas, and a third variety to a narrow intermediate zone." It is hence argued that this third or intermediate variety, on account of its existing in lesser numbers, will probably be soon overrun and exterminated by the larger populations on either side of it. But how is it possible "to adapt two varieties to two large areas, and a third [transitional] variety to a narrow intermediate zone," in the face of free intercrossing on a continuous area? Let A, B, and C represent the three areas in question. According to the argument, variety A passes first into variety B, and then into variety C, while variety B eventually becomes exterminated by the inroads both from A and C. But how can all this have taken place with nothing to prevent intercrossing throughout the entire area A, B, C? I confess that to me it seems this argument can only hold on the supposition that the analogy between varieties and species extends to the reproductive system; or, in a sense more absolute than the argument has in view, that "varieties do not essentially differ from the species" which they afterwards form, but from the first show some degree of infertility towards one another. And, if so, we have of course to do with the principles of physiological selection.
That in all such cases of species-distribution these principles have played an important part in the species-formation, appears to be rendered further probable from the suddenness of transition on the area occupied by contiguous species, as well as from the completeness of it—i. e. the absence of connecting forms. For these facts combine to testify that the transition was originally due to that particular change in the reproductive systems of the forms concerned, which still enables those forms to "interlock" without intercrossing. On the other hand, neither of these facts appears to me compatible with the theory of species-formation by natural selection alone.
But this leads us to another general fact, also mentioned by Darwin, and well recognized by all naturalists, namely, that closely allied species, or species differing from one another in trivial details, usually occupy contiguous areas; or, conversely stated, that contiguity of geographical position is favourable to the appearance of species closely allied to one another. Now, the large body of facts to which I here allude, but need not at present specify, appear to me to constitute one of the strongest of all my arguments in favour of physiological selection. Take, for instance, a large continental area, and follow across it a chain of species, each link of which differs from those on either side of it by the minute and trivial distinctions of a secondary kind, but all the links of which differ from one another in respect of the primary distinction, so that no one member of the series is perfectly fertile with any other member. Can it be supposed that in every case this constant primary distinction has been superinduced by the secondary distinctions, distributed as they are over different parts of all these kindred organisms, and yet nowhere presenting any but a trifling amount of morphological change?
For my own part, I cannot believe—any more than Darwin could believe—that all these numerous, diverse, and trivial changes have always had the accidental effect of inducing the same peculiar change in the reproductive system, and so producing it without any reference to the process of specific divergence. Nor can I believe, as Darwin incidentally and provisionally suggested, that prolonged exposure to uniform conditions of life have so generally induced an equally meaningless result. I can only believe that all the closely allied species inhabiting our supposed continent, and differing from one another in so many and such divers points of small detail, are merely so many records of the fact that selective fertility has arisen among their ancestry, and has thus given as many opportunities for the occurrence of morphological differentiations as it has furnished cases of efficient isolation. Of course, I do not deny that many, or probably most, of these trivial morphological differentiations have been produced by natural selection on account of their utility: I merely deny that they could have been so produced on this common area, but for the sexual isolation with which every distinct set of them is now found to be associated.
By topographical distribution I mean the distribution of organisms with reference to comparatively small areas, as distinguished from larger regions with reference to which the term geographical distribution is appropriate.
It will be at once apparent that a study of the topographical distribution of organic types is of even more importance for us than a study of their geographical distribution. For while the former study is conducted, as it were, with a low power of our observing microscope, the latter is conducted with a high power. The larger facts of geographical distribution yield, indeed, all the general characters which we might expect them to yield, on the theory that divergence of specific types on common areas has been in chief part determined by physiological conditions. But for the purpose of testing this theory in a still more exacting manner, it is of the first importance to consider the more detailed facts of topographical distribution, since we here come to closer quarters with the problem of specific differentiation. Therefore, as we have already considered this problem under the most general points of view, we will now consider it under more special points of view.
It is self-evident, as we have seen in the preceding section, that the greater the number of individuals of the same species on a given area, the less must be the power of natural selection to split that species into two or more allied types; because, the more crowded the population, the greater must be the uniformitarian effect of free intercrossing. This obvious fact has been insisted upon by several previous writers on Darwinism; and the only reason why it has not been recognized by all naturalists is that so few of them have observed the all-important distinction between monotypic and polytypic evolution. The denser the population, and therefore the greater the intercrossing and the severer the struggle for existence within the species, the better will it be for transmutation of the species by natural selection; but the worse it will be for differentiation of the species by this form of homogamy. On the other hand, if physiological selection be entertained as a form of homogamy, the denser the population, the better opportunity it will have of differentiating the species, first, because a greater number of individuals will be present in which the physiological change may arise, and, secondly, because, if it does arise, the severity of the struggle for existence will then give natural selection a better chance of acting rapidly and effectually on each of the isolated sections.
Hence, where the question is whether selective fertility has played any large or general part in the differentiation of specific types, the best criterion we can apply is to ascertain whether it is a general rule that closely allied species occur in intimate association, so that their individual members constitute, as it were, a single population, or, on the other hand, whether they occur rather on different sides of physical barriers. If they occur intimately associated, the form of homogamy to which their differentiation was due must have presumably been the physiological form; whereas, if they are proved to be correlated with physical barriers, the form of homogamy which was concerned in their differentiation must presumably have been the geographical form.
Now, at first this consideration was a trouble to me, because Moritz Wagner had strenuously argued—and supported his argument by a considerable wealth of illustration—that allied species are always found correlated with physical barriers or discontinuous areas. Weismann's answer, indeed, had shown that Wagner's statement was much too general: nevertheless, I was disappointed to find that so much could be said in favour of the geographical (or topographical) form of isolation where closely allied species are concerned. Subsequently, however, I read the writings of Nägeli on this subject, and in them I find a very different state of matters represented.
Seeing as clearly as Wagner that it is impossible under any circumstances for natural selection to cause specific differentiation unless assisted by some other forms of homogamy, but committing the same oversight as Wagner and Weismann in supposing that the only other form of homogamy in nature is geographical isolation, Nägeli, with great force of reasoning, and by many examples, founded his argument against the theory of natural selection on the ground that in the vegetable kingdom closely allied species are most frequently found in intimate association with one another, not, that is to say, in any way isolated by means of physical barriers. This argument is everywhere logically intact; and, as he sustains it by a large knowledge of topographical botany, his indictment against natural selection as a cause of specific differentiation appeared to be insurmountable. And, in point of fact, it was insurmountable; so that the whole problem of the origin of species by differentiation on common areas has hitherto been left in utter obscurity. Nor is there now any escape from this obscurity, unless we entertain the "supplementary factor" of selective fertility. And, apparently, the only reason why this has not been universally recognized, is because Darwinians have hitherto failed to perceive the greatness of the distinction between the differentiation and the transmutation of species; and hence have habitually met such overwhelming difficulties as Nägeli presented by an illogical confounding of these two totally distinct things.
But if the idea of selective fertility had ever occurred to Nägeli as a form of segregation which gives rise to specific differentiation, I can have no doubt that so astute and logical a thinker would have perceived that his whole indictment against natural selection was answered. For it is incredible that he should not have perceived how this physiological form of homogamy (supposing it to arise before or during, and not after the specific differentiation) would perform exactly the same function on a continuous area, as he allowed that "isolation" does on a discontinuous one.
However, be this as it may, there cannot be any question touching the immense value of his facts and arguments as evidence in favour of physiological selection—albeit this evidence was given unconsciously, or, as it were, prophetically. Therefore I will here quote a few examples of both, from his paper Du Développement des Espèces Sociales[24].
After stating the theory of natural selection, he says that if the theory is (of itself) a true explanation of the origin (or divergence) of specific forms, it ought to follow that
two closely allied forms, derived the one from the other, would necessarily occupy two different geographical areas [or topographical stations], since otherwise they would soon become blended. Until they had already become sufficiently consolidated as distinct species to render mutual intercrossing highly improbable, they could not be intermingled without disadvantage [to differentiation]. Had Darwin endeavoured to support his hypothesis by facts, he would, at least in the vegetable kingdom, have found little to favour his cause. I can cite many hundreds of cases, in which species in every stage of development have been found closely mingling with one another, and not in any way isolated. Therefore, I do not think that one can rightly speak of natural selection in the Darwinian sense in the vegetable kingdom; and, in my estimation, there is a great difference between the formation of species by nature and the production of stock by a breeder.... (p. 212).
Of the two kinds of distribution (i. e. growing apart and growing together), Synoicy (or growing together) is by far the most usual in nature. I reckon that out of a hundred allied vegetable forms, at least ninety-five would be found to be synoical (p. 219).
This is a most important point. That so enormous a proportion of vegetable species should have originated in intimate association with their parent or sister types, is clearly unintelligible on the theory of natural selection alone; there obviously must be some other form of homogamy which, whether or not in all places associated with natural selection, is the primary condition to the differentiation. Such I hold with Nägeli, is a logical necessity; and this whether or not I am right in believing the other form of homogamy in question to be selective fertility. But I go further and say, Surely there can be no rational question that this other form of homogamy must have been, at any rate as a highly general rule, the one which I have assigned. For how is it that in these ninety-five per cent. of cases, where vegetable species are growing intimately associated with their nearest allies, there is no hybridizing, or blending and relapsing to the original undifferentiated types? We know well the answer. These are fully differentiated species, and, as such, are protected from mutual intercrossing by the barrier of mutual sterility. But now, if this bar is thus necessary for preserving the specific distinctions when they have been fully developed, much more must it have been so to admit of their development; or, otherwise stated, since we know that this barrier is associated with "synoical" species, and since we clearly perceive that were it withdrawn these species would soon cease to exist, can we reasonably doubt that their existence (or origin) is due to the previous erection of this barrier? If synoical species were comparatively rare, the validity of such reasoning might be open to question; or, even if we should not doubt it in such cases, at any rate we might well doubt the importance or extent of selective fertility as a factor in the origination of species. But the value of Nägeli's writings on the present subject consists in showing that synoical species constitute so overwhelming a majority of the vegetable kingdom, that here, at all events, it appears impossible to rate too highly the importance of the principle I have called physiological selection.
In the last section we have considered the topographical distribution of closely allied species. I now propose to go still further into matters of detail, by considering the case of natural varieties. And here we come upon a branch of our inquiry where we may well expect to meet with the most crucial tests of our theory. For if it should appear that these nascent species more or less resemble fully developed species in presenting the feature of cross-infertility, the theory would be verified in the most direct and conclusive manner possible. These nascent species may be called embryo species, which are actually in course of differentiation from their parent-type; and therefore, if they do not exhibit the feature in relation to that type which the present theory infers to be necessary for the purposes of differentiation, the theory must be abandoned. On the other hand, if they do exhibit this feature, it is just the feature which the theory predicted as one that would be found highly characteristic of such embryo types. Contrariwise, the theory of natural selection can have no reason to form any such anticipation; or rather its anticipation would necessarily require to be the exact opposite. For, according to this theory, the cross-infertility of allied species is due, either to correlation with morphological changes which are being produced by the selection, or else, as Darwin supposed, to "prolonged exposure to uniform conditions of life"; and thus, in either case, the sterility variation ought to be, as a general rule at all events, subsequent to the specific differentiation, and, according to Darwin's view, long subsequent. Thus we ought not to find that the physiological change is ever, on any large or general scale, the initial change; nor ought we to find that it is, on any such scale, even so much as a contemporary change: there ought, in fact, to be no constant or habitual association between divergence of embryo-types and the concurrence of cross-infertility.
Now, it will be my endeavour to prove that there is an extraordinarily general association between varietal divergence and cross-infertility, wherever common areas are concerned; and in as far as this can be proved, I take it that the evidence will make wholly in favour of physiological selection as the prime condition to specific divergence, while at the same time they will make no less wholly, and quite independently, against natural selection as the unaided cause of such divergence.
I shall begin with some further quotations from Nägeli.
Species may be synoical at all stages of relationship. We come across varieties, scarcely distinguishable from one another, growing in the same locality (as, for example, the Cirsium heterophyllum, with smooth or jagged leaves, the Hieracium sylvaticum, with or without caulinary leaves); again, we meet other varieties more accentuated (as the H. hoppeanum, with under ligules of white or red, the Campanula, with white or lilac flowers, &c.), other varieties even more marked, which might almost be elevated to the rank of species (Hieracium alpinum, with hairs and glands, and the new form H. holadenium, which has only glands, Campanula rotundifolia with smooth and hairy leaves), or forms still more distinct, up to well-defined species. I could enumerate endless examples at all stages.
It will be seen that in my definition of synoicy I do not mean to assert that all allied forms are invariably found together, but that they are much more often seen in groups than singly. Take, for instance, nine forms closely related (A to I). A, E, H will be found side by side at one point, B, D at another, C, F at a third, &c. These facts are plainly opposed to the theory of isolation and amixia, and make, on the contrary, in favour of the social development of species (loc. cit., p. 221).
Not to multiply quotations to the same general effect, I will supply but one other, referring to a particular case.
At one spot (Rothwand) much exposed to the sun, and difficult of access, I remarked two closely allied forms, so nearly related to H. villosum that this would seem to be an intermediary form between the two. One of these (H. villosissimum) is distinguished by its tongue and thick pubescence, its tolerably large capitula, and by the lengthened and separated scales of the involucrum; the other, on the contrary (H. elongatum), is less pubescent, has smaller capitula, and more compact scales on the involucrum than H. villosum. Both are finally distinguishable from the type by their longer stalks, which are more decidedly aphyllous, and by their later flowering. At the spot where I found them the two forms were closely intermingled, and each was represented by a considerable number of plants. I did not find them anywhere else on the mountain, nor could I find at the spot where these were growing a single specimen of the true H. villosum, nor a single hybrid from these two.
I concluded that these two new forms had, by joining their forces, expelled the H. villosum from its primitive abode, but had not succeeded in displacing one another. As to their origin, they had evidently developed in two different directions from a common point of departure, namely H. villosum. They had succeeded, not only in separating themselves from the original form, but also in preventing any intermediary form from interposing. I thought myself therefore justified in considering this as a case of varieties which have come into existence subsequently to the Glacial epoch. The morphological characteristics of the three forms are sufficiently distinct for them to be designated as species by a good many writers. They are better defined than some of MM. Frolich and Fries' weaker species, and as well defined as some of MM. Koch and Grisebach's (p. 222).
Now it is clear, without comment, that all this is exactly as it ought to be, if allied species have been differentiated on common areas by selective fertility. For if, as Nägeli elsewhere says, "one meets forms in nature associated with one another, and severally distinguished by every possible degree of differentiation," not only as Nägeli adds, does this general fact lead to the inference that species are (usually) developed when plants grow intimately associated together; but as certainly it leads to the further inference that such development must be due to a prior development of cross-infertility between the diverging varietal forms, cross-infertility which is therefore afterwards so characteristic of the allied species, when these are found, in their fully differentiated condition, still occupying the same area in large and intimately mingled populations.
To my mind there could not be any inference more strongly grounded than this, because, with the one exception of the physiological form, no other form of homogamy can be conceived which shall account for the origin and permanence of these synoical varieties, in all degrees of differentiation up to well-defined synoical species. Least of all, as we have seen, can natural selection alone have had anything to do with such a state of matters; while, as we have likewise seen, in all its details it is exactly the state of matters which the theory of physiological selection requires.
Nevertheless, although this inference is so strongly grounded, we ought to remember that it is only an inference. In order fully to verify the theory of physiological selection, we ought to prove by experiment the fact of cross-infertility between these synoical varieties, as we learn that it afterwards obtains between synoical species. It is to be regretted that the theory of physiological selection did not occur to the mind of Nägeli, because he would then, no doubt, have ascertained this by actual experiment. As it is, the great value of his observations goes no further than establishing a strong presumption, that it must be selective fertility which causes the progressive differentiation of synoical varieties; and also that, if so, this must be the principal factor in the differentiation of vegetable species, seeing that some ninety-five per cent. are of synoical origin.
My paper on Physiological Selection pointed out that the whole theory would have to stand or fall with the experimental proof of the presence or the absence of cross-infertility between varieties of the same species growing on common areas. From the facts and considerations which we have hitherto been dealing with, it did indeed appear to me that there was the strongest conceivable ground for inferring that cross-infertility between such varieties would be found by experiment to be a phenomenon of highly general occurrence—amply sufficient ground to prove that allied species on common areas for the most part owed their origin to this character of mutual sterility, and not vice versa as previously supposed. At that time I was not aware that any experiments had been made in this direction. Soon after the paper was published, however, my attention was directed to a laborious research which had been directed to this very point, and carried on for more than thirty years, by M. Jordan[25]. This had not attracted the general notice which it undoubtedly deserved; and I have since ascertained that even Darwin began to look into it only a few months before his death.
Having devoted his life to closely observing in divers stations multitudes of different species of plants—annuals and perennials, bulbous and aquatic, trees and shrubs—M. Jordan has been able to satisfy himself, and the French school of botanists to which this line of observation has given rise, that in most cases (or "nearly everywhere"), when a Linnean species is indigenous to a country and is there of common occurrence, this species within that district is represented by more or less numerous and perfectly constant varieties. These varieties are constituted by such minute differences of morphological character that their very existence eluded the observation of botanists, until M. Jordan began to search specially for them as the special objects of his scrutiny. Moreover, these varieties of a Linnean species occupy common areas, and there grow in intimate association with one another, or as M. Jordan says, "pêle-mêle." So far, be it noticed, Jordan was proceeding on exactly the same lines as Nägeli; only he carried his observations over a still wider range of species on the one hand, and into a still minuter search for varieties on the other. But the all-important point for us is, that he further proceeded to test by experiment the physiological relations between these morphological varieties; and found, in many hundreds of cases, that they not only came true to seed (i. e. are hereditary and not merely climatic), but likewise cross-sterile inter se. For these reasons, M. Jordan, who is opposed to the theory of evolution, regards all such varieties as separately created species; and the inspiring motive of his prolonged investigations has been a desire to multiply these proofs of creative energy. But it clearly makes no difference, so far as evolutionists are concerned with them, whether all this multitude of sexually isolated forms be denominated species or varieties.
The points which are of importance to evolutionists—and of the first order of importance in the present connexion—may be briefly summarized as follows:—
(1) The research embraces large numbers of species, belonging to very numerous and very varied orders of plants; (2) in the majority of cases—although not all—indigenous species which are of common occurrence present constant varieties; (3) these varieties, nevertheless, may be morphologically so slight as to be almost imperceptible; (4) they occupy common areas and grow in intimate association; (5) although many of them have undergone so small an amount of morphological change, they have undergone a surprising amount of physiological change; for (6) not only do very many of these varieties come true to seed; but, (7) when they do, they are always more or less cross-infertile inter se.
Now, it is self-evident that every one of these seven points is exactly what the theory of physiological selection requires, while there is not one of them which it does not require. For if the theory be sound, we should expect to find large numbers of species belonging to numerous and varied orders of plants presenting constant varieties on common areas; we should expect this to be a highly general, though not a universal, rule; and we should expect it to apply only to species which are indigenous. Moreover, we should expect these varieties, although but slightly differentiated morphologically, to present a great differentiation physiologically—and this in the special direction of selective fertility, combined, of course, with heredity.
On the other hand, as I have said, this catalogue of evidences leaves nothing to be supplied. It gives us all the facts—and no more than all the facts—which my paper on Physiological Selection anticipated as the eventual result of a prolonged experimental research. And if I have to regret my ignorance of these facts when that paper was published, at any rate it now furnishes the best proof that my anticipations were not guided by the results of a verification which had already been supplied. These anticipations were deduced exclusively from the theory itself, as representing what ought to be the case if the theory were true; and, I must confess, if I had then been told that they had already been realized—that it had actually been found to be a general rule that endemic species present constant and hereditary varieties, intimately commingled on common areas, morphologically almost indistinguishable, but physiologically isolated by selective fertility—I should have felt that the theory had been verified in advance. For there are only two alternatives: either these things are due to physiological selection, or else they are due—as M. Jordan himself believes—to special creation. Which is equivalent to saying that, for evolutionists, the facts must be held to verify the former theory in as complete a manner as it is logically possible for the theory to be verified.
We have now to consider the bearing of what is called "prepotency" on the theory of physiological selection.
Speaking of the vast number of species of Compositae, Darwin says:—
There can be no doubt that if the pollen of all these species could be simultaneously or successively placed on the stigma of any one species, this one would elect with unerring certainty its own pollen. This elective capacity is all the more wonderful, as it must have been acquired since the many species of this great group of plants branched off from a common progenitor.
Darwin is here speaking of elective affinity in its fully developed form, as absolute cross-sterility between fully differentiated species. But we meet with all lower degrees of cross-infertility—sometimes between "incipient species," or permanent varieties, and at other times between closely allied species. It is then known as "prepotency" of the pollen belonging to the same variety or species over the pollen of the other variety or species, when both sets of pollen are applied to the same stigma. Although in the absence of the prepotent pollen the less potent will fertilize the seed, yet, such is the appetency for the more appropriate pollen, that even if this be applied to the stigma some considerable time after the other, it will outstrip or overcome the other in fertilizing the ovules, and therefore produce the same result on the next generation as if it had been applied to the mother plant without any admixture of the less potent pollen, although in some cases such incipient degrees of cross-infertility are further shown by the number or quality of the seeds being fewer or inferior.
Now, in different varieties and in different allied species, all degrees of such prepotency have been noticed by many observers, from the faintest perceptible amount up to complete impotency of the alien pollen—when, of course, there is absolute sterility between the two varieties or allied species. The inference is obvious. In this graduated scale of prepotency—beginning with an experimentally almost imperceptible amount of sexual differentiation between two varieties, and ending in an absolute partitioning of two allied species—we have the only remaining fact that is required to complete the case in favour of the present theory. We are here brought back to the very earliest stages of physiological differentiation or to the stages which lie behind Jordan's "Physiological Species"; and therefore, when taken in conjunction with his results, the phenomena of prepotency may be said to give us the complete and final demonstration of one continuous development, which, beginning in an almost imperceptible amount of cross-infertility, ends in absolute cross-sterility. The "elective capacity" to which Darwin alludes as having been "acquired" by all the species of Compositae since they "branched off from a common progenitor," is thus seen among innumerable other species actually in process of acquisition; and so we can perfectly well understand, what is otherwise unintelligible, that closely allied species of plants occur, in ninety-five per cent. of cases, intimately associated on common areas, while exhibiting towards one another the character of mutual sterility.
But more than this. The importance of the widespread phenomena of prepotency to the theory of physiological selection does not consist merely in thus supplying the last link in the chain of evidence touching the origin of species by selective fertility, or "elective capacity." These phenomena are of further importance as showing how in plants, at all events, physiological selection appears to be frequently capable of differentiating specific types without the necessary assistance of any other form of homogamy. In my original statement of the theory, I was careful to insist upon the great value, as differentiating agents, of even small degrees of other forms of homogamy when co-operating with physiological selection. But I also stated my belief that in many cases selective fertility is presumably of itself capable of splitting a specific type; and the reason why I still believe this is, that I do not otherwise understand these phenomena of prepotency. I cannot believe that in all the innumerable cases where they arise, they have been super-induced by some prior morphological changes going on in some other part of the organism, or by "prolonged exposure to uniform conditions of life," on the part of two well-nigh identical forms which have arisen intimately commingled in exactly the same environment, and under the operation of a previously universal intercrossing. Even if such a thing could be imagined as happening occasionally, I feel it difficult to imagine that it can happen habitually, and yet this view must be held by those who would attribute prepotency to natural selection.
It must never be forgotten that the relatively enormous changes as to size, structure, habit, &c., which are presented by our domesticated plants as results of artificial selection, do not entail the physiological character of cross-sterility in any degree, save possibly in some small number of cases. Although in wild species any correspondingly small percentage of cases (where natural selection happens to hit upon parts of the organism modifications of which produce the physiological change by way of correlation) would doubtless be the ones to survive on common areas, still it is surely incredible that such an accidental association between natural selection and cross-infertility is so habitually the means of specific differentiation as the facts of prepotency (together with the observations of Jordan and Nägeli) would necessarily demand.
Moreover, this view of the matter is still further corroborated by certain other facts and considerations. For example, the phenomena of prepotency (whether as between varieties or between closely allied species) are found to occur when the two forms occupy a common area, i.e. are growing intermingled with one another. Therefore, but for this physiological differentiation, there could be absolutely nothing to prevent free intercrossing. Yet the fact that hybrids are so comparatively rare in a state of nature—a fact which Sir Joseph Hooker has pointed out to me as otherwise inexplicable—proves the efficacy of even a low degree of such differentiation in preventing the physiologically-differentiated forms from intercrossing. Even in cases where there is no difficulty in producing artificial hybrids or mongrels between species or varieties growing on common areas, it is perfectly astonishing what an extremely small percentage of the hybrid or mongrel forms are found to occur in nature. And there can be no question that this is due to the very efficient manner in which prepotency does its work—efficient, I mean, from the point of view of the new theory; for upon any other theory prepotency is a meaningless phenomenon, which, notwithstanding its frequent occurrence, plays no part whatever in the process of organic evolution.
I attach considerable importance to the phenomena of prepotency in view of the contrast which is presented between plants and animals in the relation of their species to physical barriers. For animals—and especially the higher animals—appear to depend for their specific differentiations upon such barriers much more than in the case with plants. This is no more than we should expect; for, in accordance with our theory, selective fertility is not so likely to work alone in the case of the higher animals which mate together, as in plants which are fertilized through the agency of wind or insects. In the former case there is no opportunity given for the first rise of cross-infertility, in the form of prepotency; and even where selective fertility has gained a footing in other ways, the chances against the suitable mating of "physiological complements" must be much greater than it is in the latter case. Hence, among the higher animals, selective fertility ought much more frequently to be found in association with other forms of homogamy than it is among plants. And this is exactly what we find. Thus it seems to me that this contrast between the comparative absence and presence of physical barriers, where allied species of plants and of higher animals are respectively concerned, is entitled to be taken as a further corroboration of our theory. For while it displays exactly such a general correlation as this theory would expect, the correlation is one which cannot possibly be explained on any other theory. It is just where physiological selection can be seen to have the best opportunity of acting (viz. in the vegetable kingdom) that we find the most unequivocal evidence of its action; while, on the other hand, it is just where it can be seen to have the least opportunity of asserting itself (viz. among the higher animals) that we find it most associated with, and therefore assisted by, other forms of homogamy, i. e. not only geographical isolation, but also by sexual preference in pairing, and the several other forms of homogamy, which Mr. Gulick has shown to arise in different places as the result of intelligence.
Hitherto I have been considering, from the most general point of view, the most widespread facts and broadest principles which serve to substantiate the theory of physiological selection. I now pass to the consideration of one of those special cases in which the theory appears to have been successfully applied.
Professor Le Conte has adduced the fossil snails of Steinheim as serving to corroborate the theory of physiological selection[26].
The facts are these. The snail population of this lake remain for a long time uniform and unchanged. Then a small percentage of individuals suddenly began to vary as regards the form of their shells, and this in two or three directions at the same time, each affected individual, however, only presenting one of the variations. But after all these variations had begun to affect a proportionally large number of individuals, some individuals occur in which two or more of the variations are blended together, evidently, as Weismann says, by intercrossing of the varieties so blended. Later still, both the separate varieties and their blended progeny became more and more numerous, and eventually a single blended type, comprising in itself all the initial varieties, supplanted the parent form. Then another long period of stability ensued until another eruption of new variations took place; and these variations, after having affected a greater and greater number of individuals, eventually blended together by intercrossing and supplanted their parent form. So the process went on, comparatively short periods of variation alternating with comparatively long periods of stability, the variations, moreover, always occurring suddenly in crops, then multiplying, blending together, and in their finally blended type eventually supplanting their parent form.
Now, the remarkable fact here is that whenever the variations arose, they only intercrossed between themselves, they did not intercross with their parent form; for, if they had, not only could they never have survived (having been at first so few in number and there having been no geographical barriers in the small lake), but we should have found evidence of the fact in the half-bred progeny. Moreover, natural selection can have had nothing to do with the process, because not only are the variations in the form of the shells of no imaginable use in themselves; but it would be preposterous to suppose that at each of these "variation periods" several different variations should always have occurred simultaneously, all of which were of some hidden use, although no one of them ever occurred during any of the prolonged periods of stability. How, then, are we to explain the fact that the individuals composing each crop of varieties, while able to breed among themselves, never crossed with their parent form? These varieties, each time that they arose, were intimately commingled with their parent form, and would certainly have been reabsorbed into it had intercrossing in that direction been possible. With Professor Le Conte, therefore, I conclude that there is only one conceivable answer to this question. Each crop of varieties must have been protected from intercrossing with their parent form.
They must have been the result of a variation, which rendered the affected individuals sterile with their parent form, whilst leaving them fertile amongst themselves. The progeny of these individuals would then have dispersed through the lake, physiologically isolated from the parent population, and especially prone to develop secondary variations as a direct result of the primary variation. Thus, as we might expect, two or three variations arose simultaneously, as expressions of so many different lines of family descent from the original or physiological variety; these were everywhere prevented from intercrossing with their parent form, yet capable of blending whenever they or their ever-increasing progeny happened to meet. Thus, without going into further details, we are able by the theory of physiological selection to give an explanation of all these facts, which otherwise remain inexplicable.
In view of the evidence which has now been presented, I will now ask five questions which must be suitably answered by critics of the theory of physiological selection.
1. Can you doubt that the hitherto insoluble problem of inter-specific sterility would be solved, supposing cross-infertility were proved to arise before or during the process of specific differentiation, instead of after that process had been fully completed?
2. Can you doubt, after duly considering the circumstances under which allied species of plants have been differentiated—viz. in ninety-five per cent. of cases intimately commingled on common areas, and therefore under identical environments—that cross-infertility must have arisen before or during the specific differentiation?
3. Can you doubt, after duly considering the facts of prepotency on the one hand and those of Jordan's physiological varieties on the other, that cross-infertility does arise before or during the specific differentiation?
4. If you cannot express a doubt upon any of these points, can you explain why you refuse to accept the theory of the origin of species by means of physiological selection, together with the explanation which this theory affords of the continued cross-fertility of domesticated varieties?
5. Supposing this theory to be true, can you conceive of any other classes of facts which, either quantitatively or qualitatively, could more directly or more effectually prove its truth than those which have now been adduced?
On these five heads I entertain no doubt. I am convinced that the theory of physiological selection is the only one that can explain the facts of inter-specific sterility on the one hand, and, on the other hand, the contrast which these facts display to the unimpaired fertility of our domesticated varieties.
In conclusion, it seems desirable once more to insist that there is no antagonism or rivalry between the theories of natural and of physiological selection. For which purpose I will quote the final paragraph of my original paper.
So much, then, for the resemblances and the differences between the two theories. It only remains to add that the two are complementary. I have already shown some of the respects in which the newer theory comes to the assistance of the older, and this in the places where the older has stood most in need of assistance. In particular, I have shown that segregation of the fit entirely relieves survival of the fittest from the difficulty under which it has hitherto laboured of explaining why it is that sterility is so constantly found between species, while so rarely found between varieties which differ from one another even more than many species; why so many features of specific distinction are useless to the species presenting them; and why it is that incipient varieties are not obliterated by intercrossing with parent forms. Again, we have seen that physiological selection, by preventing such intercrossing, enables natural selection to promote diversity of character, and thus to evolve species in ramifying branches instead of in linear series—a work which I cannot see how natural selection could possibly perform unless thus aided by physiological selection. Moreover, we have seen that although natural selection alone could not induce sterility between allied types, yet when this sterility is given by physiological selection, the forms which present it would be favoured in the struggle for existence; and thus again the two principles are found playing, as it were, into each other's hands. And here, as elsewhere, I believe that the co-operation enables the two principles to effect very much more in the way of species-making than either of them could effect if working separately. On the one hand, without the assistance of physiological selection, natural selection would, I believe, be all but overcome by the adverse influences of free intercrossing—influences all the more potent under the very conditions which are required for the multiplication of species by divergence of character. On the other hand, without natural selection, physiological selection would be powerless to create any differences of specific type, other than those of mutual sterility and trivial details of structure, form, and colour—differences wholly without meaning from a utilitarian point of view. But in their combination these two principles appear to me able to accomplish what neither can accomplish alone—namely, a full and satisfactory explanation of the origin of species.
This historical sketch must begin with a consideration of Darwin's opinions on the subject; but as these were considerably modified from time to time during a period of thirty years by the publications of other naturalists, it will be impossible to avoid cross-references as between his writings and theirs. It may also be observed that the Life and Letters of Charles Darwin was not published until the year 1887, so that the various opinions which I shall quote from the letters, and which show some considerable approximation in his later years to the views which have been put forward by Mr. Gulick and myself, were not before us at the time when our papers were read.
The earliest allusion that I can find to geographical isolation in the writings of Darwin occurs in a correspondence with Sir Joseph Hooker, as far back as 1844. He there says:—
I cannot give my reasons in detail; but the most general conclusion which the geographical distribution of all organic beings appears to me to indicate is, that isolation is the chief concomitant or cause of the appearance of new forms (I well know there are some staring exceptions)[27].
And again:—
With respect to original creation or production of new forms, I have said that isolation appears the chief element[28].
Next, in the earlier editions of the Origin of Species this view is abandoned, and in its stead we meet with the opinion that geographical isolation lends a certain amount of assistance to natural selection, by preventing free intercrossing. But here we must note two things. First, the distinction between monotypic and polytypic evolution is not defined. Secondly, the levelling effect of free intercrossing in nature, and hence its antagonism to divergence of character by natural selection, is not sufficiently recognized; while, on the other hand, and in consequence of this, the importance of isolation as a factor of evolution is underrated—not only in its geographical, but likewise in all its other forms.
Taking these two points separately, the only passages in Darwin's writings, so far at least as I can find, in which any distinction is drawn between evolution as monotypic and polytypic, are those in which he deals with a somewhat analogous distinction between artificial selection as intentional and unconscious. He says, for example:—
In the case of methodical selection, a breeder selects for some definite object, and if the individuals be allowed freely to intercross, his work will completely fail. But when many men, without intending to alter the breed, have a nearly common standard of perfection, and all try to procure and breed from the best animals, improvement surely but slowly follows from this unconscious process of selection, notwithstanding that there is no separation of selected individuals. Thus it will be under nature[29].
Here we have what may perhaps be regarded as a glimmering of the distinction between monotypic and polytypic evolution. But that it is only a glimmering is proved by the immediately ensuing sentences, which apply this analogy of unconscious selection not to the case of monotypic, but to that of polytypic evolution. So likewise, in the succeeding discussion on "divergence of character," the analogy is again resorted to for the purpose of showing how polytypic evolution may occur in nature.
Thus far, then, it may be said that we have scarcely so much as a glimmering of the distinction between monotypic and polytypic evolution; and as the same discussion (with but a few verbal alterations) runs through all the editions of the Origin, it may well be asked why I should have alluded to such passages in the present connexion. Well, I have done so because it is apparent that, during the last years of his life, the distinction between selection as "methodical" and "unconscious" enabled Darwin much more clearly to perceive that between evolution as monotypic and polytypic. Thus in 1868 he wrote to Moritz Wagner (who, as we shall presently see, entirely failed to distinguish between monotypic and polytypic evolution), expressing his belief—
That in many large areas all the individuals of the same species have been slowly modified, in the same manner, for instance, as the English racehorse has been improved, that is, by the continued selection of the fleetest individuals, without any separation. But I admit that by this process two or more new species could hardly be formed within the same limited area[30].
Again, in 1876 he wrote another letter to Wagner, in which the following passage occurs:—
I believe that all the individuals of a species can be slowly modified within the same district, in nearly the same manner as man effects by what I have called the process of unconscious selection. I do not believe that one species will give birth to two or more new species as long as they are mingled together within the same district[31].
Two years later he wrote to Professor Semper:—
There are two different classes of cases, it appears to me, viz. those in which species becomes slowly modified in the same country, and those cases in which a species splits into two, or three, or more new species; and, in the latter case, I should think nearly perfect separation would greatly aid in their "specification," to coin a new word[32].
Now, these passages show a very much clearer perception of the all-important distinction between monotypic and polytypic evolution than any which occur in the Origin of Species; and they likewise show that he was led to this perception through what he supposed to be a somewhat analogous distinction between "unconscious" and "methodical" selection by man. The analogy, I need hardly say, is radically unsound; and it is a curious result of its unsoundness that, whereas in the Origin of Species it is adduced to illustrate the process of polytypic evolution, as previously remarked, in the letters above quoted we find it adduced to illustrate the process of monotypic evolution. But the fact of this analogy being unsound does not affect the validity of the distinction between monotypic and polytypic evolution to which it led Darwin, in his later years, so clearly to express[33].
Turning next to the second point which we have to notice, it is easy to show that in the earlier editions of his works Darwin did not sufficiently recognize the levelling effects of free intercrossing, and consequently failed to perceive the importance of isolation (in any of its forms) as a factor of organic evolution. This may be most briefly shown by quoting his own more matured opinion upon the subject. Thus, with reference to the swamping effects of intercrossing, he wrote to Mr. Wallace in 1867 as follows:—
I must have expressed myself atrociously: I meant to say exactly the reverse of what you have understood. F. Jenkin argued in the North British Review against single variations being perpetuated, and has convinced me, though not in quite so broad a manner as here put. I always thought individual differences more important; but I was blind, and thought that single variations might be preserved much oftener than I now see is possible or probable. I mentioned this in my former note merely because I believed that you had come to a similar conclusion, and I like much to be in accord with you. I believe I was mainly deceived by single variations offering such simple illustrations, as when man selects [i.e. isolates][34].
Again, somewhere about the same time, he wrote to Moritz Wagner:—
Although I saw the effects of isolation in the case of islands and mountain-ranges, and knew of a few instances of rivers, yet the greater number of your facts were quite unknown to me. I now see that, from the want of knowledge, I did not make nearly sufficient use of the views which you advocate[35].
Now it would be easy to show the justice of these self-criticisms by quoting longer passages from earlier editions of the Origin of Species; but as this, in view of the above passages, is unnecessary, we may next pass on to another point.
The greatest oversight that Wagner made in his otherwise valuable essays on geographical isolation, was in not perceiving that geographical isolation is only one among a number of other forms of isolation: and, therefore, that although it is perfectly true, as he insisted, that polytypic evolution cannot be effected by natural selection alone, it is very far from true, as he further insisted, that geographical isolation is the only means whereby natural selection can be assisted in this matter. Hence it is that, when Darwin said he had not himself "made nearly sufficient use" of geographical isolation as a factor of specific divergence, he quite reasonably added that he could not go so far as Wagner did in regarding such isolation as a condition, sine qua non, to divergent evolution in all cases. Nevertheless, he adds the important words, "I almost wish I could believe in its importance to the same extent with you; for you well show, in a manner which never occurred to me, that it removes many difficulties and objections." These words are important, because they show that Darwin had come to feel the force of the "difficulties and objections" with regard to divergent evolution being possible by means of natural selection alone, and how readily they could be removed by assuming the assistance of isolation. Hence, it is much to be deplored that Wagner presented a single kind of isolation (geographical) as equivalent to the principle of isolation in general. For he thus failed to present the complete—and, therefore, the true—philosophy of the subject to Darwin's mind; and in this, as in certain other respects which I shall notice later on, served rather to confuse than to elucidate the matter as a whole.
To sum up. Although in his later years, as shown by his correspondence, Darwin came to recognize more fully the swamping effects of free intercrossing, and the consequent importance of "separation" for the prevention of these effects, and although in this connexion he likewise came more clearly to distinguish between the "two cases" of monotypic and polytypic evolution, it is evident that he never worked out any of these matters—"thinking it prudent," as he wrote with reference to them in 1878, "now I am growing old, to work at easier subjects[36]." Therefore he never clearly saw, on the one hand, that free intercrossing, far from constituting a "difficulty" to monotypic evolution by natural selection, is the very means whereby natural selection is in this case enabled to operate; or, on the other hand, that, in the case of polytypic evolution, the "difficulty" in question is so absolute as to render such evolution, by natural selection alone, absolutely impossible. Hence, although in one sentence of the Origin of Species he mentions three forms of isolation (besides the geographical form) as serving in some cases to assist natural selection in causing "divergence of character" (i. e. polytypic evolution[37]), on account of not perceiving how great and how sharp is the distinction between the two kinds or "cases" of evolution, he never realized that, where "two or more new species" are in course of differentiation, some form of isolation other than natural selection must necessarily be present, whether or not natural selection be likewise so. The nearest approach which he ever made to perceiving this necessity was in one of his letters to Wagner above quoted, where, after again appealing to the erroneous analogy between monotypic evolution and "unconscious selection," he says:—"But I admit that by this process (i. e. unconscious selection) two or more new species could hardly be formed within the same limited area: some degree of separation, if not indispensable, would be highly advantageous; and here your facts and views will be of great value." But even in this passage the context shows that by "separation" he is thinking exclusively of geographical separation, which he rightly enough concludes (as against Wagner) need certainly not be "indispensable." Had he gone a step further, he must have seen that separation, in some form or another, is "indispensable" to polytypic evolution. Instead of taking this further step, however, two years later he wrote to Semper as follows:—
I went as far as I could, perhaps too far, in agreement with Wagner [i. e. in the last edition of the Origin of Species]; since that time I have seen no reason to change my mind; but then I must add that my attention has been absorbed on other subjects[38].
And he seems to have ended by still failing to perceive that the explanation which he gives of "divergence of character" in the Origin of Species, can only hold on the unexpressed assumption that free intercrossing is in some way prevented at the commencement, and throughout the development, of each diverging type.
Lastly, we have to consider Darwin's opinion touching the important principle of "Independent Variability." This, it will be remembered, is the principle which ensures that when a portion (not too large) of a species is prevented from interbreeding with the rest of the species, sooner or later a divergence of type will result, owing to the fact that the average qualities of the separated portion at the time of its separation cannot have been exactly the same as the average qualities of the specific type as a whole. Thus the state of Amixia, being a state of what Mr. Gulick calls Independent Generation, will of itself—i.e. even if unassisted by natural selection—induce divergence of type, in a ratio that has been mathematically calculated by Delbœuf.
Darwin wrote thus to Professor Weismann in 1872:—
I have now read your essay with very great interest. Your view of the origin of local races through "Amixia" is altogether new to me, and seems to throw an important light on an obscure question[39].
And in the last edition of the Variation of Animals and Plants he adds the following paragraph:—
This view may throw some light on the fact that the domestic animals which formerly inhabited the several districts in Great Britain, and the half-wild cattle lately kept in several British parks, differed slightly from one another; for these animals were prevented from wandering over the whole country and intercrossing, but would have crossed freely within each district or park[40].
Now, although I allow that Darwin never attributed to this principle of Amixia, or Independent Variability, anything like the degree of importance to which, in the opinion of Delbœuf, Gulick, Giard, and myself, it is entitled, the above passage appears to show that, as soon as the "view" was clearly "suggested" to his mind, he was so far from being unfavourably disposed towards it, that he added a paragraph to the last edition of his Variation for the express purpose of countenancing it. Nevertheless, later on the matter appears to have entirely escaped his memory; for in 1878 he wrote to Semper, that he did "not see at all more clearly than I did before, from the numerous cases which he [Wagner] has brought forward, how and why it is that a long isolated form should almost always become slightly modified[41]." I think this shows entire forgetfulness of the principle in question, because, if the latter is good for explaining the initial divergence of type as between separated stocks of "domesticated animals," much more must it be competent to explain the further divergence of type which is "almost always" observable in the case of "a long isolated form" under nature. The very essence of the principle being that, when divergence of type has once begun, this divergence must ipso facto proceed at an ever-accelerating pace, it is manifestly inconsistent to entertain the principle as explaining the first commencement of divergence, and then to ignore it as explaining the further progress of divergence. Hence, I can only conclude that Darwin had forgotten this principle altogether when he wrote his letter to Semper in 1878—owing, no doubt, as he says in the sentence which immediately follows, to his having "not attended much of late years to such questions."
So much, then, for Darwin's opinions. Next in order of time we must consider Moritz Wagner's essays on what he called the "Law of Migration[42]." The merit of these essays was, first, the firm expression of opinion upon the swamping effects of free intercrossing; and, second, the production of a large body of facts showing the importance of geographical isolation in the prevention of these effects, and in the consequent differentiation of specific types. On the other hand, the defect of these essays was, first, not distinguishing between evolution as monotypic and polytypic; and, second, not perceiving that geographical isolation is only one among a number of other forms of isolation. From these two radical oversights—which, however, were shared by all other writers of the time, with the partial exception of Darwin himself, as previously shown—there arose the following and most lamentable errors.
Over and over again Moritz Wagner insists, as constituting the fundamental doctrine of his attempted reform of Darwinism, that evolution by natural selection is impossible, unless natural selection be assisted by geographical isolation, in order to prevent the swamping effects of intercrossing[43]. Now, if instead of "evolution" he had said "divergence of type," and if instead of "geographical isolation" he had said "prevention of intercrossing," he would have enunciated the general doctrine which it has been the joint endeavour of Mr. Gulick and myself to set forth. But by not perceiving that "evolution" is of two radically different kinds—polytypic and monotypic—he entirely failed to perceive that, while for one of its kinds the prevention of intercrossing is an absolute necessity, for the other of its kinds the permission of intercrossing is a necessity no less absolute. And, again, in missing the fact that geographical isolation is but one of the many ways whereby intercrossing may be prevented, he failed to perceive that, even as regards the case of polytypic evolution, he greatly erred in representing this one form of isolation as being universally a necessary condition to the process. The necessary condition to this process is, indeed, the prevention of intercrossing by some means or another; but his unfortunate insistence on geographical separation as the only possible means to this end—especially when coupled with his no less unfortunate disregard of monotypic evolution—caused him to hinder rather than to advance a generalization which he had only grasped in part. And this generalization is, as now so repeatedly stated, that while the form of isolation which we know as natural selection depends for its action upon the intercrossing of all the individuals which it isolates (i. e. selects), when acting alone it can produce only monotypic evolution; but that when it is supplemented by any of the other numerous forms of isolation, it is furnished with the necessary condition to producing polytypic evolution—and this in as many lines of divergent change as there may be cases of this efficient separation.
Nevertheless, while we must lament these shortcomings on the part of Wagner, we ought to remember that he rendered important services in the way of calling attention to the swamping effects of free intercrossing, and, still more, in that of showing the high importance of geographical isolation as a factor of organic evolution. Therefore, although in an elaborate criticism of his views Weismann was easily able to dispose of his generalizations in the imperfect form that they presented, I do not think it was just in Weismann to remark, "if Wagner had confined himself to the statement that geographical isolation materially assists the process of natural selection, and thus also promotes the origination of new species, he would have met with little or no opposition; but then, of course, in saying this much, he would not have been saying anything new." No doubt, as I have just shown, he ought thus (as well as in other and still more important respects not perceived by Prof. Weismann) to have limited his statement; but, had he done so, it does not follow that he would not have been saying anything new. For, in point of fact, in as far as he said what was true, he did say a great deal that was also new. Thus, most of what he said of the principle of separation (apogamy) was as new as it was true, although, as we have seen, he said it to very little purpose on account of his identifying this principle as a whole with that of but one of its forms. Again, notwithstanding this great error, or oversight, he certainly showed of the particular form in question—viz. geographical isolation—that it was of considerably more importance than had previously been acknowledged. And this was so far a valuable contribution to the general theory of descent.