Darwin, and After Darwin, Volume 3 of 3 / Post-Darwinian Questions: Isolation and Physiological Selection

by George John Romanes

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.

Evidence from Prepotency.

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.