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

by George John Romanes

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.

Evidence from Experimental Research.

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.