Much more to the point are certain cases which were adduced long ago by Weismann in his essay previously considered. Nevertheless, although this essay was published as far back as 1872, and, although it expressly deals with the question of divergence of character through the mere prevention of intercrossing (Amixia), Mr. Wallace nowhere alludes to these cases per contra, which are so much more weighty than his own "test case" of Ireland. Of such are four species of butterflies, belonging to three genera[56], which are identical in the polar regions and in the Alps, notwithstanding that the sparse Alpine populations have been presumably separated from their parent stocks since the glacial period; or of certain species of fresh water crustaceans (Apus), the representatives of which are compelled habitually to form small isolated colonies in widely separated ponds, and nevertheless exhibit no divergence of character, although apogamy has probably lasted for centuries. These cases are unquestionably of a very cogent nature, and appear of themselves to prove that apogamy alone is not invariably capable of inducing divergence—at any rate, so rapidly as we might expect. There appears, however, to be another factor, the presence or absence of which makes a great difference. This as stated in the text, is the degree in which a specific type is stable or unstable—liable or not liable to vary. Thus, for example, the Goose is what Darwin calls an "inflexible" type as compared with most other domesticated birds. Therefore, if a lot of geese were to be indiscriminately isolated from the rest of their species, the probability is that in a given time their descendants would not have diverged from the parent type to such an extent as would a similar lot of ducks under similar circumstances: the more stable specific type would require a longer time to change under the influence of apogamy alone. Now, the butterflies and crustaceans quoted by Weismann may be of a highly stable type, presenting but a small range of individual variability; and, if so, they would naturally require a long time to exhibit any change of type under the influence of apogamy alone. But, be this as it may, Weismann himself adduces these cases merely for the sake of showing that there are cases which seem to tell against the general principle of modification as due to apogamy alone—i.e. the general principle which, under the name amixia, he is engaged in defending. And the conclusion at which he himself arrives is, that while it would be wrong to affirm that apogamy must in all cases produce divergence, we are amply justified in affirming that in many cases it may have done so; while there is good evidence to prove that in not a few cases it has done so, and therefore should be accepted as one of the factors of organic evolution[57].
My view from the very first has been that variations in the way of cross-infertility are of frequent occurrence (how, indeed, can they be otherwise, looking to the complex conditions that have to be satisfied in every case of full fertility?); and, therefore, however many of such variations are destined to die out, whenever one arises, "under suitable conditions," "it must inevitably tend to be preserved as a new natural variety, or incipient species." Among the higher animals—which are "comparatively few in number"—I think it probable that some slight change of form, colour, habit, &c., must be usually needed either to "superinduce," or, which is quite a different thing, to coincide with the physiological change But in the case of plants and the lower invertebrata. I see no reason for any frequent concomitance of this kind; and therefore believe the physiological change to be, "as a general rule," the primordial change. At the same time, I have always been careful to insist that this opinion had nothing to do with "the essence of physiological selection"; seeing that "it was of no consequence" to the theory in what proportional number of cases the cross-sterility had begun per se, had been superinduced by morphological changes, or only enabled to survive by happening to coincide with any other form of homogamy. In short, "the essence of physiological selection" consists in all cases of the diversifying effect of cross-infertility, whensoever and howsoever it may happen in particular cases to have been caused.
Thus I emphatically reaffirm that "from the first I have always maintained that it makes no essential difference to the theory in what proportional number of cases they [the physiological variations] have arisen 'alone in an otherwise undifferentiated species'"; therefore, "even if I am wrong in supposing that physiological selection can ever act alone, the principle of physiological selection, as I have stated it, is not thereby affected. And this principle is, as Mr. Wallace has re-stated it, 'that some amount of infertility characterizes the distinct varieties which are in process of differentiation into species'—infertility whose absence, 'to obviate the effects of intercrossing, may be one of the usual causes of their failure to become developed into distinct species.'"
These last sentences are quoted from the correspondence in Nature[58], and to them Mr. Wallace replied by saying, "if this is not an absolute change of front, words have no meaning"; that "if this is 'the whole essence of physiological selection,' then physiological selection is but a re-statement and amplification of Darwin's views"; that such a "change of front" is incompatible, not only with my term "physiological selection," but also with my having "acknowledged that Mr. Catchpool had 'very clearly put forward the theory of physiological selection'"; and much more to the same effect.
Now, to begin with, it is due to Mr. Catchpool to state that his only publication upon this subject is much too brief to justify Mr. Wallace's, inference, that he supposes variations in the way of cross-infertility always to arise "alone in an otherwise undifferentiated species." What Mr. Catchpool's opinion on this point may be, I have no knowledge; but, whatever it is, he was unquestionably the first writer who "clearly stated the leading principles" of physiological selection, and this fact I am very glad to have "acknowledged." In my correspondence with Mr. Wallace, however, I not only named Mr. Catchpool: I also named—and much more prominently—Mr. Gulick. For even if I were to grant (which I am far indeed from doing) that there was any want of clearness in my own paper touching the point in question, I have now repeatedly shown that it is simply impossible for any reader of Mr. Gulick's papers to misunderstand his views with regard to it. Accordingly, I replied to Mr. Wallace in Nature by saying:—
Not only have I thus from the first fully recognized the sundry other causes of specific change with which the physiological variations may be associated; but Mr. Gulick has gone into this side of our common theory much more fully, and elaborately calculated out the high ratio in which the differentiating agency of any of these other causes must be increased when assisted by—i. e. associated with—even a moderate degree of the selective fertility, and vice versa. Therefore, it is simply impossible for Mr. Wallace to show that "our theory" differs from his in this respect. Yet it is the only respect in which his reply alleges any difference. (Vol. xliii. p. 127.)
I think it is to be regretted that, in his answer to this, Mr. Wallace alludes only to Mr. Catchpool, and entirely ignores Mr. Gulick—whose elaborate calculations above alluded to were communicated to the Linnaean Society by Mr. Wallace himself in 1887.
The time has now come to prove, by means of quotations, that I have from the first represented the "principle," or "essence," of physiological selection to consist in selective fertility furnishing a needful condition to specific differentiation, in at least a large proportional number of allied species which afterwards present the reciprocal character of cross-sterility; that I have never represented variations in the way of this selective fertility as necessarily constituting the initial variations, or as always arising "alone, in an otherwise undifferentiated species"; and that, although I have uniformly given it as my opinion that these variations do in some cases thus arise (especially among plants and lower invertebrata), I have as uniformly stated "that it makes no difference to the theory in what proportional number of cases they have done so"—or even if, as Mr. Wallace supposes, they have never done so in any case at all[59]. These statements (all of which are contradictory of the only points of difference alleged) have already been published in my article in the Monist of October, 1890. And although Mr. Wallace, in his reply to that article, ignores my references to the "original paper," it is scarcely necessary to quote the actual words of the paper itself, since the reader who is further interested in this controversy can readily refer to it in the Journal of the Linnaean Society (vol. xix. pp. 337-411).
Having arrived at these results with regard to the theory of Isolation in general and of Physiological Isolation in particular, I arrive also at the end of this work. And if, while dealing with the post-Darwinian period, I have imparted to any general reader the impression that there is still a great diversity of expert opinion; I must ask him to note that points with reference to which disagreement still exists are but very subordinate to those with regard to which complete agreement now prevails. The noise of wrangling disputations which has so filled the camp of evolutionists since the death of their captain, is apt to hide from the outside world the solid unanimity that prevails with regard to all the larger and more fundamental questions, which were similarly the subjects of warfare in the past generation. Indeed, if we take a fair and general view of the whole history of Darwinism, what must strike us as the really significant fact is the astonishing unanimity which has been so rapidly attained with regard to matters of such immeasurable importance. It is now but little more than thirty years since the publication of the Origin of Species; and in that period not only have all naturalists unequivocally embraced the doctrine of descent considered as a fact; but, in one degree or another, they have all as unequivocally embraced the theory of natural selection considered as a method. The only points with regard to which any difference of opinion still exist, have reference to the precise causation of that mighty stream of events which, under the name of organic evolution, we have now all learnt to accept as scientifically demonstrated. But it belongs to the very nature of scientific demonstration that, where matters of great intricacy as well as of high generality are concerned, the process of demonstration must be gradual, even if it be not always slow. It is only by the labours of many minds working in many directions that, in such cases, truth admits of being eventually displayed. Line upon line, precept upon precept, here a little and there a little—such is the course of a scientific revelation; and the larger the subject-matter, the more subtle and the more complex the causes, the greater must be the room for individual differences in our reading of the book of Nature. Now, if all this be true, must we not feel that in the matter of organic evolution the measure of agreement which has been attained is out of all proportion to the differences which still remain—differences which, although of importance in themselves, are insignificant when compared with those which once divided the opinions of not a few still living men? And if we are bound to feel this, are we not bound further to feel that the very intensity of our disputations over these residual matters of comparative detail, is really the best earnest that can be given of the determination of our quest—determination which, like that of our fathers, cannot fail to be speedily rewarded by the discovery of truth?
Nevertheless, so long as this noise of conflict is in the Senate, we cannot wonder if the people are perplexed. Therefore, in conclusion, I may ask it to be remembered exactly what are the questions—and the only questions—which still divide the parties.