Volume Iii Part 8 (1/2)
[51] _Animal Life and Intelligence_, pp. 98, 99 (1890-1891).
Similarly, and still more recently, Professor Le Conte writes:--
It is evident, then, as Romanes claims, that natural selection alone tends to _monotypic_ evolution. Isolation of some sort seems necessary to _polytypic_ evolution. The tree of evolution under the influence of natural selection alone grows palm-like from its terminal bud. Isolation was necessary to the starting of lateral buds, and thus for the profuse ramification which is its most conspicuous character[52].
[52] _The Factors of Evolution_ (1891).
In order to complete this historical review, it only remains to consider Mr. Wallace's utterances upon the subject.
It is needless to say that he stoutly resists the view of Weismann, Delbuf, Gulick, and myself, that specific divergence can ever be due--or, as I understand him, even so much as a.s.sisted--by this principle of indiscriminate isolation (apogamy). It will be remembered, however, that Mr. Gulick has adduced certain general principles and certain special facts of geographical distribution, in order to prove that apogamy eventually leads to divergence of character, provided that the isolated section of the species does not contain any very large number of individuals. Now, Mr. Wallace, without making any reference to this argument of Mr. Gulick, simply states the reverse--namely, that, as a matter of fact, indiscriminate isolation is not found to be a.s.sociated with divergence of character. For, he says, ”there is an entire absence of change, where, if this were a _vera causa_, we should expect to find it[53].” But the only case which he gives is that of Ireland.
[53] _Darwinism_, p. 151.
This, he says, furnishes ”an excellent test case, for we know that it [Ireland] has been separated from Britain since the end of the glacial epoch: ... yet hardly one of its mammals, reptiles, or land molluscs has undergone the slightest change[54].” Here, however, Mr. Wallace shows that he has failed to understand ”the views of those who, like Mr.
Gulick, believe isolation itself to be a cause of modification of species”; for it belongs to the very essence of these views that the efficiency of indiscriminate isolation as a ”_vera causa_” of organic evolution varies inversely with the number of individuals (i. e. the size of the species-section) exposed to its influence. Therefore, far from being ”an excellent test case,” the case of Ireland is unsatisfactory. If we are in search of excellent test cases, in the sense intended by Mr. Wallace, we ought not to choose a large island, which from the time of its isolation must have contained large bulks of each of the geographically separated species concerned: we ought to choose cases where as small a number as possible of the representatives of each species were in the first instance concerned. And, when we do this, the answer yielded by any really ”excellent test case” is unequivocal.
[54] _Ibid._
No better test case of this kind has ever been furnished than that of Mr. Gulick's land-sh.e.l.ls, which Mr. Wallace is specially considering in the part of his book where the sentence above quoted occurs. How, then, does he meet this case? He meets it by a.s.suming that in all the numerous adjacent valleys of a small island there must be as many differences of environment, each of which is competent to induce slight varietal changes on the part of its occupants by way of natural selection, although in no one case can the utility of these slight changes be surmised. Now, against this explanation there are three overwhelming considerations. In the first place, it is purely gratuitous, or offered merely in order to save the hypothesis that there _can_ be no other cause of even the most trivial change in species than that which is furnished by natural selection. In the second place, as Mr. Gulick writes to me in a private letter, ”if the divergence of Sandwich Island land molluscs is wholly due to exposure to different environments, as Mr. Wallace argues on pages 147-150, then there must be completely occult influences in the environment that vary progressively with each successive mile. This is so violent an a.s.sumption that it throws doubt on any theory that requires such support.” In the third place, the a.s.sumption that the changes in question must have been due to natural selection, is wholly incompatible with the facts of isolation elsewhere--namely, in those cases where (as in that of Ireland) a large section of species, instead of a small section, has been indiscriminately isolated. Mr. Wallace, as we have seen, inadvertently alludes to these ”many other cases of isolation” as evidence against apogamy being _per se_ a cause of specific change. But although, for the reason above stated, they are without relevancy in this respect, they appear to me fatal to the explanation which he gives of specific changes under apogamy where only small sections of species are concerned. For example, can it be rationally maintained that there are more differences of environment between every two of the many contiguous valleys of a small island, such as Mr. Gulick describes, than there are in the incomparably larger area of the whole of Ireland? But, if not, and if natural selection is able to work such ”occult” wonders in each successive mile on the Sandwich Islands, why has it so entirely lost this magic power in the case of Ireland--or in the ”many other cases of isolation” to which Mr. Wallace refers? On his theory there is no coherent answer to be given to this question, while on our theory the answer is given in the very terms of the theory itself. The facts are plainly just what the theory requires that they should be; and therefore, if they were not as they are, the theory would be deprived of that confirmation which it now derives from them.
Thus, in truth, though in an opposite way, the case of Ireland is, as Mr. Wallace says, ”an excellent test case,” when once the theory of apogamy as a ”_vera causa_” of specific change is understood; and the effect of applying the test is fully to corroborate this theory, while at the same time it as fully negatives the other. For the consideration whereby Mr. Wallace seeks to explain the inactivity of natural selection in the case of Ireland is not ”coherent.” What he says is, ”That changes have not occurred through natural selection, is perhaps due to the less severe struggle for existence, owing to the smaller number of competing species[55].” But even with regard to molluscs alone, there is a greatly larger number of species in Ireland than occurs in any one valley of the Sandwich Islands; while if we have regard to all the other cla.s.ses of animal life, comparison entirely fails.
[55] _Loc. cit._, p. 151.
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 b.u.t.terflies, belonging to three genera[56], which are identical in the polar regions and in the Alps, notwithstanding that the spa.r.s.e 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 circ.u.mstances: the more stable specific type would require a longer time to change under the influence of apogamy alone. Now, the b.u.t.terflies 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].
[56] Namely, _Lycaena denzelii_, _L. pheretes_, _Argynnis pales_, _Erebia mante_.
[57] Since the above was written, I have heard of some cases which seem to present greater difficulties to our theory than those above quoted. These refer to some of the numerous species of land mollusca which inhabit the isolated rocks near Madeira (Dezertas). My informant is Dr. Grabham, who has himself investigated the matter, and reports as follows:--
”It is no uncommon thing to meet with examples of the same species, sub-fossil, recent, and living upon one spot, and presenting no variation in the long record of descent.” Then, after naming these examples, he adds, ”All seem to vary immediately on attaining new ground, a.s.suming many aspects in different districts.”
Unquestionably these statements support, in a very absolute manner, Mr. Wallace's opinion, while making directly against my own. It is but fair, however, to add that the cases are not numerous (some half-dozen at the most, and all within the limits of a single genus), and that, even in the opinion of my informant himself, the facts have not hitherto been sufficiently investigated for any decisive judgement to be formed upon them.
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 h.o.m.ogamy. 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.
[58] Vol. xliii. p. 127.
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:--