The three objections to which I allude are, (1) that a large proportional number of specific, as well as of higher taxonomic characters, are seemingly useless characters, and therefore do not lend themselves to explanation by the Darwinian theory; (2) that the most general of all specific characters—viz. cross-infertility between allied species—cannot possibly be due to natural selection, as is demonstrated by Darwin himself; (3) that the swamping effects of free intercrossing must always render impossible by natural selection alone any evolution of species in divergent (as distinguished from serial) lines of change.
These three objections have been urged from time to time by not a few of the most eminent botanists and zoologists of our century; and from one point of view I cannot myself have the smallest doubt that the objections thus advanced are not only valid in themselves, but also by far the most formidable objections which the theory of natural selection has encountered. From another point of view, however, I am equally convinced that they all admit of absolute annihilation. This strong antithesis arises, as I have said, from differences of standpoint, or from differences in the view which we take of the theory of natural selection itself. If we understand this theory to set forth natural selection as the sole cause of organic evolution, then all the above objections to the theory are not merely, as already stated, valid and formidable, but as I will now add, logically insurmountable. On the other hand, if we take theory to consist merely in setting forth natural selection as a factor of organic evolution, even although we believe it to have been the chief factor or principal cause, all the three objections in question necessarily vanish. For in this case, even if it be satisfactorily proved that the theory of natural selection is unable to explain the three classes of facts above mentioned, the theory is not thereby affected: facts of each and all of these classes may be consistently left by the theory to be explained by causes other than natural selection—whether these be so far capable or incapable of hypothetical formulation. Thus it is evident that whether the three objections above named are to be regarded as logically insurmountable by the theory, or as logically non-existent in respect to it, depends simply upon the manner in which the theory itself is stated.
In the next volume a great deal more will have to be said upon these matters—especially with regard to the causes other than natural selection which in my opinion are capable of explaining these so-called “difficulties.” In the present connexion, however, all I have attempted to show is, that, whatever may be thought touching the supplementary theories whereby I shall endeavour to explain the facts of inutility, cross-sterility, and non-occurrence of free intercrossing, no one of these facts is entitled to rank as an objection against the theory of natural selection, unless we understand this theory to claim an exclusive prerogative in the field of organic evolution. This, as we have previously seen, is what Mr. Wallace does claim for it; while on the other hand, Mr. Darwin expressly—and even vehemently—repudiates the claim: from which it follows that all the three main objections against the theory of natural selection are objections which vitally affect the theory only as it has been stated and upheld by Wallace. As the theory has been stated and upheld by Darwin, all these objections are irrelevant. This is a fact which I had not myself perceived at the time when I mentioned these objections in a paper entitled Physiological Selection, which was published in 1886. The discussions to which that paper gave rise, however, led me to consider these matters more closely; and further study of Darwin’s writings, with these matters specially in view, has led me to see that none of the objections in question are relevant to his theory, as distinguished from that of Mr. Wallace. This, I acknowledge, I ought to have perceived before I published the paper just alluded to; but in those days I had had no occasion to follow out the differences between Darwin and Wallace to all their consequences, and therefore adopted the prevalent view that their theories of evolution were virtually identical. Now, however, I have endeavoured to make it clear that the points wherein they differ involve the important consequences above set forth. All these the most formidable objections against the theory of natural selection arise simply and solely from what I conceive to be the erroneous manner in which the theory has been presented by Darwin’s distinguished colleague.
I have now considered, as impartially as I can, all the main criticisms and objections which have been brought against the theory of natural selection; and the result is to show that, neither singly nor collectively, are they entitled to much weight. On the other hand, as we have seen in the preceding chapter, there is a vast accumulation of evidence in favour of the theory. Hence, it is no wonder that the theory has now been accepted by all naturalists, with scarcely any one notable exception, as at any rate the best working hypothesis which has ever been propounded whereby to explain the facts of organic evolution. Moreover, in the opinion of those most competent to judge, the theory is entitled to be regarded as something very much more than a working hypothesis: it is held to be virtually a completed induction, or, in other words, the proved exhibition of a general law, whereby the causation of organic evolution admits of being in large part—if not altogether—explained.
Now, whether or not we subscribe to this latter conclusion ought, I think, to depend upon what we mean by an explanation in the case which is before us. If we mean only that, given the large class of known facts and unknown causes which are conveniently summarized under the terms Heredity and Variability, then the further facts of Struggle and Survival serve, in some considerable degree or another, to account for the phenomena of adaptive evolution, I cannot see any room to question that the evidence is sufficient to prove the statement. But it is clear that by taking for granted these great facts of Heredity and Variability, we have assumed the larger part of the problem as a whole. Or, more correctly, by thus generalizing, in a merely verbal form, all the unknown causes which are concerned in these two great factors of the process in question, we are not so much as attempting to explain the precedent causation which serves as a condition to the process. Much more than half the battle would already have been won, had Darwin’s predecessors been able to explain the causes of Heredity and Variation; hence it is but a very partial victory which we have hitherto gained in our recent discovery of the effects of Struggle and Survival.
Yet partial though it be in relation to the whole battle, in itself, or considered absolutely, there can be no reasonable doubt that it constitutes the greatest single victory which has ever been gained by the science of Biology. For this very reason, however, it behoves us to consider all the more carefully the extent to which it goes. But my discussion of this matter must be relegated to the next volume, where I hope to give abundant proof of the soundness of Darwin’s judgment as conveyed in the words:—"I am convinced that natural selection has been the main, but not the exclusive, means of modification.”