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." CHAPTER X. THE THEORY OF SEXUAL SELECTION, AND CONCLUDING REMARKS.