In view of all these considerations taken together, I freely confess that the difficulty presented by this case appears to me of a magnitude and importance altogether unequalled by that of any other single case--or any series of cases--which has hitherto been encountered by the theory of natural selection. So that, if there were many other cases of the like kind to be met with in nature, I should myself at once allow that the theory of natural selection would have to be discarded. But inasmuch as this particular case stands so far entirely by itself, and therefore out of analogy with thousands, or even millions, of other cases throughout the whole range of organic nature, I am constrained to feel it more probable that the electric organ of the skate will some day admit of being marshalled under the general law of natural selection--in just the same way as proved to be the case with the conspicuous colouring of those caterpillars, which, as explained in the last chapter, at one time seemed to constitute a serious difficulty to the theory, and yet, through a better knowledge of all the relations involved, has now come to constitute one of the strongest witnesses in its favour. * * * * * I have now stated all the objections of any importance which have hitherto been brought against the theory of natural selection, excepting three, which I left to be dealt with together because they form a logically connected group. With a brief consideration of these, therefore, I will bring this chapter to a close. 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.