To illustrate this, let us imagine a slight variety to arise spontaneously, we do not know how. Having arisen there is a ‘prepotent influence’ about it which enables it to secure a considerable proportion of offspring having its own characteristics. Now, suppose that the characteristics are such as to adapt the individuals possessing them more perfectly to the conditions of nature which surround them. When, by breeding amongst themselves, the new variety is rendered permanent, the members of this variety will, therefore, have an advantage over their elder brethren so far as certain conditions of nature are concerned, will in fact succeed better in the struggle for existence, and will ultimately displace the elder branches. Thus the struggle for existence bears to natural selection the same relation as man bears to artificial selection. 171. We now come to the real point of difficulty, or at least the unproved point, in the Darwinian hypothesis. We may cross one race with another, but we do not obtain, so far as we know, those phenomena of infertility which are exhibited when we cross distinct species with each other. The Ancon sheep were perfectly fertile when matched with their elder brethren, and the dray-horse and the Arab, or the pouter and the tumbler, breed together as easily as if they were of the same race. But if we cannot produce infertility, how can we apply the results of artificial selection to account for the origin of species? This difficulty is met by Darwin and his followers in this way:—‘It is not as yet proved,’ says Professor Huxley, ‘that a race ever exhibits, when crossed with another race of the same species, those phenomena of hybridisation which are exhibited by many species when crossed with other species. On the other hand, not only is it not proved that all species give rise to hybrids infertile _inter se_, but there is much reason to believe that, in crossing, species exhibit every gradation from perfect sterility to perfect fertility.’ This appears to carry weight; the old theory went with a leap from perfect fertility to perfect sterility, and did not contemplate the possibility of a continuous gradation from the one extreme to the other; at least its argument was founded upon the neglect of such a gradation. But if there be a gradation of this kind, it follows that infertility will merely represent the results of crossing two species whose functional characteristics are very different from each other; and, on the other hand, the reason why artificially produced varieties are not infertile when crossed with one another may only be that the experiment has not been continued long enough. Time, in fact, is the essential requisite in all such attempts to imitate nature.