Now, in view of this conspicuous contrast, Darwin suggested that species in a state of nature "will have been exposed during long periods of time to more uniform conditions than have domesticated varieties, and [that] this may well make a wide difference in the result." Now we have to remember that species, living and extinct, are numbered by millions, and represent every variety of type, constitution, and habits; is it probable, then, that this one peculiarity of the reproductive system should be due, in so many cases, to some merely incidental effect produced on that system by uniform conditions of life? Again, _ex hypothesi_, at the time when a variety is first forming, the influence exercised by uniform conditions of life (whatever in different cases this may happen to be) cannot be present as regards that variety: yet this is just the time when its infertility with the parent (or allied) form is most likely to have arisen; for it is just then that the nascent variety would otherwise have been most liable to extinction by free intercrossing--even supposing that in the presence of such intercrossing the variety could ever have come into existence at all. Mr. Wallace meets the difficulty by arguing that sterility between allied species may have been brought about by the direct influence of natural selection. But, as previously remarked, this view is expressly opposed to that of Darwin, who held that Wallace's contention is erroneous. It will be seen, then, that both Darwin, and Wallace, fully recognize the necessity of finding some explanation of the infertility of allied species, over and above the mere reaction of morphological differentiation on the physiology of the reproductive system, and they both agree in suggesting additional causes, though they entirely disagree as to what these causes are. Now, the theory of physiological selection likewise suggests an additional cause--or, rather, a new explanation--and one which is surely the most probable. For what is to be explained? The very general association of a certain physiological peculiarity with that amount of morphological change which distinguishes species from species, of whatever kind the change may be, and in whatever family of the animal or vegetable kingdom it may occur. Well, the theory of physiological selection explains this very general association by the simple supposition that, at least in a large number of cases, it was the physiological peculiarity which first of all led to the morphological divergence, by interposing the bar of sterility between two sections of a previously uniform species; and by thus isolating the two sections one from another, started each upon a subsequently independent course of divergent evolution.