Now, the remarkable fact here is that whenever the variations arose, they only intercrossed between themselves, they did not intercross with their parent form; for, if they had, not only could they never have survived (having been at first so few in number and there having been no geographical barriers in the small lake), but we should have found evidence of the fact in the half-bred progeny. Moreover, natural selection can have had nothing to do with the process, because not only are the variations in the form of the shells of no imaginable use in themselves; but it would be preposterous to suppose that at each of these "variation periods" several different variations should always have occurred simultaneously, all of which were of some hidden use, although no one of them ever occurred during any of the prolonged periods of stability. How, then, are we to explain the fact that the individuals composing each crop of varieties, while able to breed among themselves, never crossed with their parent form? These varieties, each time that they arose, were intimately commingled with their parent form, and would certainly have been reabsorbed into it had intercrossing in that direction been possible. With Professor Le Conte, therefore, I conclude that there is only one conceivable answer to this question. Each crop of varieties must have been _protected from intercrossing with their parent form_. They must have been the result of a variation, which rendered the affected individuals sterile with their parent form, whilst leaving them fertile amongst themselves. The progeny of these individuals would then have dispersed through the lake, physiologically isolated from the parent population, and especially prone to develop secondary variations as a direct result of the primary variation. Thus, as we might expect, two or three variations arose simultaneously, as expressions of so many different lines of family descent from the original or physiological variety; these were everywhere prevented from intercrossing with their parent form, yet capable of blending whenever they or their ever-increasing progeny happened to meet. Thus, without going into further details, we are able by the theory of physiological selection to give an explanation of all these facts, which otherwise remain inexplicable. * * * * * In view of the evidence which has now been presented, I will now ask five questions which must be suitably answered by critics of the theory of physiological selection. 1. Can you doubt that the hitherto insoluble problem of inter-specific sterility would be solved, supposing cross-infertility were proved to arise before or during the process of specific differentiation, instead of after that process had been fully completed?