Success in the attainment of reproduction depends upon the successful discharge of the sexual function; and the discharge of the sexual function depends primarily upon an individual of one sex coming into contact with one of the opposite sex at the appropriate season and when its appropriate organic condition arises. Now the power of locomotion is so highly developed in birds that it may seem unreasonable to suppose that males and females would have any difficulty in meeting when their inherited nature required that they should do so, still less reasonable to suggest that this power might even act as a hindrance to successful mating. Nevertheless, if we try to picture to ourselves the conditions which would obtain if the movements of both sexes were in no wise controlled, and mating were solely dependent upon fortuitous gatherings, we shall come, I fancy, to no other conclusion than that much loss of valuable time and needless waste of energy would often be incurred in the search, and that many an individual would fail to breed just because its wanderings took it into districts in which, at the time, there happened to be too many of this sex or too few of that. And as the power of locomotion increased and the distribution of the sexes became more and more irregular, so the opportunity would be afforded for the development of any variation which would have tended to facilitate the process of pairing, and by so doing have conferred upon the individuals possessing it, some slight advantage over their fellows. What would have been the most likely direction for variation to have taken? Any restriction upon the freedom of movement of both sexes would only have added to the difficulties of mating; but if restriction had been imposed upon one sex, whilst the other had been left free to wander, some order would have been introduced into the process. That the territory serves to restrict the movements of the males and to distribute them uniformly throughout all suitable localities, there can be no question; and since the instinctive behaviour in relation to it is timed to appear at a very early stage in the seasonal sexual process, the males are in a position to receive mates before the impulse to mate begins to assert itself in the female.