imagine that, in the course of a journey prolonged through some 50° or 60° of latitude, the stronger individuals should outstrip the weaker by a very perceptible distance, and it can hardly be doubted that in most species the males are stouter, as they are bigger than the females." Granting that the males are the stronger, how can this account for their outstripping the females by a week, ten days, or even a fortnight, in a journey of perhaps 1500 miles? To expect the birds to accomplish such a distance in seven days is surely not estimating their capabilities too highly, and any slight inequality in the power of flight or endurance could give the males an advantage of a few hours only. But this explanation, based upon inequalities in the power of flight and endurance on the one hand, and the magnitude of the distance traversed on the other, cannot afford a solution of the behaviour of the resident males, and is less likely, therefore, to be a true solution of that of the migrants.
There is another theory, simple enough in its way, which will probably occur to many. It is based on the assumption that the males reach sexual maturity before the females; and it is contended that the functioning of the instincts which contribute towards the biological end of reproduction depend upon the organic changes which the term "sexual maturity" is held to embrace, and that, inasmuch as the migratory instinct belongs to the group of such instincts,
