of the young--a
period which must be determined by the season of the year." Newton
suggested the following explanation[2]: "It is not difficult to
imagine that, in the course of a journey prolonged through some 50 deg.
or 60 deg. 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, the males must be the
first to leave their winter quarters.

What is meant by the "migratory instinct"? To speak of it as one of the
instincts concerned in reproduction is not enough. Reproduction involves
the actual discharge of the sexual function, which involves the
females; but the first visible manifestation of organic change in the
male is its desertion of the females. Yet this is the behaviour which is
referred to as the "migratory instinct," and which comes into play,
according to this theory, because the bird has reached sexual maturity.
Manifestly we must have some clear understanding as to what these terms
represent. That organic changes determine the functioning of certain
definite instincts at certain specified times there can be no doubt;
that these changes may occur at a somewhat earlier date in the male than
in the fema