wever, we assume that the revival of the sexual
instinct in the migratory male is coincident in time with its return to
the breeding quarters; and we do so because the act of migrating is
believed to be the first step in the breeding process. But it is well to
bear in mind just how much of this assumption is based upon fact, and
how much is due to questionable inference. All that can be definitely
asserted is this, that appropriate dissection reveals in most of the
migrants, upon arrival at their destination, unquestionable evidence of
seasonal increase in the size of the sexual organs. Beyond this there is
nothing to go upon. Yet if the term "sexual instinct" is held to
comprise the whole series of complex relationships which are manifest to
us in numerous and specialised modes of behaviour, which ultimately lead
to reproduction, and which have gradually become interwoven in the
tissue of the race, there can be little doubt that the assumption is a
reasonable one. To some, the term may recall the fierce conflicts which
are characteristic of the season; to others, emotional response; to not
a few, perhaps, the actual discharge of the sexual function--all of
these, it is true, are different aspects of the one instinct; but at the
same time each one marks a stage in the process, and the different
stages follow one another in ordered sequence. However, we are not
concerned at the moment with the term in its wider application; we wish
to know the precise stage at which the disposition to mate influences
the behaviour of the male. Is the female to him, from the moment the
seasonal change in his sexual organs takes place, a goal that at all
costs must be attained? Or is it only when the cycle of events which
leads up to reproduction is nearing completion that she looms upon his
horizon? One would like to be in a position to answer these questions,
but there is nothing in the way of experimental evidence to go upon; and
if I say that there is reason to believe that, in the earlier stages,
the female is but a shadow in the external environment of the male, it
must be taken merely as an expression of opinion, though based in some
measure upon a general observation of the behaviour of various species.

Before attempting to explain the difference in the times of arrival of
the male and female migrant, let us examine the behaviour of some
resident species at a corresponding period. My investigations have been
made principally amongs