, some control over the
local distribution of species is of paramount importance. Nevertheless,
if all the different forms that require similar conditions of existence
were intolerant of one another in a like degree, the smaller bird would
have no chance in competition with the larger. This, however, is not
the case. Some, as we saw, arouse little or no animosity in others, in
fact the more closely related the rivals, the more responsive their
pugnacious nature seems to become.

To return now to the view that the fighting is not really serious, but,
on the contrary, that it is either vestigial and has no longer any part
to play in furthering the life of the individual, or that it is a
by-product of the seasonal sexual condition to which no meaning can be
attached. First, there is the relationship with the territory, and this,
it seems to me, is a fact of some importance; for if the fighting were
merely an exuberant manifestation of sexual emotion, one would expect to
find it occurring under all conditions, and not merely under one
particular condition in the life of the bird. The hostility is too
widespread, however, and too uniform in occurrence for us to suppose
that it has no root in the inherited constitution of the bird; and if it
served some useful purpose in the past, the instinct might still
persist, so long as it were not harmful. Thus the view that the
behaviour is vestigial is not perhaps unreasonable. But manifestly it
makes no difference whether it be vestigial or a by-product of sexual
emotion, whether the battle be fierce or so trivial as to appear to us
to be more in the nature of "play," so long as some change in the
relative prospects of the opponents is the result.

For us, then, the main consideration lies in the question: Is the
behaviour serviceable now in furthering the life of the individual?
Whether the evidence which we have examined affords sufficient ground
for the belief that the hostility is genuine and has a part to play in
the whole scheme of reproduction, each must judge for himself.

CHAPTER VII

THE RELATION OF THE TERRITORY TO MIGRATION

Coincident in time with the growth of appropriate conditions in the
environment, organic changes take place rendering certain instincts
susceptible to stimulation; and the stimulus being applied, the Warbler
leaves the country wherein it had passed the winter and finds its way
back, with apparently little difficulty, to the district in wh