evelop similar
songs, whole areas might be left without their complement of pairs just
because this male mistook the voice of that, and avoided it when there
was no necessity for doing so. So that just as from the point of view of
"recognition" each female must be able to distinguish the voice of its
own kind, so likewise the warning can only be adequate providing that
the sounds are specifically distinct. A point, however, arises here in
regard to closely related forms. Some species require similar food and
live under similar conditions of existence; they meet in competition and
fight with one another; and, if they did not do so, the food-supply of a
given area would be inadequate to support the offspring of all the pairs
inhabiting that area. Generally speaking, the more closely related the
forms happen to be, the more severe the competition tends to become; and
it may be argued that in such cases a similar song would contribute to
more effective distribution and in some measure provide against the
necessity of physical encounter; that, in fact, it would stand in like
relation to the success of all the individuals concerned, as does the
song to the individuals of the same species. But we must bear in mind
that the primary purpose of song is to direct the females to those males
that are in a position to breed; and to risk the possibility of prompt
recognition in order that the males of closely related species should
fight the less, would be to sacrifice that which is indispensable for a
more remote and less important advantage.

What meaning does the song convey to a male that is unestablished? Does
the bird recognise that it is forestalled; does it foresee and fear the
possibility of a conflict, and conclude that the attempt to settle is
not worth while? I do not imagine that it thinks about it at all. How
then does the warning warn? We will endeavour to answer this question,
but, in order to do so, we must review the stages by which a territory
is secured.

We take as our starting point the internal organic changes which are
known to occur. These changes are correlated with other changes,
manifested by a conspicuous alteration in behaviour--to wit, the
disappearance of sociability and its replacement by isolation. Having
found a station which meets the requirements of its racial
characteristics, the male establishes itself for a season, becomes
vociferous, displays hostility towards others of its kind, and in due
c