t of
other males of its own species, and examined more especially the claims
of the "territory" as opposed to those of the "female." But here we
start on firmer ground, because the one factor which introduced an
element of uncertainty--the female--can be definitely excluded; at least
it seems so to me, for granting even that her presence is the condition
under which the pugnacious nature of the male is rendered susceptible to
stimulation, it is difficult to see why a male of a different species
should supply that stimulus, or what biological purpose could be served
by its doing so.

When dealing with the attitude of a male towards others of its kind, we
attached considerable significance to the fact that its pugnacious
nature gained or lost susceptibility according to the position which it
happened to occupy. We found, it will be remembered, that the same bird
that was pugnacious in its own territory took no further interest in its
opponent when the boundary was passed; and, moreover, that if it
happened to wander into an adjoining one, it made no real effort to
defend itself when attacked, but returned forthwith to its own
headquarters. It remains to be shown whether the rivalry between
different kinds of birds is similarly related to the position which the
opponents happen to occupy at the time.

First, then, there is the general consideration, namely, that the enmity
occurs for the most part just at the time when the territories are in
process of being established. During autumn and winter, many birds of
more or less close affinity assemble together in flocks, wherever the
supply of food is abundant, and are then not only sociable, but, so
there is reason to believe, are mutually helpful both in discovering the
necessary means of subsistence which are often none too plentiful, and
in affording protection from enemies, which, on the contrary, are often
numerous. That the different units of which these flocks are composed
should live on amicable terms is therefore as necessary for the welfare
of the whole community at this particular season as that the different
individuals of the same species should do so. But just as the sociable
relations, which obtain between these individuals throughout the winter,
undergo a marked change at the commencement of the breeding season, so,
too, do different species, which habitually associate together, suddenly
become hostile to one another. This change is coincident in time with
th