drous,
and the voice of the male is not sufficient by itself to bring to pass
the union of the sexes; and in which, consequently, if the purpose of
song be that of recognition, we should expect to find, as we do find,
that the female had a distinct and penetrating call.

We now come to the question of "warning," by no means the least
important purpose of song. I pointed out that one of the chief
differences between the call-notes and the song was that the former were
socially serviceable, whereas the latter was only serviceable to certain
individuals; and in making this statement, I had in mind the direct
benefits to the community which proceeded from an appreciation of sounds
having a mutually beneficial meaning, not the indirect, though none the
less beneficial, consequences to the species as a whole. Biologically
considered, song, if it acts as a warning and thereby leads in one way
or another to more complete success in the rearing of offspring, may be
spoken of as socially serviceable; but it is legitimate to draw a
distinction between the prospective value of remote relationships which
we can foresee, and the mutual assistance which the individuals of a
community derive from their close association.

If there were always sufficient breeding ground to support the offspring
of all the individuals of each species, if the individuals were always
so distributed that there was no possibility of overcrowding in any
particular area, and if the conditions of existence of different
species were so widely divergent that the presence of this one in no way
affected the interests of that, no opportunity would be afforded for the
development of so complex a system as is involved in the "territory" and
all that appertains to it. But the available breeding ground is by no
means unlimited. The supply of food, which is a determining factor in
the environment, is always fluctuating according to the climate and
according to the changes in the earth's surface; and so the distribution
of the bird population in any given area, though it may be suitably
adjusted for one year or even for a period of years, is bound in the
course of time to require readjustment. Now there cannot be readjustment
without competition, nor competition without combat. But the appeal to
physical force is only a means to an end, and, since no male can endure
incessant warfare and the perpetual strain of always being on the alert,
without experiencing such physica