tory is essential if the
offspring are to be successfully reared; that, since the available
breeding ground is limited, competition for it is severe, and that the
male is precluded from leaving the ground which he has selected, and is
obliged, in order to secure a mate, to make himself conspicuous. That
was our general result. Now there are two ways by which the male can
make himself conspicuous--by occupying such a position that he can be
readily seen, or by producing some special sound which will be audible
to the female and direct her to the spot. The former, by itself, is
insufficient; in the dim light of the early dawn, when life is at its
highest, and mating proceeds apace, what aid would it be for a male to
perch on the topmost branch of a tree, how slender a guide in the depth
of the forest? But whether in the twilight or in the dark, in the
thicket or the jungle, on the mountain or on the moor, the voice can
always be heard--and the voice is the principal medium through which the
sexes are brought into contact.

Well now, we come back to the question, why, if all species have a
serviceable recognition call, that call should not be sufficient for the
purpose, just as, without a doubt, it is adequate for all purposes at
other seasons? The answer is, I think, clear. The recognition call is
not confined to one sex, nor only to breeding birds; it is the common
property of all the individuals of the species, and if the female were
to rely upon it as a guide she might at one moment pursue another
female, at another a non-breeding male; she might even be guided to a
paired female or to a paired male, and time would be wasted and much
confusion arise. So that no matter how much a male might advertise
himself by cries and calls which were common alike to all the
individuals of the species, it would not assist the biological end which
we have in view. Something else is therefore required to meet the
peculiar circumstances, some special sound bearing a definite meaning by
which the female can recognise, amongst the host of individuals of no
consequence to her, just those particular males in a position to breed
and ready to receive mates. Hence the vocal powers, the power of
producing sounds instrumentally, and the power of flight, have been
organised to subserve the biological end of "recognition."

And this view is strengthened, it seems to me, by the erratic behaviour
of certain species, more particularly by one remarka