, in the winter also, just as others betray, in the autumn,
symptoms of emotional manifestation peculiar to the spring; but just as
the manifestation of the latter is feeble and vestigial, so, too, does
the song of the former lack the vigour and persistency which is
characteristic of the spring. Again, in contrast with the call-notes,
which are common alike to both sexes, song is confined to one sex--a
peculiar property of the males.

Now all, I think, will agree that it must serve some biological
purpose--this at least seems to be the conclusion to be drawn from the
two outstanding features of "diversity" and "specific character"; and
since the voices of different individuals of the same species vary, it
has been suggested that, by creating a more effective pairing situation,
it is serviceable in furthering the life of the individual. I do not
propose at the moment to enquire whether this doctrine be true, but
rather to direct attention to other ways in which the song may be
useful.

Is the instinct susceptible to stimulation under all conditions during
the season of reproduction, or only under some well-defined condition?
This is the question to which we will first direct inquiry.

Song in its full development belongs, as we have seen, to the season of
reproduction; it is heard at the dawn of the seasonal sexual process,
and is the most conspicuous outward manifestation of the internal
organic changes which ultimately lead to reproduction. These changes
would appear, at first sight, to be the primary condition which renders
the instinct susceptible to appropriate stimulation. But while this is
true up to a point, in so far, that is to say, as organic changes are a
necessary antecedent of all behaviour connected with the attainment of
reproduction, closer acquaintance with the circumstances under which the
instinct is allowed full play leads to the belief that they are not
alone sufficient to account for the facts as observed. In order to
arrive at a decision we must seek out the specific factors in the
external environment with which "song" is definitely related.

Some birds cross whole continents on their way to the breeding grounds,
others travel many miles, others again find suitable accommodation in a
neighbouring parish--nearly all have a journey to perform, it may be
short or it may be long. The flocks of Finches gradually decrease and we
observe the males scattering in different directions in search of
terr