ng acting as a "warning," we do not
mean that it arouses any sensation of fear; it is but a stimulus to
that part of the inherited nature of the hearer which predominates at
the moment.

Are we then justified in the use of such terms as "warning,"
"significance," or even "meaning," when it is but a matter of stimulus
and response? In what does the impulse to avoid other males consist?
There is no reason to suppose that there is any sensation of fear in the
first stage, and the course of behaviour demonstrates that there is none
in the later stages. But it is difficult to conceive of an impulse which
has, as its end, the isolation of the individual from members of its own
sex and kind, without some feeling-tone, the reverse of pleasurable,
entering into the situation; just as it is difficult to believe that the
female experiences no pleasurable sensation when she hears the voice of
the male that directs her search. So that the song may be actually
repellent in the one case and attractive in the other; and it is none
the less repellent when, as in the later stages, it attracts a
neighbouring male, for the attraction is then of a different order,
determined by the presence of the condition which renders the pugnacious
nature susceptible and leads to attack. In a sense, therefore, we can
speak of "meaning"--though not perhaps of "significance"--and of
"warning," when we refer to the prospective value of the behaviour.

So much for the purpose of "song"; there still remains the more
difficult question--the question of origin. Let me make clear what I
mean by origin. As we have already seen, there is infinite diversity in
the sexual voice of different species; some are harsh and others
monotonous, and some strike the imagination by their novelty whilst
others are melodious; and to the naturalist each, in its particular way
and in a particular degree, probably makes some appeal according to the
associations that it arouses. But just why a Marsh-Warbler is gifted
with a voice that is so beautiful and varied, whilst the
Grasshopper-Warbler must perforce remain content with a monotonous
trill; just why the tail feathers of the Snipe have developed into an
instrument, whilst the Pied Woodpecker has developed muscles which
enable it to make use of a decayed branch as an instrument--we know no
more than we do of the nature of the forces which lead the Reed-Warbler
to weave its nest to reeds, or the caterpillar of the Elephant Hawk