in the nature of stimulation
absent, but, and this is a remarkable fact, the other items in the
environment which formerly evoked response no longer do so in quite the
same way. Is there any awareness on the part of the male of the relation
between his voice and the mate that is to be, or is it merely that as
the sexual situation increases in complexity some inhibiting influence
comes into play? These are questions which lead up to difficult
problems. But it is no part of my task to discuss the psychological
aspect of the behaviour; my purpose is merely to show that the situation
on the arrival of a female undergoes marked modification, that the
instinct of the male is then less susceptible to stimulation, and that
the factors in the external environment which formerly elicited response
become relatively neutral.

Hence the appearance of the female on the scene marks the opening of a
new stage in the life-history of the male, and, to judge by the course
of events, it would seem as if the song with its network of
relationships had now served its main biological purpose.

And now, what is the purpose, and what the origin, of song? Is it, as
some naturalists have conceived, a means of raising the emotional tone
of the female, of creating a more effective pairing situation, and so
of removing a barrier to the successful discharge of the sexual
function; or, is the emphasis here too much upon the emotional, too
little upon the strictly utilitarian, aspect? All, I think, will agree
that it must serve some biological purpose, and the position we have so
far reached is that the determining condition of its manifestation is
not merely the establishment, but the actual occupation of a territory,
and that there are no factors in the external environment which can
evoke response in the absence of such condition. This being so, the
further questions arise as to whether it contributes towards the
attainment of the end for which the whole territorial system has been
built up, and what precisely is the way in which it does so.

Everyone knows that in the spring the shyest of birds no longer practise
the art of concealment. The Curlew soars to a great height, and upon
outstretched wings hovers in the air whilst uttering its plaintive wail;
the cock Grouse, as if dissatisfied with its "crowing," springs into the
air and becomes a conspicuous object of the moor; the wary Redshank,
poised on flickering wings, forgets its mournful alarm