the altered
behaviour of the male when it joins the flock, and if, as I believe,
song and hostility are intimately associated, forming part of an
inter-related whole which, for biological interpretation, has, as its
end, the attainment of reproduction, it is not surprising that
circumstances which lead to the modification of the one should likewise
affect the other; I offer no apology, therefore, for adverting to this
aspect of behaviour once again.

Now a male may leave its territory for three reasons--to pursue an
intruder, to join the flock on neutral ground, or to find the necessary
means of subsistence on other feeding grounds. On each of these
occasions it hears the song of, and is in close contact with, other
males; and if the relationship of which we are speaking be really
exclusive of cross-correlation, its instinct ought to respond with the
customary freedom. But what happens? A male pursues its rival, betraying
much emotion and singing extravagantly, until the boundary is passed,
when emotion subsides and it is silent; or, it flies to the flock on
neutral ground, and, although surrounded by the very males that a short
time previously evoked response, is there unresponsive; or again, it
goes in search of food and collects with other males bent on a similar
errand, and in presence of what we know would be an exciting influence
under other circumstances, it nevertheless remains silent. Hence the
relationship between the song and a male rival seems, as in the case of
the headquarters, to depend in the first instance upon the occupation
of a territory.

So that the relationship between the song and the territory as a whole
is clearly of a different order from that which obtains between the song
and the headquarters, or the song and a male rival; for the first, as
far as can be judged by observation, is exclusive of, whilst the second
and the third involve, cross-correlation. How are these facts to be
explained? We have already seen that it belongs to the nature of the
male during the season of reproduction to establish itself in a definite
place, and this action is just as much a part of its hereditary nature
as the building of the nest is of that of the female, and it is just as
necessary for the successful attainment of reproduction. What exactly
the stimulus is to this mode of behaviour we do not know; we can go no
further back than the internal organic changes which are known to occur
and which we assume, no