t. Next, we found that the hostility was peculiar to a certain
season--and that one the season of reproduction. And if the question
were asked: What condition would then be most likely to render the
instinct susceptible, the answer that would most certainly be given
would be--the presence of a female. And in reply to a further question
as to the particular nature of the stimulus to which the instinct would
respond, we should be told--the presence of another male of the same
species. Now the possible influence of the female on the course of the
male's behaviour was the subject of inquiry in the second chapter,
wherein we endeavoured to explain the hostility between males of the
same species, and we came to the conclusion that it was not alone
sufficient to account for the facts disclosed. Still less likely,
therefore, is it that her presence can bear any direct relation to the
hostility between different species, the more so since the biological
end of securing a mate is definitely excluded. And we have something in
the nature of proof of the correctness of this view in the fact that she
accompanies her mate when he joins the flock, and that there his
instinct is not susceptible to stimulation. We then proceeded to examine
certain cases in which all the indications pointed to the fact that the
"will to fight" was present in only one of the opponents; and we
attached considerable importance to this circumstance, because we knew
from experience that the same bird which seemed to lack courage, could
at other times and in other situations be most aggressive. If then we
ask what condition was present on the one occasion that was absent on
the other, we have no difficulty in finding a reply--on every occasion
on which the opponents appeared to be unevenly matched, one was in
occupation of a territory and the other was not. And if we inquire
further as to which of the two was the aggressor, the answer is again
clear, namely, the bird that occupied a territory. Finally we considered
some particular instances in which the "will to fight" was present alike
in both opponents, and in which the battles were protracted and severe.

But the fact that a bird has established a territory is not in itself
sufficient to render its hostile nature susceptible; it must be actually
in occupation if a response is to be elicited. We reach this conclusion
step by step: the behaviour of the migrant, that lacks the "will to
fight" when on migration b