Though it be true that every instinct requires for its functioning a stimulus of an appropriate kind, yet it is also true that the condition which will render it responsive must be present. What we have then to consider is whether the phenomena which we have explored give us any clue as to the particular nature of that condition. In the first place, we have the general fact that the hostility is not confined to a few species belonging to a few families, but that it is of wide application--birds of prey, Warblers, Woodpeckers, all supply us with evidence which serves to show, in greater or less degree, its nature and extent. 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.