What we have, then, to consider is, Do these battles between different species contribute towards the attainment of the end for which the whole territorial system has been evolved? Let us take the individual and see whether we can establish any relation between the hostility it displays towards members of other species and its general disposition to secure a territory. We must remember that a male can have no knowledge of the prospective value of its behaviour, nor is it likely that it has any ulterior purpose in ejecting other males, beyond the pleasure it derives from satisfying its impulse to do so. The proximate end of its behaviour is to attack, nothing more, and this, of course, it can only do just in so far as the intruder evokes the appropriate instinct. Now the arguments we shall employ will, on the whole, be similar to those which we made use of in the second chapter, wherein we attempted to ascertain the conditions under which a male becomes intolerant of other males of its own species, and examined more especially the claims of the "territory" as opposed to those of the "female." But here we start on firmer ground, because the one factor which introduced an element of uncertainty--the female--can be definitely excluded; at least it seems so to me, for granting even that her presence is the condition under which the pugnacious nature of the male is rendered susceptible to stimulation, it is difficult to see why a male of a different species should supply that stimulus, or what biological purpose could be served by its doing so. When dealing with the attitude of a male towards others of its kind, we attached considerable significance to the fact that its pugnacious nature gained or lost susceptibility according to the position which it happened to occupy. We found, it will be remembered, that the same bird that was pugnacious in its own territory took no further interest in its opponent when the boundary was passed; and, moreover, that if it happened to wander into an adjoining one, it made no real effort to defend itself when attacked, but returned forthwith to its own headquarters. It remains to be shown whether the rivalry between different kinds of birds is similarly related to the position which the opponents happen to occupy at the time.