To return now to the view that the fighting is not really serious, but, on the contrary, that it is either vestigial and has no longer any part to play in furthering the life of the individual, or that it is a by-product of the seasonal sexual condition to which no meaning can be attached. First, there is the relationship with the territory, and this, it seems to me, is a fact of some importance; for if the fighting were merely an exuberant manifestation of sexual emotion, one would expect to find it occurring under all conditions, and not merely under one particular condition in the life of the bird. The hostility is too widespread, however, and too uniform in occurrence for us to suppose that it has no root in the inherited constitution of the bird; and if it served some useful purpose in the past, the instinct might still persist, so long as it were not harmful. Thus the view that the behaviour is vestigial is not perhaps unreasonable. But manifestly it makes no difference whether it be vestigial or a by-product of sexual emotion, whether the battle be fierce or so trivial as to appear to us to be more in the nature of "play," so long as some change in the relative prospects of the opponents is the result. For us, then, the main consideration lies in the question: Is the behaviour serviceable now in furthering the life of the individual? Whether the evidence which we have examined affords sufficient ground for the belief that the hostility is genuine and has a part to play in the whole scheme of reproduction, each must judge for himself. CHAPTER VII THE RELATION OF THE TERRITORY TO MIGRATION Coincident in time with the growth of appropriate conditions in the environment, organic changes take place rendering certain instincts susceptible to stimulation; and the stimulus being applied, the Warbler leaves the country wherein it had passed the winter and finds its way back, with apparently little difficulty, to the district in which it was reared or had previously reared offspring. What is the nature of these changes and of the impulse which is first brought into functional activity; whence comes the stimulus; and what directs the bird on its journey--these are all different aspects of one great problem, the problem of migration. I do not propose to discuss all these various aspects, for indeed I have no suggestions to offer which are in the least likely to be helpful, but I seek rather to ascertain whether the phenomena which we have explored bear any relation to the problem as a whole; whether, that is to say, the competition for territory and all that appertains to it can have supplied the conditions under which, in the process of time, this complex and definite mode of behaviour has evolved.