After enumerating instances of hostility, sufficient in number, so it seemed, to constitute reasonable ground for the belief that they had a part to play in the life-history of the individual, the two questions we set ourselves to examine in this chapter were: Is there any circumstance in the life behaviour of the individual with which the hostility can be definitely related; and, will the hostility lead to the securing of a greater measure of success in the attainment of reproduction?
Many different species assemble together in winter and roam from place to place in search of food. But in spring their behaviour undergoes a remarkable transformation; they avoid one another and become quarrelsome, so much so that whereas the outstanding feature of the winter is sociability, that of the spring is hostility. With this general fact before us, we proceeded to investigate this change of behaviour still further. First of all we took the case of a migrant, and, comparing its behaviour, as it journeyed, with that when finally it reached its destination, we found that the bird which was notoriously pugnacious when in occupation of a territory betrayed no interest in other species as it travelled to the accustomed breeding ground. Not only so, but even though it was attacked, we found that its pugnacious instinct still failed to respond. Here, however, it may be contended, and with reasonable justification, that in the interval which elapses before the ultimate destination is reached, some change in the organic condition of the bird may occur which will account for its altered behaviour; or, it may be urged, with no less justification, that whereas on migration the bird is unpaired, when the destination is reached it is probably in possession of a mate and is therefore quarrelsome. Now, at the most, the interval can only be a matter of a few days, and it is unlikely that organic changes sufficient to bring about so important an alteration of behaviour could occur in so short a time, still less likely that they could be timed to come into functional activity just at the moment when the bird reaches its breeding ground. And with regard to the suggestion that the change can be accounted for by the presence of a mate, we shall do well to remember not only that males as a rule precede the females by some days, but that a male may even remain in its territory, mateless, for some weeks, and yet display hostility.
Nevertheless the case of the migrant did not, by itself, afford sufficient evidence upon which to base any conclusion. We therefore inquired into the behaviour of some of the residents at a corresponding period. The Bunting served as an illustration. Early in the season it establishes a territory, and because food is then scarce it is forced to seek it elsewhere than on the small plot of ground which it has acquired; and so it makes its way to some spot where the supply is abundant, and there, meeting with other species bent on a similar errand, forms with them a flock. Part of its time is then spent in the territory and part on the feeding ground, and between these two points a highway is formed by the bird passing constantly to and fro. But the attention which it pays to other species is very different on these two occasions—when in the territory it is intolerant of strangers, but when it accompanies the flock it displays no interest in their movements. From hour to hour its nature seems to change. But, as we saw, the inborn constitution of the bird cannot change, and therefore we came to the conclusion that an explanation of the altered behaviour was to be found in the fact that the pugnacious instinct is only rendered susceptible under a certain condition. So that all the evidence tended to confirm the impression which we had gained from the course of events in the life of the migrant, namely, that the hostility bears a direct relation to the occupation of a territory.
Finally we were led to inquire whether the hostility was serviceable in promoting the welfare of the individuals. We saw that many different species require similar conditions of existence, that ancient breeding haunts disappear and that new ones come into being, and that in the ordinary course of events such species must often assemble in the same area for the purpose of reproduction. So that even though a male might be successful in protecting its ground from intruders of its own kind, yet it might still fail to rear offspring, just because it happened to choose a position in which other kindred forms had gathered. Hence if the territory is adequately to serve its purpose, some control over the local distribution of species is of paramount importance. Nevertheless, if all the different forms that require similar conditions of existence were intolerant of one another in a like degree, the smaller bird would have no chance in competition with the larger. This, however, is not the case. Some, as we saw, arouse little or no animosity in others, in fact the more closely related the rivals, the more responsive their pugnacious nature seems to become.
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.