Page:The British Warblers A History with Problems of Their Lives - 9 of 9.djvu/35

 their offspring by the parents, and the numbers of unpaired individuals of both sexes. On the other, we can detect in the search for new quarters, which necessarily follows the banishment of one individual after a conflict with a rival, the inception of a movement which must ultimately lead to an extension of breeding range, and possibly form the foundation of those extensive journeys undertaken each spring for the purpose of procreation. If the struggle for territory were confined solely to the males of the same species, it would be sufficiently interesting, but it becomes more so when we recollect that the males of closely related forms contest the question of ownership with one another. Such racial strife is more frequent than one might suppose to be the case—in fact it is only within recent years that close observation has revealed how common is that which previously I deemed of rare occurrence, and it is not difficult to imagine what far-reaching' effects this extension of territorial warfare may have had on the past history of bird life.

With the advent of a female in the territory of a given male we witness the commencement of the period of sexual activity, productive of such striking emotional behaviour. This emotion is expressed by movements of the wings, tail, and feathers, and by extravagant antics correlated in some instances with an extravagant use of the vocal powers. Generally speaking, each species has a definite type of response which is peculiar to all its members, but we can often observe a great similarity, or even identity, in the overt expressional movements of different species. We have the slow flapping flight; we have, without actual flight, the rapid fluttering of wings, the slow flapping of wings, the outspread motionless wings; we have the outspread tail, and the sidelong motion of the tail. All of these movements—and others might be added—characterise the emotion which accompanies the sexual instinct. A consideration of these activities led to some discussion of the theory of sexual selection and the modifications of that theory, which from