But while it would be quite untrue to say that this routine is never departed from, and equally profitless to attempt to find a point beyond which the bird will under no circumstances wander, yet there is enough definition and more than enough to answer the purpose for which the territory has, I believe, been evolved, that is to say the biological end of reproduction. Again, however, the process of adjustment is a complex one. Habit plays its part in determining the boundaries in a rough and ready manner, but the congenital basis, which is to be found in the behaviour adapted to a particular environment, is an important factor in the situation. For example, if instead of resting content with just a bare position sufficient for the purpose of reproduction, the Guillemot were to hustle its neighbours from adjoining ledges, the Guillemot as a species would probably disappear; or if instead of securing an area capable of supplying sufficient food both for itself and its young, the Chiffchaff were to confine itself to a single tree, and, after the manner of the Guillemot, trust to spasmodic excursions into neutral ground for the purpose of obtaining food, the Chiffchaff as a species would probably not endure. All such adjustments have, however, been brought about by relationships which have gradually become interwoven in the tissue of the race. The intolerance that the male displays towards other individuals, usually of the same sex, leads to a vast amount of strife. Nowhere in the animal world are conflicts more frequent, more prolonged, and more determined than in the sexual life of birds; and though they are acknowledged to be an important factor in the life of the individual, yet there is much difference of opinion as to the exact position they occupy in the drama of bird life. Partly because they frequently happen to be in evidence, partly because they are numerically inferior, and partly, I suppose, because the competition thus created would be a means of maintaining efficiency, the females, by common consent, are supposed to supply the condition under which the pugnacious nature of the male is rendered susceptible to appropriate stimulation. And so long as the evidence seemed to show that battles were confined to the male sex, so long were there grounds for hoping that their origin might be traced to such competition. But female fights with female, pair with pair, and, which is still more remarkable, a pair will attack a single male or a single female; moreover, males that reach their destination in advance of their prospective mates engage in serious warfare. How then is it possible to look upon the individuals of one sex as directly responsible for the strife amongst those of the other, or how can the female supply the necessary condition? As long as an attempt is made to explain it in terms of the female, the fighting will appear to be of a confused order; regard it, however, as part of a larger process which