Page:The British Warblers A History with Problems of Their Lives - 5 of 9.djvu/54

 reason, so far as we are able to judge, why, in many instances, more members should not breed in proximity to one another than actually do so at the present time. Have we not an explanation in the constant struggle for territory—that struggle which is such an essential to reproduction, a motive force, as it were, dispersing the species in every direction? But let this, which I have already pointed out, be remembered, that the territories need in no sense be regarded as a chain with numberless links, and that it is not essential for every available plot of ground to be occupied before the species can extend, but that by reason of the struggle being so relentless it must happen, and not infrequently, that an individual will be unnecessarily urged to seek ground beyond the immediate neighbourhood.

Now there is no reason to believe that this struggle for territory has only recently arisen. Possibly it has been even more severe in the past. But if proof of its early origin be required it can be found in this fact, that it is severally common to and inseparable from the habits of widely separate species. Therefore let us glance at the past history of an imaginary species, and, bearing in mind the severity of the struggle on the one hand, and the long periods of time on the other, attempt to trace the course of events in so far as they might have been affected by this law. We must suppose that the species is of comparatively small numerical strength, and of limited range, and, further, that a point has been reached when the individuals that do not allow others to breed in proximity to them have gained an advantage. Each recurring spring the conflict for territory would take place, causing an extension in the breeding range. In some directions the conditions of existence might be such as would make it impossible for the species to survive, in others large areas might be unoccupied, owing possibly to their being passed over unheeded, or to the individuals settled therein having been from one cause or another exterminated and not replaced, and yet in others the succession of breeding territories might be