Page:The British Warblers A History with Problems of Their Lives - 5 of 9.djvu/57

 upon, one male crossing its boundary and entering the area of its neighbour. And, lastly, in the case of two males occupying adjoining territories, one paired and the other unpaired, we find that the unpaired male frequently attacks not only the neighbouring male but also the female when either of them intrudes upon its territory. Any one who will take the pains to keep under observation two pairs of Chiff-chaffs, or of Blackcaps, living in proximity to one another, or, better still, some large sheet of water inhabited by a number of different pairs of Moorhens, can observe for himself these struggles which are of almost daily occurrence. And how great an influence they must have exercised upon the history of any one given species we can well imagine, when, on the one hand, we remember how incessant they must in reality be in order that we may so readily observe them, and, on the other, bear in mind the enormous period of time through which species have been slowly evolving.

The strength or weakness of the theory lies in the evidence upon which it is based, that is to say, upon the observation of one person, and herein, at least, lies a weakness. But although it is probable that in the details there may be some exaggeration—and it is doubtful whether the human powers of observation are capable of always giving an accurate statement of the facts—yet the essential facts I believe to be true; and I appeal to Nature herself for corroboration. Let these principal points be borne in mind: that whatever species I have hitherto studied I have found the question of territory in some degree present; that the problems with regard to unpaired birds, rate of increase, extension of breeding range, and, in the case of the migrants, the hurrying forward of the males in spring, on this principle become capable of solution, and once more, and finally, that although the evidence is drawn from the lives of comparatively few species, yet some of those species are not closely allied, but belong to widely separate genera.