Page:The British Warblers A History with Problems of Their Lives - 5 of 9.djvu/40

 a fatal struggle; and if a territory is won a mate is won also. No fact has impressed itself upon me more strongly than this latter, for we know that not all the males, nor all the females, arrive at the same moment, and so it happens that one male will commence to breed a week, a fortnight, or perhaps a month before its neighbour; yet notwithstanding this difference in time, which, when we take into consideration how short is the whole period of sexual reproduction, is a very considerable one, I have not found a single case of a male securing a territory and not ultimately securing a mate. It may be urged that the evidence of so small a number of species is insufficient, but let it be remembered that I have purposely studied species very widely separated in the phylogenetic tree, and have found a similar law in operation; and if it were not a general rule that territory and reproduction were synonymous, some instances, some evidence at least, ought to have come under my notice. It seems evident, therefore, that a territory is essential to the individual male if it is to attain to reproduction, and inasmuch as the final appeal for possession is to the law of battle, we can well understand that on the average it will be the stronger birds that will leave offspring to inherit their congenital tendencies; the weaker will either perish, or which amounts to the same thing not attain to reproduction. At the same time we must not always assume that the bird that is defeated is, although weaker, not so fit to reproduce, or not so likely to produce healthy offspring, since it is quite conceivable that up to a certain age the older birds will have an advantage, due solely to laws of growth, but, on the average, it will be the stronger individuals that leave offspring. Thus there is constantly at work each recurring spring a process of elimination, whereby the species is not only maintained in a healthy condition, but is gradually being brought to a greater state of perfection. And so in its immediate results, the law of territory is nothing more than a form of natural selection. Before going further, however, it will be necessary to