Page:The British Warblers A History with Problems of Their Lives - 4 of 9.djvu/63

 namely, that supplied by fortuitous variation and selection; and in order that this may have been effectual, it must be taken for granted that any one given species has suffered constant persecution at the hands of some natural enemy or enemies. What the enemies are, which have been so misled by these proceedings that advantage has accrued to the individuals thus practising them, I find it difficult to imagine. It surely cannot be thought that the human element has been a factor; for consider, in the case of a little Warbler, how immense a period of time would be necessary, and how constant the persecution must have been before selection could have completed its task, and the variation have become fixed, and finally established as an instinct uniform throughout the species. Nor, after carefully surveying the life-history of the species in which the habit is prevalent, is it more satisfactory to regard carnivorous quadrupeds as the principal factor; yet it is to them that the selectionist must look. At the same time we must bear in mind the past history of the species, the enormous periods of time, and the possibility of the habit having been formed at some very early date when the conditions of existence were more strenuous, and enemies more numerous, but even if we grant all this imperfection of our knowledge, it is but a frail foundation upon which to raise a theory.

Let us look at the facts more closely. I have described a number of cases in which—and this no doubt is true of every case of the same kind—similar stimulating circumstances produce similar results, and in whatever way we may regard any one particular case, this much must be granted, that no distinction is scientifically possible between the behaviour of the Avocet on the one hand, and that of the Lesser Whitethroat on the other; we cannot, that is to say, point to the apparently injured leg or wing of the Avocet, and, because it thus approaches more nearly our conception of a wounded bird, proceed to argue that therefore its behaviour is different in kind, and capable of a different explanation from that of the