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

 and Chiff-chaff, for instance, are closely related, live under the influence of a similar environment, require similar food and construct similar nests, yet the former builds upon the ground, the latter in some suitable undergrowth a foot or so above it. Can we then say that the ground leads to success in the one case but brings disaster in the other? Surely not when we bear in mind the identity of the conditions of existence of the two species. Or again, of the numberless channels into which sexual behaviour has been guided, can we say that each has some definite value attached to it? Can we even say that it is strictly correlated with something which has utility? Granting for the moment that emotional manifestation does play a part, granting that attitudes and antics create on the whole a more effective pairing situation, that beautiful plumage adds to the effect and that song contributes to an increased emotional tone—granting all of this, can anyone seriously suggest that wings raised a little more or a little less, slowly flapped or quickly fluttered, partially or fully expanded, a tail waved with a sidelong or an up-and-down motion, a song produced as the bird ascends or descends in the air, and a host of other trifling but specific forms of behaviour, depend for their usefulness on being cast in just this or that particular mould. If the life behaviour as a whole has utility, does it follow that every detail is independently useful?

In much of the behaviour which centres round the securing and defence of a territory, the building of a nest, and the care of the young, wherein a psychical accompaniment seems to be required as a contributory factor, one cannot help being impressed with the difficulties of evolutionary interpretation in terms of fluctuating variation or mutation only. How can we appeal to the slow accumulation of fluctuating variations only in a case where a definite organic structure in A depends for its success on a definite psychical factor in B? How can we appeal to variations of definite origin in explanation of the guidance of behaviour which is accommodating the organism to new and changed conditions of existence? How can we