BRITISH WARBLERS
although the differences in their structure are but slight, and we have no reason, therefore, to anticipate any striking dissimilarity in behaviour, yet we can observe without difficulty quite a number of specific types of reaction. And examining the five pairs separately we notice that the reactions are alike in one case only, but in the remaining four widely divergent. No one who observes and compares the behaviour of the Blackcap and Garden Warbler, the Willow Warbler and Chiff-Chaff, the Marsh and Reed Warbler, or the Whitethroat and Lesser Whitethroat can deny that in each case the former bird is a superior exponent of the art of demonstration. Bearing in mind then the essential points of the theory—namely, that the reactions have been developed solely because they reflect vitality and are consequently useful in indicating the fitness of the individual to take its share in reproduction, and that they have been evolved in conjunction with an external factor—how shall we explain this remarkable disparity in their development? We can only do so if we say that a comparison between one species and another need not necessarily hold good, since it matters not how many different degrees of emotion bird life may supply so long as there exists for each separate species a certain specific standard of response to which it is necessary for every individual to attain if it is to have an average chance of reproducing. Of two closely allied forms, this one is excitable, flaps its expanded wings, spreads its tail and works itself almost into a frenzy; that one is passive, similar circumstances produce in its case only a modicum of excitement, only slight use is made of the wings, the tail is only moderately spread and the song is but little different from that heard on any ordinary occasion. In each case the behaviour must have a similar meaning through which it has reached its present development. And since in both cases it must be assumed, ex hypothesi, to represent equivalent emotion and equivalent strength and to serve its purpose equally well in its own particular sphere, the explana-
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