BRITISH WARBLERS 



although the differences in their structure are but slight, and 

 we have no reason, therefore, to anticipate any striking dis- 

 similarity in behaviour, yet we can observe without difficulty 

 quite a number of specific types of reaction. And examin- 

 ing 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 Wil- 

 low Warbler and Chiff-Chaff, the Marsh and Eeed 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 com- 

 parison 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 hypotliesi, 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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