BRITISH WARBLERS 



the two, since the possession of certain congenital nervous 

 dispositions determines the actions which are adapted to pro- 

 mote the welfare of the species ; but of the nature of that 

 relation we are ignorant. Did the change of environment 

 precede, was it coincident with, or subsequent to the change 

 in the nervous system ? Our answer to this question will 

 depend in a great measure upon the view we hold with regard 

 to the development of living forms from others of a simpler 

 type. From our inquiries into the life-histories of the two 

 species we believe that they may have been at some earlier 

 time even more closely related than they are to-day, and we 

 believe that the Marsh Warbler is the more recent modification. 

 According to the theory of continuous variation this result 

 must have been brought about by a number of successive steps 

 •or gradations, each one of which possessed some definite value 

 in the struggle for existence and was thus left free to develop. 

 And those who hold that this is the only means by which one 

 form can have passed into another will point to the variation 

 in the nesting instinct in proof of their assertion ; and doubt- 

 less they will be justified in doing so, for there we have 

 indisputable evidence of the process at work. But the fact 

 of the nesting instinct being in a condition of instability 

 shows that sufficient time has not yet elapsed to complete the 

 adaptation, and consequently the change from one environ- 

 ment to the other must be regarded from this point of view 

 as of more or less recent occurrence. A difficulty arises here. 

 The vocal powers and motor reactions of the two species are 

 separated as regards development by a very considerable 

 interval, and in the case of the Marsh Warbler they are not 

 subject to variation to such an extent as the nesting instinct 

 but seem to have reached a condition of comparative stability. 

 On the hypothesis of gradual transition such an interval must 

 represent an infinite number of gradations, which, in their 

 turn, must have required a long period of time for their com- 

 pletion, and I find it difficult to imagine that time which was 

 insufficient to complete such a simple but nevertheless very 



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