BRITISH WARBLERS 



a factor cannot well be admitted, w T e must fall back upon racial 

 preparation of some description ; and if this be granted, then 

 in the elimination which threatens the offspring of the careless 

 worker we observe the process at work. Another example is 

 afforded by a comparison of the Eeed Warbler and Marsh 

 Warbler. The peculiarly close relationship between the two 

 birds has been referred to at some length, and judging by 

 external characters only we may freely admit the possibility 

 of no intermediate forms ever having arisen, but the differ- 

 ences of behaviour indicate something in the structure of the 

 nervous system which we cannot explain at the present time in 

 terms of any known theory. Here w T e are concerned with that 

 particular aspect of behaviour which centres round the building 

 of the nest. I have given my reasons for believing that at 

 some earlier period the Marsh Warbler dwelt amongst reeds ; 

 to-day its environment is different from that of the Eeed 

 Warbler, and no one type of nest seems yet to have been 

 evolved which adequately meets the present conditions of 

 existence. Variation meets us in many directions, nests 

 on the one hand needlessly secure, and on the other so ill 

 adapted to their surroundings that disaster is liable to befall 

 the offspring. Unless therefore I am interpreting the facts 

 wrongly, we have here presented to us a process of elimination 

 leading to a gradual development in a definite direction. 



While firmly convinced of the importance of natural 

 selection, I am not one of those who regard it as the exclusive 

 means of organic process, nor do I hold the doctrine of utility 

 as absolute. Far from it. Why should it be necessary to 

 attempt to express whole series of phenomena relating to 

 specific behaviour in such terms ? And are there not besides 

 many facts in nature which almost compel us to look else- 

 where for an explanation ? Though it may be impossible to 

 demonstrate by actual observation the negative value of any 

 particular piece of behaviour, I nevertheless believe that a 

 comparison of certain peculiarities of closely related forms 

 almost yields the evidence we require. The Willow Warbler 



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