BRITISH WARBLERS
a factor cannot well be admitted, we 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 Reed 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 differences 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 we 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 Reed 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 elsewhere 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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