MARSH WARBLER 



present environment of the Marsh Warbler. It may be 

 remembered that the Eeed Warbler's nest is usually built 

 on from three to seven stems of the common reed; there 

 is no support for it beyond that which is supplied by the 

 architecture of the bird, and we consequently find it bound 

 securely to the reeds, the material being woven round the 

 stems and intertwined into the nest. If it were not thus 

 secured it is evident that disaster would befall the offspring ; 

 and so natural selection has eliminated the careless worker, 

 with the result that a definite type of structure has been 

 evolved. The swaying of the reeds is the indirect cause of the 

 depth, but there is no excessive swaying to be found in the 

 situations chosen by the Marsh Warbler. Young withies are 

 pliant, but do not bend before the wind like a hollow reed, 

 and what could offer greater security from this danger than 

 dense vegetation or young ash plants ? We have then to ask 

 whether it is possible to conceive of any useful purpose 

 being served by these corresponding features in the nest 

 of the Marsh Warbler ; and we can satisfy ourselves that 

 nothing is to be gained either by an excessive depth or by a 

 careful attachment to upright supports, since the present 

 environment calls for none of these factors to ensure the 

 safety of the offspring ; and surely something in the nature 

 of proof of their inutility is to be found in the fact that they 

 are constant when the nest is situated in reeds, but subject 

 to remarkable variation when recourse is had to a situation 

 of a different kind. If some particular character has been 

 developed by selection owing to its having served some useful 

 purpose in relation to the environment, and if upon new 

 conditions arising the selective agency is withdrawn, the 

 stability of that character will be replaced by fluctuating 

 variability and gradually fade away during the process of 

 modification to suit the new conditions. Bearing this in 

 mind, what traces of the former nesting instinct should we 

 expect to find, if the Marsh Warbler had at some earlier 

 period resorted to reed beds for the purpose of procreation, 



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