Page:The British Warblers A History with Problems of Their Lives - 7 of 9.djvu/73

 present environment of the Marsh Warbler. It may be remembered that the Reed 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served some useful purpose in relation to the environment, and if upon new conditions arising the selective agency is withdrawn, the stability of that character will be replaced by fluctuating variability and gradually fade away during the process of modification to suit the new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,