BRITISH WARBLERS
that the action of climate could never have produced so remarkable an alteration instantaneously, and, moreover, are confident that no perceptible change does take place throughout the season, how can we explain the fact that the song of the same species has diverged to so large an extent? To this there can be but one reply—namely, that the development in any one direction has been continuous through a long period of time. We have good reasons to think that a bird returns more or less to the same district year after year, and supposing this to be true, it is possible that the change has been wrought by degrees, but by what means we cannot tell, for in my opinion it is a case in which natural selection must be excluded, since it is unreasonable to suppose that slight changes of tone can ever have been of sufficient importance to constitute their presence a matter of life and death in the struggle for existence; and, moreover, it is demonstrable that they have not been so, since we are able to recognise innumerable variations in the song of any one species scattered throughout Europe, and at the same time find that species as relatively plentiful in one district as another. Climate could never have been a cause of song, but by some such means it may have determined the lines along which any particular development has taken place.
I wish it to be clearly understood that I regard this discussion solely as a preliminary foundation upon which further investigation can be based; for I can find but passing reference to the subject in any work on natural history, and then only to the possibility of the existence of some variation in the song. All I claim to have shown, beyond the fact that this variation is considerable, is the possibility, perhaps probability, of there being some connection between the type of the song and the climate. Further than this my remarks are entirely presumptive.
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