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 through- 
out 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 import- 
ance 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 dis- 
cussion 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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