BRITISH WARBLERS 
menced he would immediately join in, the clicking of all the 
bills making quite an unusual noise. 
These examples show under what diverse conditions the 
inquisitiveness occurs, and all members of the species appear 
to be equally susceptible to the same stimulating circum- 
stances. By the use of the term “ inquisitiveness’”’ I do not 
wish to imply an intelligent appreciation on the part of the 
bird of its various actions; the construction intended to be 
placed upon it is a figurative one, the purpose being to make 
clear the type of action when viewed from the human stand- 
point. 
It is so generally accepted that plasticity of instinct? has 
been established as a law of Nature that I almost refrain 
from questioning the correctness of this view; nevertheless, 
the more I reflect upon the facts, which I have from time 
to time collected, the more I feel disposed to regard as 
doubtful, not so much the possibility of isolated cases of 
variation arising, though there is little evidence in support 
of this, but whether the extent to which it has been 
thought to have been shown that they do occur, is suffi- 
client to justify our accepting such a law as beyond dispute. 
Owing, perhaps, to the great difficulty of establishing this 
on the basis of actual observation, it seems to have been 
assumed too readily that they do occur, but whatever 
difficulty there may be in this direction can only arise from 
the rarity or possible absence of true variation, and we ought 
on this account to be very cautious before arriving at any 
definite decision. 
1 Anecdotes of animals in a domesticated or semi-domesticated state 
have furnished most of the evidence upon which this theory is based. 
This is much to be regretted, since it tends not only to confuse the issue, 
but also to transfer attention from the only true source of information 
available, viz., an impartial investigation of wild Nature. It must be borne 
in mind that the question to be solved is not how far instinct can become 
plastic under a guiding human intelligence, but what method is adopted 
by Nature, and has been adopted ab initio. Even the very animals from 
whose actions evidence is taken are indirectly the outcome of human faculty. 
26 
