152 
THE DESCENT OF MAN. 
Part L. 
through the increased or decreased use of other parts, 
to other changes of a quite unexpected nature. It is- 
also well to reflect on such facts, as the wonderful 
growth of galls on plants caused by the poison of an 
insect, and on the remarkable changes of colour in the 
plumage of parrots when fed on certain fishes, or in- 
oculated with the poison of toads ; 80 for we can thus 
see that the fluids of the system, if altered for some 
special purpose, might induce other strange changes.. 
We should especially bear in mind that modifications 
acquired and continually used during past ages for 
some useful purpose would probably become firmly 
fixed and might be long inherited. 
Thus a very large yet undefined extension may safely 
be given to the direct and indirect results of natural 
selection ; but I now admit, after reading the essay by 
Nageli on plants, and the remarks by various authors 
with respect to animals, more especially those recently 
made by Professor Broca, that in the earlier editions of 
my ‘ Origin of Species 5 I probably attributed too much 
to the action of natural selection or the survival of the 
fittest. I have altered the fifth edition of the Origin 
so as to confine my remarks to adaptive changes of 
structure. I had not formerly sufficiently considered 
the existence of many structures which appear to be, 
as far as we can judge, neither beneficial nor injurious 
and this I believe to be one of the greatest oversights as 
yet detected in my work. I may be permitted to say 
as some excuse, that I had two distinct objects in view, 
firstly, to shew that species had not been separately 
created, and secondly, that natural selection had been 
the chief agent of change, though largely aided by the 
80 1 The Variation of Animals and Plants under Domestication/ voL 
ii. p. 280, 282. 
