The Descent of Man

gether. Independently of correlation, a change
in one part often leads, 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 inoculated with the poison of
toads (95. The 'Variation of Animals and Plants
under Domestication,' vol. ii. pp. 280, 282.); for
we can thus see that the fluids of the system, if
altered for some special purpose, might induce
other 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 large yet undefined extension may safely
be given to the direct and indirect results of
natural selection; but I now admit, after reading