The Descent of Man

We can further see why a great amount of modification
in some one character ought not to
lead us to separate widely any two organisms.
A part which already differs much from the
same part in other allied forms has already,
according to the theory of evolution, varied
much; consequently it would (as long as the
organism remained exposed to the same exciting
conditions) be liable to further variations
of the same kind; and these, if beneficial, would
be preserved, and thus be continually augmented.
In many cases the continued development
of a part, for instance, of the beak of a bird, or
of the teeth of a mammal, would not aid the
species in gaining its food, or for any other object;
but with man we can see no definite limit
to the continued development of the brain and
mental faculties, as far as advantage is concerned.
Therefore in determining the position of
man in the natural or genealogical system, the
extreme development of his brain ought not to