As all the species of the same genus are supposed, on my theory, to
have descended from a common parent, it might be expected that they
would occasionally vary in an analogous manner; so that a variety of
one species would resemble in some of its characters another species;
this other species being on my view only a well-marked and permanent
variety. But characters thus gained would probably be of an
unimportant nature, for the presence of all important characters will
be governed by natural selection, in accordance with the diverse
habits of the species, and will not be left to the mutual action of
the conditions of life and of a similar inherited constitution. It
might further be expected that the species of the same genus would
occasionally exhibit reversions to lost ancestral characters. As,
however, we never know the exact character of the common ancestor of a
group, we could not distinguish these two cases: if, for instance, we
did not know that the rock-pigeon was not feather-footed or
turn-crowned, we could not have told, whether these characters in our
domestic breeds were reversions or only analogous variations; but we
might have inferred that the blueness was a case of reversion, from
the number of the markings, which are correlated with the blue tint,
and which it does not appear probable would all appear together from
simple variation. More especially we might have inferred this, from
the blue colour and marks so often appearing when distinct breeds of
diverse colours are crossed.
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