o 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. Hence, though under nature
it must generally be left doubtful, what cases are reversions to an
anciently existing character, and what are new but analogous variations,
yet we ought, on my theory, sometimes to find the varying offspring of
a species assuming characters (either from reversion or from analogous
variation) which already occur in some other members of the same group.
And this undoubtedly is the case in nature.

A considerable part of the difficulty in recognising a variable species
in our systematic works, is due to its varieties mocking, as it were,
some of the other species of the same genus. A considerable catalogue,
also, could be given of forms intermediate between two other forms,
which themselves must be doubtfully ranked as either varieties or
species; and this shows, unless all these forms be considered as
independently created species, that the one in varying has assumed some
of the characters of the other, so as to produce the intermediate form.
But the best evidence is af