But
characters exclusively due to analogous variation would probably be of an
unimportant nature, for the preservation of all functionally important
characters will have been determined through natural selection, in
accordance with the different habits of the species. It might further be
expected that the species of the same genus would occasionally exhibit
reversions to long-lost characters. As, however, we do not know the common
ancestor of any natural group, we cannot distinguish between reversionary
and analogous characters. If, for instance, we did not know that the
parent rock-pigeon was not feather-footed or turn-crowned, we could not
have told, whether such characters in our domestic breeds were reversions
or only analogous variations; but we might have inferred that the blue
colour was a case of reversion from the number of the markings, which are
correlated with this tint, and which would not probably have all appeared
together from simple variation. More especially we might have inferred
this from the blue colour and the several marks so often appearing when
differently coloured breeds are crossed. Hence, although under nature it
must generally be left doubtful, what cases are reversions to formerly
existing characters, and what are new but analogous variations, yet we
ought, on our theory, sometimes to find the varying offspring of a species
assuming characters which are already present in other members of the same
group.
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