most rarely a blue and black-barred bird, there has been a tendency in
each generation in the plumage to assume this colour. This view is
hypothetical, but could be supported by some facts; and I can see no
more abstract improbability in a tendency to produce any character
being inherited for an endless number of generations, than in quite
useless or rudimentary organs being, as we all know them to be, thus
inherited. Indeed, we may sometimes observe a mere tendency to produce
a rudiment inherited: for instance, in the common snapdragon
(Antirrhinum) a rudiment of a fifth stamen so often appears, that this
plant must have an inherited tendency to produce it.
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. 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.
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