some semi-domestic breeds and some apparently truly wild breeds have,
besides the two black bars, the wings chequered with black. These
several marks do not occur together in any other species of the whole
family. Now, in every one of the domestic breeds, taking thoroughly
well-bred birds, all the above marks, even to the white edging of the
outer tail-feathers, sometimes concur perfectly developed. Moreover,
when two birds belonging to two distinct breeds are crossed, neither of
which is blue or has any of the above-specified marks, the mongrel
offspring are very apt suddenly to acquire these characters; for
instance, I crossed some uniformly white fantails with some uniformly
black barbs, and they produced mottled brown and black birds; these I
again crossed together, and one grandchild of the pure white fantail
and pure black barb was of as beautiful a blue colour, with the white
rump, double black wing-bar, and barred and white-edged tail-feathers,
as any wild rock-pigeon! We can understand these facts, on the
well-known principle of reversion to ancestral characters, if all the
domestic breeds have descended from the rock-pigeon. But if we deny
this, we must make one of the two following highly improbable
suppositions. Either, firstly, that all the several imagined aboriginal
stocks were coloured and marked like the rock-pigeon, although no other
existing species is thus coloured and marked, so that in each separate
breed there might be a tendency to revert to the very same colours and
markings. Or, secondly,
that each breed, even the purest, has within a dozen or, at most,
within a score of generations, been crossed by the rock-pigeon: I say
within a dozen or twenty generations, for we know of no fact
countenancing the belief that the child ever reverts to some one
ancestor, removed by a greater number of generations. In a breed which
has been crossed only once with some distinct breed, the tendency to
reversion to any character derived from such cross will naturally
become less and less, as in each succeeding generation there will be
less of the foreign blood; but when there has been no cross with a
distinct breed, and there is a tendency in both parents to revert to a
character, which has been lost during some former generation, this
tendency, for all that we can see to the contrary, may be transmitted
undiminished for an indefinite number of generations. These two
distinct cases are often confounded in treatises on inheritance.
Lastly, the hybrids or mongrels from between all the domestic breeds of
pigeons are perfectly fertile. I can state this from my own
26