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 birds belonging to two or more distinct breeds
are crossed, none of which are blue or have any of the above-specified
marks, the mongrel offspring are very apt suddenly to acquire these
characters. To give one instance out of several which I have observed: I
crossed some white fantails, which breed very true, with some black barbs--
and it so happens that blue varieties of barbs are so rare that I never
heard of an instance in England; and the mongrels were black, brown and
mottled. I also crossed a barb with a spot, which is a white bird with a
red tail and red spot on the forehead, and which notoriously breeds very
true; the mongrels were dusky and mottled. I then crossed one of the
mongrel barb-fantails with a mongrel barb-spot, and they produced a bird of
as beautiful a blue colour, with the white loins, 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 are descended from the
rock-pigeon. But if we deny this, we must make one of the two following
highly improbable suppositions. Either, first, 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.
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