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
observations, purposely made on the most distinct breeds. Now, it is
difficult, perhaps impossible, to bring forward one case of the hybrid
offspring of two animals CLEARLY DISTINCT being themselves perfectly
fertile. Some authors believe that long-continued domestic