ere
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, {26} 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 domestication eliminates this strong tendency
to sterility: from the history of the dog I think there is some probability
in this hypothesis, if applied to species closely related together, though
it is unsupported by a single experiment. But to extend the hypothesis so
far as to suppose that species, aboriginally as distinct as carriers,
tumblers, pouters, and fantails now are, should yield offspring perfectly
fertile, _inter se_, seems to me rash in the extreme.

From these several reasons, namely, the improbability of man having
formerly got seven or eight supposed {27} species of pigeons to breed
freely under domestication; these supposed species being quite unknown in a
wild state, and their becoming nowhere feral; these species having very
abnormal characters in certain respects, as compared with all other
Columbidae, though so like in most other respects to the rock-pigeon;