derived from such a 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, and there is a tendency in the breed to revert to
a character which was 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 of
reversion are often confounded together by those who have written on
inheritance.

Lastly, the hybrids or mongrels from between all the breeds of the pigeon
are perfectly fertile, as I can state from my own observations, purposely
made, on the most distinct breeds. Now, hardly any cases have been
ascertained with certainty of hybrids from two quite distinct species of
animals being perfectly fertile. Some authors believe that long-continued
domestication eliminates this strong tendency to sterility in species.
>From the history of the dog, and of some other domestic animals, this
conclusion is probably quite correct, if applied to species closely related
to each other. But to extend it 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 made seven or eight supposed species of pigeons to breed freely
under domestication--these supposed species being quite unknown in a wild
state, and their not having become anywhere feral--these species presenting
certain very abnormal characters, as compared with all other Columbidae,
though so like the rock-pigeon in most other respects--the occasional
reappearance of the blue colour and various black marks in all the breeds,
both when kept pure and when crossed--and lastly, the mongrel offspring