t to any character 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 being perfectly fertile--from these several reasons,
taken together, we may safely conclude that all our domestic breeds are
descended from the rock-pigeon or Columba livia with its geographical
sub-species.

In favour of this view, I may add, firstly, that the wild C. livia has
been found capable of domestication in Europe and in India; and that it
agrees in habits and in a great number of points of structure with all
the domestic breeds. Secondly, that although an English carrier or a
short-faced tumbler differs immensely i