Charles Darwin: The Origin of Species by means of Natural Selection

CHAPTER I. VARIATION UNDER DOMESTICATION. 4. BREEDS OF THE DOMESTIC PIGEON, THEIR DIFFERENCES AND ORIGIN. (continued)

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.