42 ORIGIN OF SPECIES 



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 no instance is 

 known of crossed descendants reverting to an ancestor of 

 foreign blood, removed by a greater number of generations. 

 In a breed which has been crossed only once, the tendency to 

 revert to any character derived from such a cross will nat- 

 urally 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 genera- 

 tion, 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 inheri- 

 tance. 



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 do- 

 mestic animals, this conclusion is probably quite correct, if 

 applied to species closely related to each other. But to ex- 

 tend it so far as to suppose that species, aboriginally as dis- 

 tinct as carriers, tumblers, pouters, and fantails now are, 

 should yield offspring perfectly fertile inter se, would be 

 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 sup- 

 posed 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 

 Columbidse, though so like the rock-pigeon in most respects ; 

 — the occasional re-appearance of the blue colour and various 



