VARIATION UNDER DOMESTICATION 19 



and it so happens that blue varieties of barbs are so rare that I 

 never heard of an instance in England; and the mongrels were 

 black, brown, and mottled. I also crossed a barb with a spot, 

 which is a white bird with a red tail and red spot on the forehead, 

 and which notoriously breeds very true; the mongrels were dusky 

 and mottled. I then crossed one of the mongrel barb-fantails with 

 a mongrel barb-spot, and they produced a bird of as beautiful a 

 blue color, with the white loins, double black wing-bar, and barred 

 and white-edged tail-feathers, as any wild rock-pigeon! We can 

 understand these facts, on the well-known principle of reversion to 

 ancestral characters, if all the domestic breeds are descended from 

 the rock-pigeon. But, if we deny this, we must make one of the 

 two following highly improbable suppositions. Either, first, that 

 all the several imagined aboriginal stocks were colored and marked 

 like the rock-pigeon, although no other existing species is thus 

 colored and marked, so that in each separate breed there might be 

 a tendency to revert to the very same colors 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 naturally become less and 

 less, as in each succeeding generation there will be less of the for- 

 eign blood; but when there has been no cross, and there is a 

 tendency in the breed to revert to a character which was lost dur- 

 ing some former generation, this tendency, for all that we can see 

 to the contrary, may be transmitted undiminished for an in- 

 definite number of generations. These two distinct cases of rever- 

 sion 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 ob- 

 servations, 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 prob- 

 ably quite correct, if applied to species closely related to each 

 other. But to extend it so far as to suppose that species, aborigi- 

 nally as distinct as carriers, tumblers, pouters, and fan tails now 



