On the Origin of Species, 107 



crossed, neither of which is blue or has any of the above-specified 

 marks, the mongrel offspring are very apt suddenly to acquire 

 these characters ; for instance, I crossed some uniformly white 

 fantails with some uniformly black barbs, and they produced 

 mottled brown and black birds ; these I again crossed together, 

 and one grandchild of the pure white fantail and pure black barb 

 was of as beautiful a blue colour, with the white rump, double 

 black wing-bar, and barred with 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 have descended from the rock- pigeon. But if we 

 deny this, we must make one of the two following highly impro- 

 bable suppositions. Either, firstly, that all the several imagined 

 aboriginal stocks were coloured, and marked like the rock-pigeon, 

 although no other existing species is thus coloured and marked, 

 so that in each separate breed there might be a tendency to re- 

 vert to the very same colours and markings. Or, secondly, that 

 each breed, even the purest, has within a dozen or, at most, with- 

 in a score of generations, been crossed by the rock pigeon : I say 

 within a dozen or twenty generations, for we know of no fact 

 countenancing the belief that the child ever reverts to some one an- 

 cestor, removed by a greater number of generations. In a breed 

 which has been crossed only once with some distinct breed, the 

 tendency to reversion to any character derived from such cross 

 will naturally become less and less, as in each succeeding genera- 

 tion there will be less of the foreign blood ; but when there has 

 been no cross within a distinct breed, and there is a tendency in 

 both parents to revert to a character, which has been 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 are often con- 

 founded in treatises on inheritance." 



" Lastly, the hybrids or mongrels from between all the domestic 

 breeds of pigeons are perfectly fertile. I can state this from my 

 own observations, purposely made on the most distinct breeds. 

 Now, it is difficult, perhaps impossible, to bring forward one case 

 of the hybrid offspring of two animals clearly/ distinct being 

 themselves perfectly fertile. Some authors believe that long-con- 

 tinued domestication eliminates this strong tendency to sterility: 

 from the history of the dog I think there is some probability in 

 this hypothesis if applied to species closely related together, 



