DARWIN 



this fact obviously shows that they are still the same 

 species since their young are not sterile, and, what is 

 even more significant, the pigeons from such cross- 

 breeding, in a few generations, still revert back to the 

 original ancestral type. This tendency to revert to the 

 original stock is so strong that even in pure strains 

 we find "the occasional appearance in all the breeds 

 of slaty-blue birds with two black bars on the wings, 

 white loins, a bar at the end of the tail, with the outer 

 feathers externally edged near their bases with 

 white";"" these are the characteristics of the rock 

 pigeon {Columba Livia). Darwin explains this rever- 

 sion as follows: "After twelve generations, the pro- 

 portion of blood, to use a common expression, from 

 one ancestor, is only one in 2048 ; and yet, as we see, 

 it is generally believed that a tendency to reversion is 

 retained by this remnant of foreign blood. In a breed 

 which has not been crossed, but in which both parents 

 have lost some character which their progenitor pos- 

 sessed, the tendency, whether strong or weak, to pro- 

 duce the lost character might, as was formerly re- 

 marked, for all that we can see to the contrary, be 

 transmitted for almost any number of generations. 

 When a character which has been lost in a breed, re- 

 appears after a great number of generations, the most 

 probable hypothesis is, not that one individual sud- 

 denly takes after an ancestor removed by some hun- 

 dred generations, but that in each successive genera- 



-^Ibid., vol. I, p. 195. 



C 219 3 



