VARIATION UNDER DOMESTICATION 55 



pigeon! We can understand ihese facts, on the ^ndl- 

 kiiown principle of reversion to ancestral characters, if 

 all the domestic breeds are descended from the rook- 

 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 genera- 

 tions, 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 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 generation, 

 this tendency, for all that we can see to the contrary, 

 may be transmitted undiminished for an indefinite num- 

 ber of generations. These two distinct cases of reversion 

 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 observations, purposely made, on the most 

 distinct breeds. Now, hardly any cases have been ascer- 

 tained with certainty of hybrids from two quite distinct 



