160 LAWS OF VARIATION. Chap. V. 



rump, a bar at the end of the tail, with the outer 

 feathers externally edged near their bases with white. 

 As all these marks are characteristic of the parent rock- 

 pigeon, I presume that no one will doubt that this is 

 a case of reversion, and not of a new yet analogous 

 variation appearing in the several breeds. We may I 

 think confidently come to this conclusion, because, as 

 we have seen, these coloured marks are eminently liable 

 to appear in the crossed offspring of two distinct and 

 differently coloured breeds ; and in this case there is 

 nothing in the external conditions of life to cause the 

 reappearance of the slaty-blue, with the several marks, 

 beyond the influence of the mere act of crossing on the 

 laws of inheritance. 



No doubt it is a very surprising fact that characters 

 should reappear after having been lost for many, perhaps 

 for hundreds of generations. But when a breed has 

 been crossed only once by some other breed, the offspring 

 occasionally show a tendency to revert in character to 

 the foreign breed for many generations — some say, for 

 a dozen or even a score of generations. After twelve 

 generations, the proportion of blood, to use a common 

 expression, of any one ancestor, is only 1 in 2048 ; and 

 yet, as we see, it is generally believed that a tendency 

 to reversion is retained by this very small proportion of 

 foreign blood. In a breed which has not been crossed, 

 but in which both parents have lost some character 

 which their progenitor possessed, the tendency, whether 

 strong or weak, to reproduce the lost character might 

 be, as was formerly remarked, for all that we can see 

 to the contrary, transmitted for almost any number of 

 generations. When a character which has been lost in 

 a breed, reappears after a great number of generations, 

 the most probable hypothesis is, not that the offspring sud- 

 denly takes after an ancestor some hundred generations 



