196 DISTINCT SPECIES PEESENT [Chap. V. 



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. 



Xo doubt it is a very surprising fact that characters 

 should reappear after having been lost for many, 

 probably for hundreds of generations. But when a 

 breed has been crossed only once by some other breed, 

 the offspring occasionally show for many generations a 

 tendencv to revert in character to the foreign breed — 

 some say, for a dozen or even a score of generations. 

 After twelve generations, the proportion of blood, to use 

 a common expression, from 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 remnant 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, as 

 was formerly remarked, 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, reappears after a great number of generations, 

 the most probable hypothesis is, not that one individual 

 suddenly takes after an ancestor removed by some 

 hundred generations, but that in each successive 

 generation the character in question has been lying 

 latent, and at last, under unknown favourable condi- 

 tions, is developed. With the barb-pigeon, for instance, 

 which very rarely produces a blue bird, it is probable 



