196 DISTINCT SPECIES PRESENT [Chap. V. 



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 influ- 

 ence of the mere act of crossing on the laws of inherit- 

 ance. 



No doubt it is a very surprising fact that characters 

 should reappear after having "been lost for many, prob- 

 ably 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 

 tendency 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 hoth 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 gen- 

 erations. 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 hun- 

 dred generations, but that in each successive generation 

 the character in question has been lying latent and at 

 last, under unknown favourable conditions, is developed. 

 With the barb-pigeon, for instance, which very rarely 

 produces a blue bird, it is probable that there is a latent 



