12(5 DISTINCT SPECIES PRESENT [Chap. V 



pigeon, I presume that no cne 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 re- 

 appear 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 

 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 pos- 

 sessed, 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 gener- 

 ations. 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 conditions, is developed. With the 

 barb-pigeon, for instance, which very rarely produces a blue bird, it 

 is probable that there is a latent tendency in each generation tc 

 produce blue plumage. The abstract improbability of such a ten- 

 dency being transmitted through a vast number of generations, is 

 not greater than that of quite useless or rudimentary organs being 

 similarly transmitted. A mere tendency to produce a rudiment is 

 indeed sometimes thus inherited. 



As all the species of the same genus are supposed to be descended 

 from a common progenitor, it might be expected that they would 

 occasionally vary in an analogous manner ; so that the varieties of 

 two or more species would resemble each other, or that a variety 

 of one species would resemble iz certain characters another and 

 distinct species, — this other species being, according to our view, 

 only a well-marked and permanent variety. But characters exclu- 



