222 THE ORIGIN OF SPECIES 



for hundreds, of generations. But when a breed has been 

 crossed only once by some other breed, the offspring occa- 

 sionally 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 cotmmon expression, 

 from one ancestor, is only 1 in 2,048; 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 favorable conditions, is de- 

 veloped. 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 to produce blue 

 plumage. The abstract improbability of such a tendency 

 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 ex- 

 pected that they would occasionally vary in an analogous 



