118 DISTIX-CT SPECIES PRESENT [CHAP. V. 



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 lot h 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 

 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 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 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 in certain 

 characters another and distinct species, this other species being, 

 according to our view, only a well-marked and permanent variety. 

 But characters exclusively due to analogous variation would 

 probably be of an unimportant nature, for the preservation of 

 all functionally important characters will have been determined 

 through natural selection, in accordance with the different habits 

 of the species. It might further be expected that the species of 

 the same genus would occasionally exhibit reversions to long lost 

 characters. As, however, we do not know the common ancestor 

 of any natural group, we cannot distinguish between revisionary 

 and analogous characters. If, for instance, we did not know that 

 the parent rock-pigeon was not feather-footed or turn-crowned, 

 we could not have told, whether such characters in our domestic 

 breeds were reversions or only analogous variations; but we 

 might have inferred that the blue colour was a case of reversion 

 from the number of the markings, which are correlated with this 



