LAWS OF VARIATION 129 



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 con- 

 ditions, 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 de- 

 scended 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 on an unimportant nature, for the preservation of all func- 

 tionally 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 charac- 

 ters. As, however, we do not know the common ancestor of any 

 natural group, we cannot distinguish between reversionary 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 color was a case of reversion from the 

 number of the markings, which are correlated with this tint, and 

 which would not probably have all appeared together from sim- 

 ple variation. More especially we might have inferred this from 

 the blue color and the several marks so often appearing when 

 differently colored breeds are crossed. Hence, although under na- 

 ture it must generally be left doubtful, what cases are reversions 

 to formerly existing characters, and what are new but analogous 

 variations, yet we ought, on our theory, sometimes to find the 



