Chap, v.] ANALOGOUS VARIATIONS. 197 



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 in- 

 herited. 



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 anal- 

 ogous 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 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 colour 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 simple 

 15 



