142 DISTINCT SPECIES PRESENT 



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 anal- 

 ogous 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 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 simple 

 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 nature 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 varying offspring of a species assum- 

 ing characters which are already present in other members 

 of the same group. And this undoubtedly is the case. 



The difficulty in distinguishing variable species is largely 

 due to the varieties mocking, as it were, other species of the 

 same genus. A considerable catalogue, also, could be given 

 of forms intermediate between two other forms, which them- 

 selves can only doubtfully be ranked as species ; and this 

 shows, unless all these closely allied forms be considered as 



