170 ORIGIN OF SPECIES 



than that of quite useless or rudimentary organs being simi- 

 larly 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 analo- 

 gous variation would probably be of an unimportant nature, 

 for the preservation of all functionally important characters 

 will have been determined through natural selection, in ac- 

 cordance 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 varia- 

 tions; 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 variation. More 

 especially we might have inferred this, from the blue colour 

 and the several marks so often appearing when differently 

 coloured 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 analo- 

 gous variations, yet we ought, on our theory, sometimes to 

 find the varying offspring of a species assuming 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- 



