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 reversionary and analogous characters.