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 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 themselves can only doubtfully be ranked as species; and this shows, unless all these closely allied forms be considered as independently created species, that they have in varying assumed some of the characters of the others.

But the best evidence of analogous variations is afforded by parts or organs which are generally constant in character, but which occasionally vary so as to resemble, in some degree, the same part or organ in an allied species.

I have collected a long list of such cases; but here, as before, I lie under the great disadvantage of not being able to give them.

I can only repeat that such cases certainly occur, and seem to me very remarkable.
I will, however, give one curious and complex case, not indeed as affecting any important character, but from occurring in several species of the same genus, partly under domestication and partly under nature.
It is a case almost certainly of reversion.