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. The ass sometimes has very distinct
transverse bars on its legs, like those on the legs of a zebra. It has
been asserted that these are plainest in the foal, and from inquiries which
I have made, I believe this to be true.
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