Mr. Bates may almost be said to have actually
witnessed the process by which the mimickers have come so closely to
resemble the mimicked; for he found that some of the forms of Leptalis
which mimic so many other butterflies, varied in an extreme degree. In one
district several varieties occurred, and of these one alone resembled, to a
certain extent, the common Ithomia of the same district. In another
district there were two or three varieties, one of which was much commoner
than the others, and this closely mocked another form of Ithomia. From
facts of this nature, Mr. Bates concludes that the Leptalis first varies;
and when a variety happens to resemble in some degree any common butterfly
inhabiting the same district, this variety, from its resemblance to a
flourishing and little persecuted kind, has a better chance of escaping
destruction from predaceous birds and insects, and is consequently oftener
preserved; "the less perfect degrees of resemblance being generation after
generation eliminated, and only the others left to propagate their kind."
So that here we have an excellent illustration of natural selection.
Messrs. Wallace and Trimen have likewise described several equally striking
cases of imitation in the Lepidoptera of the Malay Archipelago and Africa,
and with some other insects. Mr. Wallace has also detected one such case
with birds, but we have none with the larger quadrupeds.
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