perhaps a slight advantage; for although at short distances this
variety would be easily distinguished and devoured, yet at a longer
distance it might be mistaken for one of the uneatable group, and so be
passed by and gain another day's life, which might in many cases be
sufficient for it to lay a quantity of eggs and leave a numerous
progeny, many of which would inherit the peculiarity which had been the
safeguard of their parent.

Now, this hypothetical case is exactly realized in South America. Among
the white butterflies forming the family Pieridae (many of which do not
greatly differ in appearance from our own cabbage butterflies) is a
genus of rather small size (Leptalis), some species of which are white
like their allies, while the larger number exactly resemble the
Heliconidae in the form and colouring of the wings. It must always be
remembered that these two families are as absolutely distinguished from
each other by structural characters as are the carnivora and the
ruminants among quadrupeds, and that an entomologist can always
distinguish the one from the other by the structure of the feet, just as
certainly as a zoologist can tell a bear from a buffalo by the skull or
by a tooth. Yet the resemblance of a species of the one family to
another species in the other family was often so great, that both Mr.
Bates and myself were many times deceived at the time of capture, and
did not discover the distinctness of the two insects till a closer
examination detected their essential differences. During his residence
of eleven years in the Amazon valley, Mr. Bates found a number of
species or varieties of Leptalis, each of which was a more or less exact
copy of one of the Heliconidae of the district it inhabited; and the
results of his observations are embodied in a paper published in the
Linnean Transactions, in which he first explained the phenomena of
"mimicry" as the result of natural selection, and showed its identity in
cause and purpose with protective resemblance to vegetable or inorganic
forms.

The imitation of the Heliconidae by the Leptalides is carried out to a
wonderful degree in form as well as in colouring. The wings have become
elongated to the same extent, and the antennae and abdomen have both
become lengthened, to correspond with the unusual condition in which
they exist in the former family. In colouration there are several types
in the different genera of Heliconidae. The genus Mechanitis is gen