256 
DARWINISM 
CHAP. 
closely allied forms—those that require minute examination 
to discriminate them as distinct species—are most generally 
found in separate areas, and are what are termed representative 
forms. 
The extension we have now given to the theory of mimicry 
is important, since it enables us to explain a much wider 
range of colour phenomena than those which were first im¬ 
puted to mimicry. It is in the richest butterfly region in the 
world—the Amazon valley—that we find the most abundant 
evidence of the three distinct sets of facts, all depending on 
the same general principle. The form of mimicry first 
elucidated by Mr. Bates is characterised by the presence in 
each locality of certain butterflies, or other insects, themselves 
edible and belonging to edible groups, which derived protec¬ 
tion from having acquired a deceptive resemblance to some 
of the inedible butterflies in the same localities, which latter 
were believed to be wholly free from the attacks of in¬ 
sectivorous birds. Then came the extension of the principle, 
by Dr. F. Muller, to the case of species of distinct genera 
of the inedible butterflies resembling each other quite as 
closely as in the former cases, and like them always found 
in the same localities. They derive mutual benefit from 
becoming, in appearance, one species, from which a certain 
toll is taken annually to teach the young insectivorous birds 
that they are uneatable. Even when the two or more species 
are approximately equal in numbers, they each derive a 
considerable benefit from thus combining their forces ; but 
when one of the species is scarce or verging on extinction, the 
benefit becomes exceedingly great, being, in fact, exactly appor¬ 
tioned to the need of the species. 
The third extension of the same principle explains the 
grouping of allied species of the same genera of inedible 
butterflies into sets, each having a distinct type of coloration, 
and each consisting of a number of species which can hardly 
be distinguished on the wing. This must be useful exactly 
in the same way as in the last case, since it divides the 
inevitable toll to insectivorous birds and other animals 
among a number of species. It also explains the fact of the 
great similarity of many species of inedible insects in the 
same locality—a similarity which does not obtain to anything 
