In South America there are, as we have already said, many mimics of the
immune Ithomiinae (or as Bates called them Heliconidae). Among these there
occur not merely species which are edible, and thus require the protection
of a disguise, but others which are rejected on account of their
unpalatableness. How could the Ithomiine dress have developed in their
case, and of what use is it, since the species would in any case be immune?
In Eastern Brazil, for instance, there are four butterflies, which bear a
most confusing resemblance to one another in colour, marking, and form of
wing, and all four are unpalatable to birds. They belong to four different
genera and three sub-families, and we have to inquire: Whence came this
resemblance and what end does it serve? For a long time no satisfactory
answer could be found, but Fritz Muller (In "Kosmos", 1879, page 100.),
seventeen years after Bates, offered a solution to the riddle, when he
pointed out that young birds could not have an instinctive knowledge of the
unpalatableness of the Ithomiines, but must learn by experience which
species were edible and which inedible. Thus each young bird must have
tasted at least one individual of each inedible species and discovered its
unpalatability, before it learnt to avoid, and thus to spare the species.
But if the four species resemble each other very closely the bird will
regard them all as of the same kind, and avoid them all. Thus there
developed a process of selection which resulted in the survival of the
Ithomiine-like individuals, and in so great an increase of resemblance
between the four species, that they are difficult to distinguish one from
another even in a collection. The advantage for the four species, living
side by side as they do e.g. in Bahia, lies in the fact that only one
individual from the MIMICRY-RING ("inedible association") need be tasted by
a young bird, instead of at least four individuals, as would otherwise be
the case. As the number of young birds is great, this makes a considerable
difference in the ratio of elimination.