only lie in selection, which
preserves and intensifies in each species the favourable variations
that present themselves. The great faithfulness of the copy is
astonishing in these cases, for it is not _the whole_ wing which is
transparent; certain markings are black in colour, and these contrast
sharply with the glass-like ground. It is obvious that the pursuers of
these butterflies must be very sharp-sighted, for otherwise the
agreement between the species could never have been pushed so far. The
less the enemies see and observe, the more defective must the
imitation be, and if they had been blind, no visible resemblance
between the species which required protection could ever have arisen.

A seemingly irreconcilable contradiction to the mimicry theory is
presented in the following cases, which were known to Bates, who,
however, never succeeded in bringing them into line with the principle
of mimicry.

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 Mueller,[51] 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 in