I must not pass over a discovery of Poulton's which is of great theoretical
importance--that mimetic butterflies may reach the same effect by very
different means. ("Journ. Linn. Soc. London (Zool.)", Vol. XXVI. 1898,
pages 598-602.) Thus the glass-like transparency of the wing of a certain
Ithomiine (Methona) and its Pierine mimic (Dismorphia orise) depends on a
diminution in the size of the scales; in the Danaine genus Ituna it is due
to the fewness of the scales, and in a third imitator, a moth (Castnia
linus var. heliconoides) the glass-like appearance of the wing is due
neither to diminution nor to absence of scales, but to their absolute
colourlessness and transparency, and to the fact that they stand upright.
In another moth mimic (Anthomyza) the arrangement of the transparent scales
is normal. Thus it is not some unknown external influence that has brought
about the transparency of the wing in these five forms, as has sometimes
been supposed. Nor is it a hypothetical INTERNAL evolutionary tendency,
for all three vary in a different manner. The cause of this agreement can
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.