wasps are dreaded for their sting, and
they are copied by harmless flies of the genera Eristalis and Syrphus,
and these mimics often occur in swarms about flowering plants without
damage to themselves or to their models; they are feared and are
therefore left unmolested.

In regard also to the _faithfulness of the copy_ the facts are quite
in harmony with the theory, according to which the resemblance must
have arisen and increased _by degrees_. We can recognise this in many
cases, for even now the mimetic species show very _varying degrees of
resemblance_ to their immune model. If we compare, for instance, the
many different imitators of _Danaida chrysippus_ we find that, with
their brownish-yellow ground-colour, and the position and size, and
more or less sharp limitation of their clear marginal spots, they have
reached very different degrees of nearness to their model. Or compare
the female of _Elymnias undularis_ with its model _Danaida genutia_;
there is a general resemblance, but the marking of the Danaida is very
roughly imitated in Elymnias.

Another fact that bears out the theory of mimicry is, that even when
the resemblance in colour-pattern is very great, the _wing-venation_,
which is so constant, and so important in determining the systematic
position of butterflies, is never affected by the variation. The
pursuers of the butterfly have no time to trouble about entomological
intricacies.

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.[50] 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 Itune 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