58 The Selection Theory 
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 entomo- 
logical 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’. Thus the glass-like trans- 
parency 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 colour- 
lessness and transparency, and to the fact that they stand upright. 
In another moth mimic (Anthomyza) the arrangement of the trans- 
parent 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 favour- 
able 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 other- 
wise 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 irreconcileable 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 Heli- 
conidae). 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 
1 Journ. Linn. Soc. London (Zool.), Vol. xxv1. 1898, pp. 598—602. 
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