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 1 . Thus the glass-like trans- 

 parency of the wing of a certain Ithomiine (Methona) and its Pierine 

 mimic {Dismorphia orlse) 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. xxvi. 1898, pp. 598 — 602. 



