292 Colour and the Struggle for Life 



Mimicry and Sex. 



Ever since Wallace's classical memoir on mimicry in the Malayan 

 Swallowtail butterflies, those naturalists who have written on the 

 subject have followed his interpretation of the marked prevalence of 

 mimetic resemblance in the female sex as compared with the male. 

 They have believed with Wallace that the greater dangers of the 

 female, with slower flight and often alighting for oviposition, have 

 been in part met by the high development of this special mode of pro- 

 tection. The fact cannot be doubted. It is extremely common for a 

 non-mimetic male to be accompanied by a beautifully mimetic female 

 and often by two or three different forms of female, each mimicking a 

 different model. The male of a polymorphic mimetic female is, in fact, 

 usually non-mimetic (e.g. Papilio dardanus = merope), or if a mimic 

 (e.g. the Nymphaline genus Euripus), resembles a very different model. 

 On the other hand a non-mimetic female accompanied by a mimetic 

 male is excessively rare. An example is afforded by the Oriental 

 Nymphaline, Cethosia, in which the males of some species are rough 

 mimics of the brown Danaines. In some of the orb-weaving spiders 

 the males mimic ants, while the much larger females are non-mimetic. 

 When both sexes mimic, it is very common in butterflies and is also 

 known in moths, for the females to be better and often far better 

 mimics than the males. 



Although still believing that Wallace's hypothesis in large part 

 accounts for the facts briefly summarised above, the present writer 

 has recently been led to doubt whether it offers a complete explana- 

 tion. Mimicry in the male, even though less beneficial to the species 

 than mimicry in the female, would still surely be advantageous. 

 Why then is it so often entirely restricted to the female ? While the 

 attempt to find an answer to this question was haunting me, I re-read 

 a letter written by Darwin to Wallace, April 15, 1868, containing the 

 following sentences: "When female butterflies are more brilliant than 

 their males you believe that they have in most cases, or in all cases, 

 been rendered brilliant so as to mimic some other species, and thus 

 escape danger. But can you account for the males not having 

 been rendered equally brilliant and equally protected ? Although 

 it may be most for the welfare of the species that the female should 

 be protected, yet it would be some advantage, certainly no dis- 

 advantage, for the unfortunate male to enjoy an equal immunity from 

 danger. For my part, I should say that the female alone had happened 

 to vary in the right manner, and that the beneficial variations had 

 been transmitted to the same sex alone. Believing in this, I can 

 see no improbability (but from analogy of domestic animals a strong 

 probability) that variations leading to beauty must often have occurred 



