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 Huripus), 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 
