294 Colour and the Struggle for Life 



(1) Mimicry may occasionally arise in one sex because the 

 differences which distinguish it from the other sex happen to be such 

 as to afford a starting-point for the resemblance. Here the male 

 is at no disadvantage as compared with the female, and the rarity 

 of mimicry in the male alone (e.g. Cethosia) is evidence that the great 

 predominance of female mimicry is not to be thus explained. 



(2) The tendency of the female to dimorphism and polymorphism 

 has been of great importance in determining this predominance. 

 Thus if the female appear in two different forms and the male in only 

 one it will be twice as probable that she will happen to possess a 

 sufficient foundation for the evolution of mimicry. 



(3) The appearance of melanic or partially melanic forms in the 

 female has been of very gi*eat service, providing as it does a change of 

 ground-colour. Thus the mimicry of the black generally red-marked 

 American "Aristolochia swallowtails" (Pharmacophagns) by the 

 females of Papilio swallowtails was probably begun in this way. 



(4) It is probably incorrect to assume with Haase that mimicry 

 always arose in the female and was later acquired by the male. Both 

 sexes of the third section of swallowtails (Cosmoclesmus) mimic 

 Pharmacophagns in America, far more perfectly than do the females 

 of Papilio. But this is not due to Cosmodesnms presenting us with 

 a later stage of the history begun in Papilio ; for in Africa Cosmo- 

 desnms is still mimetic (of Danainac) in both sexes although the 

 resemblances attained are imperfect, while many African species of 

 Papilio have non-mimetic males with beautifully mimetic females. 

 The explanation is probably to be sought in the fact that the females 

 of Papilio are more variable and more often tend to become di- 

 morphic than those of Cosmodesmus, while the latter group has more 

 often happened to possess a sufficient foundation for the origin of 

 the resemblance in patterns which, from the start, were common to 

 male and female. 



(5) In very variable species with sexes alike, mimicry can be 

 rapidly evolved in both sexes out of very small beginnings. Thus 

 the reddish marks which are common in many individuals of Limenitis 

 arthemis were almost certainly the starting-point for the evolution of 

 the beautifully mimetic L. archijypus. Nevertheless in such cases, 

 although there is no reason to suspect any greater variability, the 

 female is commonly a somewhat better mimic than the male and 

 often a very much better mimic. Wallace's principle seems here 

 to supply the obvious interpretation. 



(6) When the difference between the patterns of the model and 

 presumed ancestor of the mimic is very great, the female is often alone 

 mimetic ; when the difiorence is comparatively small, both sexes are 

 commonly mimetic. The Nymphaline genus Hypolimnas is a good 



