THE CAUSE OF MIMETIC RESEMBLANCE. 581 



obtain. Outside these resemblances it is the rule, when any 

 difference between the sexes exists, for the female to show us 

 the ancestral type, the male the more modern development ; and 

 the male in growth from youth to maturity generally passes 

 through the condition permanently retained by the female. 



No probable interpretation of these unusual relationships can 

 be offered by any theory except natural selection. The theory 

 of external causes demands the improbable hypothesis, for which 

 no evidence can be found, that the female of mimetic species 

 (but not of others) is constitutionally more ready to respond to 

 the direct action of external forces than the male, and that the 

 difference is commonly great enough for the female to have 

 given a complete and detailed response, when the male, subject 

 to the same direct forces, does not exhibit the faintest trace of 

 the operation of any such influence. 



The facts are equally inexplicable by the theory of internal 

 causes — and not inexplicable only, but the reverse of what we 

 should expect ; for, as I have already stated, it is the female 

 which, outside these resemblances, tends to retain the ancestral 

 form. 



The theory of sexual selection also fails to account for the 

 facts. If it were valid, the selection would be that of the male, 

 for these recent developments are specially characteristic of the 

 other sex. In other cases in which male rather than female 

 selection is supposed to have acted in the production of colour 

 or pattern in butterflies, there is some direct evidence derived 

 from the observation of courtship ; but here no such support is 

 forthcoming. 



Under the theory of natural selection the facts at once receive 

 an explanation. Wallace suggested long ago that the slower 

 flight of the females " when laden with eggs, and their exposure 

 to attack while in the act of depositing their eggs upon the 

 leaves, render it specially advantageous for them to have some 

 additional protection." In animals which are hidden by 

 protective resemblance, similar causes explain why the female 

 is so often better concealed than the male. In birds the dangers 

 of incubation balance the dangers of egg-laying in insects. 

 But protective resemblances are less special than cases of 

 mimicry in the sense that the models (bark, twigs, leaves, &c.) 

 are more generally alike throughout all countries, and less 

 rapidly change their distribution than the models of mimicry and 



