INSECTS 251 



according to Poulton, is that the degenerate females are 

 incapable of exercising a choice, and therefore the more 

 beantiful males are not selected. 



In answer to this I would urge that the main point to be 

 explained is the sexual dimorphism. If one principle can 

 explain the degeneracy of the females and higher develop- 

 ment of the males in the Psychidae, it can also explain the 

 superior brilliancy of the males in the Saturniidae. It can 

 scarcely be maintained that the males in the former have 

 preferred the most degenerate females. This has not yet 

 been suggested even by Professor Poulton. It is much more 

 reasonable to suppose that the females in this family have 

 become degenerate through sedentary habits, while the males 

 have not adopted such habits. The dimorphism in this 

 family is much greater than in the Saturniidae, and if 

 difference of conditions explains the greater difference, a 

 fortiori, it probably explains the smaller. Therefore, though 

 I do not know the habits of male and female in the Emperor 

 moth, it seems to me reasonable to believe that the plainness 

 of the female in that species and the more vivid colouring of 

 the male are due not to the choice exercised by the female, 

 but to the different effects of different habits and conditions. 



From the above considerations I conclude that, in the 

 majority of cases, sexual differences in colour and marking in 

 Lepidoptera are related to differences in the conditions of 

 life, not directly to courtship or the union of the sexes. 

 There are, however, a number of sexual differences known 

 which are related to the union of the sexes, and the function 

 of most of these specialisations seems to be to enable the 

 individuals of one sex to find those of the other. In such 

 cases the process of selection need not be denied. It is 

 obvious that an individual that fails to find a mate leaves no 

 progeny, and his defects are therefore eliminated, while the 

 individual which finds a mate most easily has the best chance 



