34(5 SEXUAL SELECTION. Pakt II. 



between the male and female cocoons of the silk-moth 

 (Bombyx mori), that in France they are separated by 

 a particular mode of weighing. 13 In the lower classes 

 of the animal kingdom, the greater size of the females 

 seems generally to depend on their developing an enor- 

 mous number of ova ; and this may to a certain extent 

 hold good with insects. But Dr. Wallace has suggested 

 a much more probable explanation. He finds, after 

 carefully attending to the development of the cater- 

 pillars of Bombyx cynthia and yamamai, and especially 

 of some dwarfed caterpillars reared from a second brood 

 on unnatural food, "that in proportion as the indivi- 

 " dual moth is finer, so is the time required for its 

 " metamorphosis longer ; and for this reason the female, 

 " which is the larger and heavier insect, from having to 

 " carry her numerous eggs, will be preceded by the 

 " male, which is smaller and has less to mature." 14 Now 

 as most insects are short-lived, and as they are exposed 

 to many dangers, it w T ould manifestly be advantageous 

 to the female to be impregnated as soon as possible. 

 This end would be gained by the males being first 

 matured in large numbers ready for the advent of the 

 females ; and this again would naturally follow, as 

 Mr. A. R. Wallace has remarked, 15 through natural 

 selection ; for the smaller males would be first matured, 

 and thus would procreate a large number of offspring 

 which would inherit the reduced size of their male 

 parents, whilst the larger males from being matured 

 later would leave fewer offspring. 



There are, however, exceptions to the rule of male 

 insects being smaller than the females; and some of 



13 Kobinet, ' Vers a Soie,' 1848, p. 207. 



14 ' Transact. Ent. Soc.' 3rd series, vol. v. p. 486. 



15 < Journal of Proc. Ent. Soc' Feb. 4th, 1867, p. lxxi. 



