THE DETERMINATION OF SEX. 



45 



tendency to the production of females, which at the beginning and at the 

 end are wholly absent. In the above series of experiments, they only 

 appear when the metabolism and the nutrition were so abundant 

 that the entire development of the young wasps only occupied eighteen or 



fewer days up to the period of pupation." The peculiarity in this last case, 

 if the experiments w^ere correct, is that in parthenogenesis, where the 

 production of males is the normal condition, favourable environmental 

 influences appear to introduce females. 



Two Forms of a Common Plant- Louse or Ajjhis. — 

 This figure equally well illustrates three different 

 things, — a winged male and a wingless female ; a 

 winged and a wingless parthenogenetic female ; a 

 winged se-xual female and an ordinary wingless 

 parthenogenetic female. — From Kessler. 



(d) Case of Aphides. — -One of the most familiar illustrations 

 of the influence of nutrition upon sex, is found in the history 

 of the plant-hce or aphides, which is indeed full of other 

 suggestions in regard to the whole theory of sex and reproduc- 

 tion. Details in regard to these plant-lice, which multiply so 

 rapidly upon our rose-bushes, fruit-trees, and the like, differ 



