THE STRUGGLE FOR THE LIFE OF OTHERS 325 



plant-lice of our gardens. During the warmth 

 of summer, when food is abundant, these insects 

 produce parthenogenetically nothing but females, 

 while in the famines of later autumn they give birth 

 to males. In striking confirmation of this fact it 

 has been proved that in a conservatory where the 

 aphides enjoy perpetual summer, the parthenogenetic 

 succession of females continued to go on for four 

 years and stopped only when the temperature 

 was lowered and food diminished. Then males 

 were at once produced.^ It will no longer be said 

 that science is making no progress with this unique 

 problem when it is apparently able to determine 

 sex by turning off or on the steam in a green- 

 house. With regard to bees the relation between 

 nutrition and sex seems equally established. "The 

 three kinds of inmates in a bee-hive are known to 

 everyone as queens, workers, and drones ; or, as 

 fertile females, imperfect females, and males. What 

 are the factors determining the differences between 

 these three forms ? In the first place, it is believed 

 that the eggs which give rise to drones are not 

 fertilized, while those that develop into queens and 

 workers have the normal history. But what fate 

 rules the destiny of the two latter, determining 

 whether a given ovum will turn out the possible 

 ^ J'he Evolution of Sex^ pp. 41-6. 



