THE STRUGGLE FOR THE LIFE OF OTHERS. 253 



tion — femaleness being an accompaniment of abundant 

 food, maleness of the reverse. When Yung, to talie 

 an authentic experiment, began his observations on 

 tadpoles, he ascertained that in the ordinary natural 

 condition the number of males and females produced 

 was not far from equal— the percentage being about 

 57 female to 43 male, thus giving the females a pre- 

 ponderance of seven. But when a brood of tadpoles 

 was sumptuously fed the percentage of females rose to 

 78, and when a second brood was treated even more 

 liberally, the number amounted to 81. In a third 

 experiment with a still more highly nutritious diet, 

 the result of the high feeding was more remarkable, 

 for in this case 92 females were produced and only 8 

 males. In the case of butterflies and moths, it has 

 been found that if caterpillars are starved before en- 

 tering the chrysalis state the offspring are males, 

 while others of the same brood, when highly nourished, 

 develop into females. A still more instructive case 

 is that of the aphides, the familiar 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 confir- 

 mation of this fact it has been proved that in a con- 

 servatory 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 deter- 

 ' The Evolution of Sex, pp. 41-46. 



