CHAPTER XXV 



EXTERNAL FACTORS OF SEX DETERMINATION 



Whether the sex of an individual is determined by external 

 factors during the course of development, or whether the sex is 

 already determined in the egg, has been the subject of much dis- 

 cussion. I shall first consider the experimental evidence that 

 has seemed to show that external conditions may determine sex, 

 and then examine the evidence in favor of the view that sex is 

 predetermined in the egg, either before or after fertilization. 



Influence of Food 



Of the different external factors that have been supposed to 

 determine the sex of the individual, nutrition occupies the first 

 place. Nutrition is supposed in some cases to influence the sex 

 of the young animal, while in other cases the egg itself is supposed 

 to have been affected by the condition of nutrition of the mother. 



The work of Landois in 1867 was the first in the field. He 

 stated that he could produce, at will, males or females, of the 

 butterfly Vanessa urticae by regulating the amount of food. Six 

 years later Mrs. Treat claimed to have obtained similar results, 

 and in the same year, 1873, Gentry made a statement to the 

 same effect. Mrs. Treat made use of the caterpillars of Papilio 

 asterias. One lot deprived of food before the last moult gave 

 34 males and i female. Another lot, well fed, gave 68 females 

 and 4 males. 



Riley carried out, in 1873, a more thorough series of experi; 

 ments on the same species, as wxll as on others. He tried espe- 

 cially the effects of starving the caterpillars, for he says that it 

 is not possible to make caterpillars take more food than they do 

 naturally, and under this condition they produce both males 



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