ENVIRONMENTAL FACTORS 489 



the nutrition of the parent blowfly had no effect on the sex of 

 the offspring, and that the proportion of sexes in the larvae was 

 not affected by what they ate. In short, everything points to the 

 conclusion that the sex is fixed in these cases before hatching, 

 that the nutrition of the parents has no influence, and that 

 apparent disturbances of the normal proportions in the larvae 

 are due to differences in the rate of mortality of the two sexes 

 in unusual conditions. 



Experiments by Born, Yung, and others on tadpoles point to 

 the conclusion that the quantity and quality of the food have a 

 determining influence on the sex. It is said that the tadpole 

 remains for a considerable time in a state of what may be called 

 larval hermaphroditism, and the experiments suggest that the 

 development of the gonads and associated structures into male 

 or female reproductive organs depends on the nutrition. 



Yung found that when the tadpoles were left in natural con- 

 ditions, the percentage of females was slightly in the majority. 

 In three lots, the percentage proportions of females to males 

 were as foUows : — 54 : 46 ; 61 : 39 ; and 56 : 44. The average 

 was thus about 57 females in the number. In the first brood, by 

 feeding one set with beef, Yung raised the percentage of females 

 from 54 to "]% ; in the second, with fish flesh, the percentage rose 

 from 61 to 81 ; while in the third set, when the flesh of frogs 

 was supplied, the percentage rose from 56 to 92 ! 



The experiments of Cuenot on somewhat similar lines did not 

 corroborate Yung's results, but he points out that in his work, 

 as in that of the other's, the sex of those tadpoles that died is 

 not known, which is a fatal objection. 



It is desirable that the experiments should be repeated on a 

 large scale, to meet, if possible, the following objections : (i) that 

 the testing of the definitive sex of the tadpoles was inexact ; 

 (2) that the possibly different rate of mortality in the two sexes 

 was not allowed for ; and (3) that even in natural conditions 

 the percentage of females is sometimes very high — e.g. 100 females 



