490 HEREDITY AND SEX 



to 13 males (Pfliiger) . It should be noted, however, that finding, 

 say, 86 '8 per cent, of females in a collection of frogs from a pond 

 does not prove that Yung's results were fallacious. There may 

 have been peculiar nutritive conditions in that locality. 



Very interesting results have been pubUshed by Nussbaum in 

 reference to the common Rotifer Hydatina, in regard to which 

 he finds that the amount of food taken by the newly hatched 

 female determines whether its eggs will give rise to males or 

 females. 



Some experiments have been made — e.g. by Cuenot and 

 Schultze — on the possible influence of the nutrition of the mam- 

 malian parent on the sex of the offspring ; but the results are 

 all against the reality of this supposed influence. 



On the basis of human statistics some have tried to show that 

 abundant food for the parents favours the production of female 

 offspring, and vice versa ; but others have drawn from other 

 statistics the opposite conclusion, that the parental nutrition is 

 of no moment, unless in bringing about a differential death-rate. 

 The fact that 30 per cent, of human twins are of different sexes 

 seems enough to show that the dieting of the parent is not of 

 great importance. 



In The Evolution of Sex (revised edition, 1901), there will 

 be found a convenient summary of other data — partly experi- 

 mental and partly statistical — which have been adduced to 

 show that nutrition and temperature may have some sex-deter- 

 mining influence on the developing organism, either directly or 

 through its parents ; but it does not seem to us that any one 

 of these is conclusive, though they have a certain cumulative 

 suggestiveness which would warrant further experiment — par- 

 ticularly as regards the lower animals and the indirect influence 

 on offspring through the parents. 



Although it seems more and more clear that in the higher 

 animals — such as mammals — the environment of the embryo 

 has no sex-determining influence, it does not follow that this is 



