THE DETERMINATION OF SEX on 



the environment of the embryo or larva is largely discredited,, 

 a considerable body of evidence has been brought forward 

 to show that influences acting on the parents, particularly 

 on the mother, before fertilisation, may affect the sex of 

 the offspring ; this idea is not open to the objections which 

 appear fatal to the older view. Two of the most convincing- 

 pieces of work supporting this conclusion are those of Issako- 

 witsch on the Daphniidae * and von Malsen on Dinophilus? 

 lssakowitsch worked with the parthenogenetic females of 

 Simocephalus, von Malsen with Dinopkilus apatris, in which the 

 eggs are fertilised ; each found that differences of tempera- 

 ture caused differences in the proportion of males produced 

 and both ascribed the difference to changes in the nutrition 

 of the mother. Maupas 3 and Nussbaum 4 made somewhat 

 similar statements about Hydatina senta, in which all females 

 are from birth either male-producing or female-producing ; 

 but according to them the sex of the offspring of a partheno- 

 genetic female is determined by the conditions of temperature 

 or nutrition to which that female is subjected in the parental 

 uterus. Punnett 5 denies that temperature or nutrition has 

 any effect in the case of Hydatina and says that some stocks, 

 give rise to many arrhenotokous (male-producing) individuals, 

 others to few or none. So it seems not impossible that in this 

 case at least the evidence for the influence of environment 

 may not be as good as it appears at first and that some such 

 cause as differential mortality may bring about the results 

 observed. 6 



The idea that various external circumstances may influence the 

 proportion of the sexes does not rest only on experiments on in- 

 vertebrates ; there is a considerable mass of statistics pointing 

 in the same direction in the higher vertebrates, including man- 

 In these cases the number of young produced by one pair is 

 relatively small and in most cases the evidence takes the form 

 of figures drawn from a considerable population. The differences 



1 Biol. Centralblatt, xxv. 1905, p. 529 ; and Arch. Mikr. Anat. vol. Ixix. 1906,, 

 p. 223. 



2 Arch. Mikr. Anat. Ixix. 1906, p. 73. 



3 Comptes Rendus, cxi. 1890, pp. 310, 505. 

 * Arch. Mikr. Anat. xlix. p. 227. 



b Proc. Roy. Soc. B. lxxviii. 1906, p. 223. 



6 See Whitney, Journ. Exp. Zoo. v. 1907, p. i, for experiments on Hydatina^, 

 explaining Maupas' results. 



