474 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1910. 



for by the experimenters ; as we shall see, there is such a mass of evi- 

 dence accumulated on the other side that this idea is now largely 

 abandoned by biologists. 



But although the doctrine that sex may be influenced by the en- 

 vironment of the embryo or larva is largely discredited, a consider- 

 able body of evidence has been brought forward to show that in- 

 fluences acting on the parents, particularly on the mother, before 

 fertilization, 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 Issakowitsch on the Daphniida? 1 and von Malsen on Dinophilus. 2 

 Issakowitsch worked with the parthenogenetic females of Simocepha- 

 lus, von Malsen with DinopMlus apatris, in which the eggs are 

 fertilized; each found that differences of temperature caused differ- 

 ences 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 

 parthenogenetic female is determined by the conditions of tempera- 

 ture 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 inverte- 

 brates; 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 con- 

 siderable population. The differences due to altered environment or 

 other circumstances usually amount to only a few per cent; but if 

 they are consistent in a large population they must be taken into ac- 

 count in any theory of sex determination. There is a vast number of 

 papers of this kind, suggesting that a great variety of external cir- 



1 Biol. Centralblatt. vol. 25, 1905, p. 529; and Arch. Miki. Anat., vol. 69, 1900, p.223. 



2 Arch. jNIikr. Anat., vol. 09, 1906, p. T-l. 

 »Comptes Rendus, vol. Ill, 1890, pp. 310, 505. 



4 Arch. Mikr. Anat., vol. 49, p. l!U7. 



E l'roc. Roy. Soc. B., vol. 78, 1906, p. -1S.\. 



8 See Whitney, Journ. Exp. Zoo., vol. 5, 1907, p. 1, for experiments on Ili/thitiini, explain- 

 ing Maupas's results. [Also Shull, American Naturalist, Mar., 1910; and Journ. Exp. 

 Zool., vol. 8, 1910, p. 311 ; vol. 10, 191 1, p. 117.] 



