THE FACTORS WHICH DETERMINE SEX 631 



prevented the elimination of sugar in the urine and made the 

 metabolism more complete, Schenk claimed that he could ripen 

 the ova, and so increase the chances of male offspring. 



Influence of Environment, It has long been known that 

 parthenogenesis is the normal method of reproduction among 

 plant-lice or Aphides during the months of summer, successive 

 generations of individuals arising in this way, but that with the 

 approach of autumn males make their appearance and repro- 

 duction then becomes sexual. If, however, the Aphides be kept 

 in an environment of artificial warmth, and at the same time 

 are supplied with abundant food, the succession of partheno- 

 genetic females may be caused to continue for years without 

 the appearance of the sexual form. It is to be noted that the 

 sexual and parthenogenetic females are not identical, and also 

 that the same female may give rise to parthenogenetic and 

 sexual offspring, or to males and females, or to only one sex. 

 Moreover, Stevens has shown that male and female embryos 

 may be produced practically simultaneously by the same in- 

 dividual. It is maintained therefore by this writer that " the 

 changes in sex usually attributed to changes in external con- 

 ditions are really a change from the parthenogenetic to the 

 sexual mode of reproduction. The life cycle is often very com- 

 plicated, and in some species of Aphides there is evidence that 

 the environment (e.g. the trees on which they live) rather than 

 the temperature is responsible for the development of the sexual 

 forms. 1 



Many of the lower Crustacea undergo a somewhat similar 

 alternation of generations. For example, the water-flea 

 (Daphnia), after reproducing parthenogenetically during the 

 summer time, deposits eggs which give rise to sexual forms 

 at the commencement of autumn, and the female after impreg- 

 nation lays the winter eggs from which the new brood arises. 

 This result is generally supposed to be brought about by the 

 conditions of temperature or nutrition ; but Weismann, 2 as a 

 consequence of numerous experiments and observations, has 



1 Balbiani, " Le Phylloxera du Chene et le Phylloxera de la Vigne," &c., 

 Mem. A VAcad. des Sci., vol. xxviii., 1884. Stevens, " Studies on the 

 Germ Cells of Aphids," Carnegie Institution Publications, Washington, 1906. 



- Weismann, " Beitrage zur Naturgeschichte den Daphniden," Zeitoch. 

 f. wise. Zooloyie, vols. xxvii., xxviii., xxx., and xxxiii., 1876-79. 



