192 HEREDITARY CHARACTERS 



some organisms at any rate, external conditions may play 

 an important part in determining the sex of the individual 

 long after fertilisation has taken place. 1 Yung found that 

 tadpoles when kept under ordinary conditions produced 

 about 57 per cent, female frogs. Tadpoles fed upon beef 

 produced female frogs to the extent of 78 per cent., while 

 those fed upon the flesh of frogs produced 92 per cent, 

 females. The results of these observations have been dis- 

 puted, and similar experiments with mammals have given 

 negative results. 2 Nussbaum claimed that in the case of 

 Hydatina one of the Rotifers (wheel animalcules) the 

 amount of food determines whether the newly-hatched 

 female will produce eggs giving rise to male or female 

 individuals. 3 In other organisms it has also been claimed 

 that an abundance of food and a low temperature result in 

 the production of a large number of females in proportion to 

 males. 4 It is possible that in the case of some organisms 

 the proportions of the sexes depends to a considerable extent 

 upon the action of the environment, though the results of 

 some experiments made more recently do not agree with 

 those mentioned. 5 



A plausible explanation in such cases is not far to seek, 

 and might well be the result of natural selection. Taking 

 the case of Daphnia (water-flea), if there were a very 

 abundant food supply, it is evident that the race would 

 benefit by the production of a large number of females. 



1 Yung, E., " De I'influence des variations de milieu physico-chimique sur 

 le developpement des animaux," Arch. Sci. Phys. Nat., xiv., 1885 ; Born, G., 

 ' ' Experimentelle Untersuchungen iiber die Entstehung der Geschlechtsunter- 

 schiede," Breslauer Aerztliche Zeitschrift, 1881. 



2 Cuenot, L., " La determination du Sexe," Biblioge. Anat., iv., pp. 14-15, 

 1896; "Sur la determination du Sexe chez les Animaux," Bull. Sclent, de la 

 France et de la Belgique, xxxii., pp. 462-535, 1899. 



3 Nussbaum, Archiv f. mikro. Anat., 1897. 



4 Issakowitsch, A., " Geschlechtsbestimmende Ursachen bei den Daphniden," 

 Biol. Centralbl., xxv., 1905; Arch. mikr. Anat., Ixix., 1906; Malsen, H. von, 

 "Geschlechtsbestimmende Einflusse und Einbildung bei Dinophilus apatris/' 

 Arch. mikr. Anal., Ixix., 1906. 



5 Punnett, R. C., "Sex Determination in Hydatina," Proc. Roy. Soc., B., 

 vol. Ixxviii., 1906. 



