112 SEX 



Hi 



On the other side, there is the work of 

 Heape, who shows for some mammals and 

 birds that peculiarities in nutritive and other 

 environmental conditions may exert a selective 

 influence, affecting the numerical proportions 

 of male-producing and female-producing germ- 

 cells. Russo has made interesting experi- 

 ments in treating rabbits with lecithin (a 

 very nutritive substance forming a great part 

 of yolk of egg), as the result of which he 

 thinks he is able to increase the number of 

 female offspring, namely, by increasing the 

 production of a certain type of ova, rich 

 in nutritive reserves. His experiments have 

 been adversely criticised, and they were not 

 numerous enough to win conviction. Until 

 repetition of the experiments on a big scale 

 has shown that Russo w r as in error, his thesis 

 should be kept in mind, that the sex of the 

 offspring depends on the special metabolism 

 of the germ-cells, and that the number of ova 

 which exhibit the male or the female type of 

 metabolism is alterable by the environmental 

 conditions. This is, in fact, part of the 

 thesis of our Evolution of Sex. Remembering 

 the case of mice, however and there are 

 others equally negative we are prepared to 

 find that it is only in certain organisms that 

 the germ-cells are not predetermined very 

 early in their history. 



Many investigations have yielded results 

 quite against the view that changes in environ- 

 ment can change the numerical proportion of 

 the male and female offpsring which is in 

 most cases near equality. On the other 

 hand, there are some investigations which 



