393 PROGRESS OF SCIENCE IN THE CENTURY. 



when fertilised is probably justified; while Hensen 

 extends this notion to the male element as well. The 

 age of the parents is probably only of secondary 

 import, and the law of Hofacker and Sadler as to 

 the importance of this is not confirmed. Theories 

 of " comparative vigour " and the like must be dis- 

 missed; while Starkweather's theory of the relative 

 superiority of either sex, and of the influence of this 

 on the sex of the offspring, requires further analysis. 

 But there is much importance in Busing's explana- 

 tion of the self-regulating numerical proportion of 

 the sexes. 



It must first be recognised that a number of fac- 

 tors co-operate in the determination of sex; but the 

 most important of these may be more and more 

 resolved into plus or minus nutrition, operating upon 

 parent, sex elements, embryo, and in some cases 

 larvae, (a) Starting with the parent organisms 

 themselves, we find this general conclusion most 

 probable, that adverse circumstances, especially of 

 nutrition, but also including age and the like, tend 

 to the production of males, the reverse conditions 

 favouring females. (6) As to the reproductive ele- 

 ments, a highly nourished ovum, compared with one 

 less favourably conditioned, in every probability will 

 tend to a female rather than to a male development. 

 Fertilisation, when the ovum is fresh and vigorous, 

 before waste has begun to set in, will corroborate 

 the same tendency, (c) Then if we accept Button's 

 opinion as to a transitory hermaphrodite period in 

 most animals, from which the transition to uni- 

 sexuality is effected by the hypertrophy of the female 

 side or preponderance of the male in respective cases, 

 the vast importance of early environmental influ- 

 ences must be allowed. The longer the period of 



