THE FACTORS WHICH DETERMINE SEX 683 



"As a rule in nature the climatic forces which stimulate the 

 activity of the generative functions are also associated with a 

 plentiful supply of food; the conditions which excite the one 

 ensure the supply of the other. Among domesticated animals 

 living in the open air, on the other hand, any forcing of the breeding 

 time is brought about by special feeding. In neither case are the 

 results obtained comparable to those we have now before us, where 

 both the quality and the quantity of the food supplied is regulated 

 entirely independently of the other causes which stimulate the 

 activity of the generative system. It is to this peculiar combination 

 'I attribute the regularity of the remarkable differences shown in 

 these aviaries." 



In a still later paper 1 Heape shows that there is evidence of 

 the influence of extraneous forces upon the proportion of the sexes 

 produced by the white and coloured peoples of Cuba. Illegitimate 

 unions were found to give rise to a larger proportion of females, and 

 it is concluded that in this class of union there is an exceptionally 

 active metabolism of the mother which favourably affects the 

 development of those ovarian ova which give rise to female offspring. 



Heape suggests further that much of the evidence that has been 

 collected in regard to the influence of nutrition and other environ- 

 mental causes upon the proportions of the sexes, although it may 

 be disregarded from the point of view from which it was put forward 

 (since it is commonly assumed that the conditions directly determine 

 the sex of the embryo), may yet be well worthy of attention from 

 the standpoint adopted by him. Some of this evidence is briefly 

 referred to below. 



(:3) THEORIES WHICH LIMIT SEX-DETERMINATION TO NO PARTICULAR 

 PERIOD OF DEVELOPMENT, OR WHICH ASSERT THAT SEX MAY 

 BE ESTABLISHED AT DIFFERENT PERIODS. 



Influence of Age of Parent. Hofacker 2 and Sadler 3 arrived 

 independently at the conclusion that the sex of the offspring depends 

 on the relative ages of the parents that when the father is the 

 oldest more male births occur, and similarly when the mother is 

 the oldest there tends to be a preponderance of females. This 

 hypothesis, which is known as Hofacker and Sadler's Law, has been 

 both confirmed and contradicted, 4 but the most recent statistical 



1 Heape, " The Proportion of the Sexes Produced by Whites and Coloured 

 Peoples in Cuba," Phil. Trans., B., vol. cc., 1909. 



2 Hofacker, Ueber die Eigenschaften ivelche sick bei Menschen wnd Thieren auf 

 die Nachkommen vererben, Tubingen, 1828. 



3 Sadler, The Law of Population, London, 1830. 



4 Geddes and Thomson, loc. cit. 



