646 THE PHYSIOLOGY OF REPRODUCTION 



which stimulate the activity of the generative system. It is 

 to this pecuUar combination I attribute the regularity of the 

 remarkable differences shown in these aviaries." 



In a still later paper ^ 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 propor- 

 tion 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 

 environmental 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 ^ and Sadler ^ 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,* but 

 the most recent statistical investigation ^ on the causes con- 



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

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



^ Hofacker, Ueber die Eigenschaften welche sich hei Menschen und 

 Thieren aufdie Nachhommen vererben, Tubingen, 1828. 



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



'■ Geddes and Thomson, loc. cit. 



" Newcomb, " A Statistical Inquiry into the Probability of Causes of the 

 Production of Sex in Human Offspring," Carnegie Institution Publications , 



