THE FACTORS WHICH DETERMINE SEX 645 



larger litters, and consequently that the larger breeds have a 

 racial tendency to produce an excess of dog pups. Lastly, the 

 schedule returns strongly support the popular belief that there 

 is a tendency to prolonged gestation when the embryo is of the 

 male sex. 



In a further paper l Heape discusses the apparent influence 

 of extraneous forces on the proportion of the sexes in two aviaries 

 of canaries, kept under different conditions. One aviary was 

 kept at a regular temperature during the breeding season ; it 

 was comparatively well lighted, and the sun had access to it. 

 On the other hand, the birds did not receive specially rich nutri- 

 tion. The other aviary was kept in a room facing north and 

 east, and the temperature was allowed to vary considerably 

 during the breeding time, but the birds were always fed with 

 a plentiful supply of rich food. In the former of the two cases 

 nesting, hatching, and moulting took place earlier, only about 

 half the percentage of loss was experienced, and from the nests 

 in which all the eggs were hatched, the percentage of males 

 produced was more than three times that which was obtained 

 from the other aviary, in which the environmental conditions 

 were less favourable. The results obtained in each case could 

 not be ascribed to the particular strains of canaries, since an 

 interchange of birds between the aviaries was not followed by 

 any material alteration in the proportion of the sexes in the 

 two environments. It is concluded, therefore, that the ova 

 were subject to a selective action on which depended the 

 proportional differences produced. 



" 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 



1 Heape, " Note on the Influence of Extraneous Forces upon the Pro- 

 portion of the Sexes Produced by Canaries." Proc. Camb. Phil. 6'oe., vol. xiv., 

 1907. 



