682 THE PHYSIOLOGY OF REPRODUCTION 



therefore of the young born, is effected in all animals in which the 

 ripening of the ovarian ova is subject to selective action ; 



"(2) That when no selective action occurs in the ovary the 

 proportion of the sexes of ovarian ova produced is governed by laws 

 of heredity." 



Having arrived at these conclusions, Heape next adduces evidence 

 that certain external forces may affect the piroportion of the sexes in 

 dogs. It is shown' that amongst greyhounds, conception during the 

 period from August to N'ovember is most favourable to the production 

 of males under the conditions of breeding at present practised, and 

 this result is attributed to a selective action on the ova produced at this 

 time. There is evidence also that among dachshunds and Basset 

 hounds the seasons affect the proportions of the males and females 

 born. The bloodhound returns seem to show that an excessive 

 production of males is associated with in-breeding. Further, there is 

 statistical evidence that a higher proportion of males is produced in 

 the larger litters, that the larger dogs produce the 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 ^ 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 nutrition. 

 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 ma,terial 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. 



'■ Heape, " Note on the Influence of Extraneous Forces upon the Proportion 

 of the Sexes Produced by Canaries," Froc. Camb. Phil. Soc, vol. xiv., 1907. 



