NURTURAL INFLUENCES 501 



Careful experiments have been made, e.g. by Cuenot and 

 Schultze, on the possible influence of the nutrition of the mam- 

 malian parent (e.g. mouse) on the sex of the offspring ; but the 

 results are all against the reality of this supposed influence, in 

 which, however, some breeders strongly believe. Schultze ex- 

 tended his experiments over three generations, but the high 

 feeding of grandparents as well as parents did not seem to have 

 any influence on the proportion of the sexes among the offspring. 



Against these results, however, we have to balance the very 

 important work of Heape, who has brought forward evidence 

 for mammals and birds that peculiarities in nutrition and in 

 other environmental influences may exert a selective influence 

 on the germ-cells, affecting the proportion of male-producing 

 and female-producing gametes. "Through the medium of 

 nutrition supplied to the ovary, either by the quantity or the 

 quality of that nutrition, either by its direct effect upon the 

 ovarian ova or by its indirect effect, a variation in the proportion 

 of the sexes of the ova produced, and therefore of the young born, 

 is effected in all animals in which the ripening of the ovarian ova 

 is subject to selective action." . . . " When no selective action 

 occurs in the ovary, the proportion of the sexes of ovarian ova 

 produced is governed by the laws of heredity." 



Let us take one of Heape's interesting illustrations. Two 

 aviaries of canaries were kept under different conditions and the 

 proportions of the sexes were found to be notably different. In 

 one case, the aviary was kept at a regular temperature during 

 the breeding season ; it was comparatively well lighted and 

 sunned ; the birds did not receive specially rich food. In the 

 other case, the temperature of the aviary was allowed to vary 

 considerably during the breeding season ; it was in a room 

 facing north and east ; the birds had abundance of rich food. 

 " In the former of the two cases," to quote Marshall's summary, 

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

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



