20 WALTER HEAPE. 



in early autumn^ when the embryo does not develop beyond 

 the segmentation stage until the following spring ? (Bischoff, 

 1854). Why should the seal take eleven to twelve months for 

 gestation, while a large dog only requires three months ? If 

 there was a great difference in the size of these animals the 

 variation might to some extent be accounted for, but it is 

 not so. It is true that the newly born seal is a far more perfect 

 animal than the newly born puppy, but it cannot be that the 

 whole of the difference in the time of gestation, uamely, eight 

 to nine months, is required for the extra development of 

 the more perfect seal embryo, other factors being equal. 



Again, how is it that an unusual change of climate will 

 affect the sexual season of an animal ? This is constantly 

 observed among domesticated animals, and a very marked 

 case is recorded of cows, in Skye, after an exceptionally 

 severe winter (Youatt, 18o4). 



And how is it that the sexual seasou, for instance, of the 

 fox (Bischoff, 1863) and red deer (Cameron, 1900), is modified 

 in accordance with the nature of the country in which it lives, 

 whether high or low ground, in accordance with the age of 

 the animal, and probably also in accordance with its bodily 

 condition ? 



There seems to me ample reason for the belief that the sexual 

 season is governed directly by the influences detailed above — 

 climatic, individual, and maternal ; and that variation in the 

 rate of development of the embryo, in the length of gestatiou, 

 and in the powers of nursing, are quite sufficient to provide 

 for the launching of the young at a favourable time of the year. 



The origin of the sexual season is another matter; for a 

 solution of this question a comparative study of the phe- 

 nomena in the lower animals is necessary. 



That it is the result of a stimulus which may be effected 

 through the alimentary canal is demonstrated by the effect 

 upon ewes of certain stimulating foods; the sexual season 

 of ewes may be thus forced by " flushing " them, as it is 

 called by flockmasters. 



In the same way it is stated that a quart of milk, drawn 



