14 WALTER HEAPE. 



be also true tor cerLaiu polyoestrous mammals, as, for in- 

 stance, tlie mare, under certain circumstances. On the other 

 hand it may include several sexual seasons and several gesta- 

 tion periods, a condition to which only certain polyoestrous 

 mammals can attain, of which the rat is an example. 



The time occupied by a breeding season is very variable, 

 from a few weeks (bitch) to several months (mare), and even 

 more than a year (elephant). There may be only one 

 breeding season in the course of several years, as shown by 

 the walrus (Bell, 1874), elephant, and probably rhinoceros 

 (Willoughby, 1889). There may be one breeding season each 

 year (mare) or more than one (domestic bitch and cat). 



The result of a breeding season may be the birth of one 

 young one (mare usually), one litter of young ones (bitch), or 

 many litters (rat). 



There may be great variation in the period of gestation of 

 different species of the same order of mammals. For in- 

 stance, among Rodents, the rat goes twenty-one days in 

 young, the rabbit thirty-two days, the guinea-pig sixty-three 

 days. Among Equidte, mares carry their foals eleven months, 

 asses from three hundred and fifty-eight to three hundred 

 and eighty-five days, and Burchell's zebra over thirteen 

 months (Tegetmeier and Sutherland, 1895). Among Ovidse 

 the Barbary wild sheep goes from twelve to fifteen weeks in 

 young (Zoo.), while the domestic sheep in this country 

 averages about twenty-one weeks. 



There may even be variation in the period of gestation in 

 varieties of one species ; for instance. Merino sheep average 

 ISO'S days' gestation, while Southdowns average 144"2 days 

 (Darwin, 1875), and different breeds of cows apparently vary 

 from 277 to 288*75 days' gestation (Varigny, 1892). 



The supply of food available may influence the length of 

 time occupied by gestation. A correspondent who is a 

 sheep- breeder informed me that his ewes, when run on poor 

 land, experience an appreciably longer gestation period than 

 those run on rich land ; and I am strongly inclined to think 

 investigation will show that the supply of food, and the 



