THE CESTROUS CYCLE IN THE MAMMALIA 39 



about sixty-two days, an unusually long time for so small an animal, 

 being more than twice as long as the gestation period of the rabbit. 

 As a result the newly-born guinea-pig is well advanced in develop- 

 ment, and able to feed for itself, instead of being dependent on its 

 mother's milk. 



In most male Kodents the testes undergo a periodic increase in 

 size and descend into the sessile scrotum at the beginning of the 

 season of rut, after which they become smaller again, and are with- 

 drawn into the peritoneal cavity. In the Leporidse, however, and 

 in some other species, the testes are not so retracted, but remain 

 throughout the year in the scrotal sacs. 1 



UNGULATA 



This order contains several examples of animals which are almost 

 certainly monoestrous in a state of nature, but are polycestrous in 

 captivity or under domestication. In the latter case the increase 

 in sexual capacity appears to be due partly to the inherited effects 

 of domestication, and partly to the direct influence of a more 

 favourable environment. 



For example, the sheep presents a complete gradation from the 

 apparently monoestrous condition of some wild species to the extreme 

 degree of -polycestrum, which is reached by certain of the more 

 domesticated breeds. 2 



The Barbary wild sheep (Ovis tragelaphus) in the Zoological 

 Society's Gardens is monoestrous, breeding only once annually. 3 

 The same is stated to be the case with the Burrhel sheep (0. burrhel), 

 although the moufflon (0. musimoii) in captivity may experience two 

 or more recurrent dicestrous cycles in an annual sexual season. 4 It 

 would seem, however, from the account given by Lydekker 5 of the 

 breeding habits of 0. musimon, as well as 0. vignei, 0. ammon, and 

 0. canadensis, that these sheep in their wild condition are probably 

 moncestrous, for their annual sexual season is of short duration, and 

 occurs with great regularity. Similarly it may be inferred from 

 Prjewalsky's statements that 0. poli, 0. burrhel, and 0. argali are 

 monoestrous and breed once a year. Among wild sheep generally 

 the sexual season occurs as a rule in autumn, but it may vary with 



1 Owen, On the Anatomy of Vertebrates, vol. iii., London, 1868. 



2 Marshall, "The (Estrous Cycle, etc., in the Sheep," Phil. Trans., B., 

 vol. cxcvi., 1903. 



3 Heape, loc. cit. 



* I am indebted to Mr. F. E. Beddard, Prosector of the Zoological Society, 

 for this information. 



5 Lydekker, Wild Oxen, Sheep, and Goats of All Lands, London, 1898. 



6 Prjewalsky, Mongolia, the Tangut Country, and the Solitudes of Northern 

 Tibet (Morgan's Translation), London, 1876. 



