THE MAMMALIAN SEXUAL CYCLE 49 



Tlie object of the corpus luteiim degenerating after a brief 

 interval in polyoestrous animals when pregnancy has not super- 

 vened is (in teleological language) in order to admit of a fresh 

 batch of follicles ripening, and a new heat period taking place. 

 The purpose of the polyoestrous condition is to increase the 

 chances of the female becoming pregnant in one sexual season, 

 and therefore to promote the fecundity of the race. These 

 characteristics may be supposed to have developed in evolution- 

 ary history as a result of natural selection or the survival of the 

 fittest. 



Some species of animals appear to be monoestrous in the wild 

 state but to show varying degrees of polyoestrum under domestica- 

 tion. These degrees are partly racial and partly dependent upon 

 nutrition and other environmental factors. Thus the wild sheep 

 is said to be monoestrous, whereas the Scottish Blackface in the 

 Highlands may have two dioestrous (or short) cycles in the 

 absence of the male, and the same breed has five or six cycles 

 in the Lowlands where the conditions are more favourable ; 

 various English breeds have a, greater number of recurrent 

 dioestrous cycles, the Dorset Horn sheep, owing to such an 

 increased sexual activity, being able to have two crops of lambs 

 in one year ; while the Merinos of Xew South Wales experience 

 the most extreme polyoestrum, since they are said to pass through 

 an unbroken succession of cyclical sexual changes which, in the 

 absence of the male, may extend throughout the year. 



, In addition to the domesticated animals above mentioned, the 

 polyoestrous condition is found in a number of wild species, and 

 appears to be especially common among rodents, but the changes 

 which take place in the different organs have been studied chiefly 

 in tame varieties. The guinea-pig has been shown to have a 

 cycle of from sixteen to twenty-five days (Loeb, Stockard and 

 Papanicolaou), the rat of four days (Long and Evans), and the 

 mouse of three or four days (Allen). Besides the ovarian and 

 uterine changes. Long and Evans and others, as already 

 mentioned, have shown that there is a vaginal cycle, the 

 cornified layer of epithelial cells being desquamated at about 

 the time of oestrus and afterwards replaced. 



Among marsupials (which constitute an order of primitive 

 mammals with extremely short uterine pregnancy, the young 

 being transferred at a very early stage to the marsupium or pouch), 



