THE " SRXUAL SEASON OF MAMMALS. 29 



months in Mus rattus; and even longer, perhaps, in Mus 

 musculns and M. decuman us. 



Bell (1874) appears to think that, under favourable circum- 

 stances, the dioestrous cycles may continue all the year round 

 in these latter animals and in the rabbit, but I am inclined to 

 think such a condition is unusual in this country among* wild 

 rodents, since it is exceptional to find any of them pregnant 

 during the winter months. 



Among domesticated animals the period during which the 

 dioestrous cycles recur, in the absence of the male, lasts from 

 one month to as many as eight months for the mare, about 

 five to six months for the rabbit, from one to three months for 

 the sheep (with certain exceptions), and about two months 

 for the pig. So far as the domestic rabbit is concerned, no 

 doubt, if they are kept warm, carefully fed, and their breed- 

 ing carefully regulated throughout the spring and summer, 

 they may exhibit oestrus also in winter, but it must be recol- 

 lected that here we are treating of oestrus independent of 

 pregnancy, which is a very different matter. 



Among certain monkeys, probably in most of them, the 

 dioestrous cycle recurs all the year round (Geoffrey, St. Hilaire 

 and Cuvier, Ehrenberg, 1833, Numan, 1838, Heape, 1894, 

 1897, Keith, 1899 ; compare also Rengger, 1880, Sutton, 1880, 

 and Ellis). In the human female, as a rule, this is also the 

 case; there appear, however, to be exceptions to this rule, for 

 instance, the women of the Esquimaux peoples living between 

 the seventy-sixth and seventy-ninth parallel do not always 

 menstruate during the winter months. It is said (Cook, 1894) 

 that not more than 10 per cent, of these women menstruate 

 during the longdark winter niontlis,and itis possibletoimagine 

 that the peculiar conditions of life they experience during that 

 time may well be responsible for their peculiarity. If this be 

 so, a true anoestrous period may be experienced by women. 



Rink's (1877) account of the origin of these people, if 

 correct, precludes the probability that the occurrence of an 

 anoestrous period is a racial charactei'istic, and emphasises 

 the view that it is a variation due to climatic conditions. 



