THE "SEXUAL SEASON OF MAMMALS. 9 



In those animals which experience the dioestrons C3^cle the 

 sexual season (when conception does not take place) consists 

 of a series of such cycles, two or more ; and may occupy any 

 length of time from one month to the whole year; In the 

 former case it is limited to a definite portion of the year 

 only, while in the latter case it may be coincident with the 

 whole reproductive period [human female, under certain 

 conditions]. But when the recurrence of the dioestrous 

 cycle is limited to a definite portion of the year, the sexual 

 season is, of coui'se, also limited to that period, and there is 

 consequently a period of rest, which is the anoestrum. 



In such cases the non-pregnant female experiences a series 

 of dioestrous cycles during the sexual season, at the end of 

 which, instead of dioestrnm following metoestrum, the latter 

 is succeeded by anoestrum, which persists until the next 

 sexual season occurs. 



In order to distinguish between the two classes of female 

 mammals into which the occurrence or absence of dioestrum 

 divides them, I have called those which experience a single 

 oestrus during each sexual season, or in other words those in 

 which the anoestrous cycle only occurs, monoestrous 

 mammals; while those whose sexual season is occupied by 

 a series of dioestrous cycles, or in other words those who 

 experience a series of recurrent ocstri, I have called poly- 

 oestrous mammals. 



The complication into which an otherwise simple story is 

 thrown is due, therefore, to variation in the quiescent period. 

 In some animals this may be a very brief period, never lasting- 

 more than a few days; in others it may occupy from two to 

 eleven months each time it occurs ; while in others again both 

 these conditions are experienced at different times of the j^ear. 



Functionally, no doubt, these two varieties of the quiescent 

 period are homologous, the one is a modification of the other; 

 and the modification is probably due, as will be shown below, 

 to an increased or decreased power of reproduction. At the 

 same time, for the purposes of the present paper, the difference 

 between them is essential, and their relation to the sexual 



