16 WALTER HKAPJ;. 



course of artificial selection, added to the plentiful supply of 

 food, with warmth and shelter inseparable from domestica- 

 tion, has no doubt greatly increased their power of repro- 

 duction. 



As has been already stated, there are two forms of sexual 

 season evident in female mammals; the monoestrous, in which 

 there is only a single oestrus at one or more particular times 

 of the year (bitch), and the polyoestrous, iu which there are 

 two or more concurrent dioestrous cycles at a particular time 

 of the year (mare). 



The sexual season may be influenced by the climate of the 

 region iu which the animal lives, by the seasons of the year 

 when these are of marked variation, and by the supply of 

 food, or possibly by the nature of the food, obtainable. These 

 may be called climatic influences. 



It may also be influenced by special nervous, vascular, and 

 secretory peculiarities of the individual and by its habits of 

 life. These may be called individual influences. 



It may also be influenced by the length of gestation, the 

 claims of the newly-boru offspring on the mother (i.e. 

 nursing), and by her powers of recuperation. These may be 

 called maternal influences. 



Such influences may affect the time of year when the 

 sexual season occurs, its recurrence, and its duration. The 

 influences which affect the time of year when the sexual 

 season occurs, concern both mouoestrous and polyoestrous 

 mammals, and are essentially governed by climatic considera- 

 tions, including the supply of food. The recurrence and 

 duration of the sexual season on the other hand are affected 

 either by climatic, individual, or maternal influences, and 

 are also experienced both by monoestrous and polyoestrous 

 mammals, though in a somewhat different way by each. 



In order to understand this difference it is necessary to 

 examine briefly what occurs in these two classes of animals. 

 Among monoestrous animals there are a variable number of 

 sexual seasons each year, one or more, each composed of a 

 single oestrous of variable duration. So that the result of the 



