THE "sexual season" OP MAMMALS. 17 



different influences which affect the sexual season may be 

 either to increase or decrease the periodicity of that season, 

 or to increase or decrease the duration of each one. 



Among polyoestrous animals there is usually one sexual 

 season per annum, which is composed of two or more dioes- 

 trous cj^cles, and the result of tliese influences on such animals 

 may be, either to increase or decrease the number of con- 

 secutive dioestrous cycles in any one sexual season, or to 

 increase or decrease the duration of each cycle. 



The effect of these influences in both cases is to increase or 

 decrease the reproductive power of the animals, and they act 

 in nionoestrous animals by affecting both the periodicity and 

 duration of the sexual season, in polyoestrous animals chiefly 

 by affecting the duration, but in two different ways, namely 

 by increasing or decreasing both the number of consecutive 

 dioestrous cycles and the duration of the oestri which occur 

 therein. 



Modification of the periodicity of the sexual season, there- 

 fore, is chiefly found among moncestrous animals ; while modi- 

 fication of its duration is common to both monoesti'ous and 

 polyoestrous animals. It v^ould seem possible to simplify 

 these conditions if it were assumed that the polyoestrous 

 arose from the monocstrous condition ; if it were assumed, in 

 point of fact, that polyoestrum is simply a condition arrived 

 at by the concentration of several monocstrous sexual seasons. 



There might seem to be some reason for this when such 

 animals as the red deer, for instance, are considered ; in the 

 wild state this animal is apparently moncestrous, while in 

 captivity it is polyoestrous, at any rate in this country. 



But it may equally plausibly be argued that monoestrum is 

 simply decentralised polyoestrum. There are instances among 

 domesticated animals of moncestrous animals with a tendency 

 to polyoestrum (bitch), and of polyoestrous animals with a 

 tendency to monoestrum (mare). So also among wild animals 

 there are instances of animals which are moncestrous in one 

 climate and apparently polyoestrous in another (Sciurus 

 vulgaris) (compare Bell, 1874, and Lataste, 1887). 



VOL. 44, PART 1. — NEW SERIES. B 



