THI<] "sexual season" OF MAMMALS. 19 



ent times of the yeai% and, when gestation is of sufficiently 

 short duration, of reproduction more than once a year. An 

 increase in the duration of the sexual season may also have 

 two effects ; it gives increased opportunity for successful 

 coition, highly advantageous to those animals which live an 

 isolated life, while, among animals which experience a suffi- 

 ciently short period of gestation, it gives them opportunity 

 for reproducing several times in each season. On the other 

 hand, a decrease in the periodicity or duration of the sexual 

 season has an opposite effect. 



It would seem highly probable tliat the method of increas- 

 ing or decreasing the opportunities of reproduction varies 

 with the habits of animals, the claims of maternity, and the 

 climate in wliich they live. The different methods are not 

 necessarily peculiar to particular groups or classes of animals, 

 and they may vary, within limits, in the same species in 

 different localities <'ind in the same individual under different 

 circumstances. Climatic and maternal influences may be 

 observed in wild, captive, and domesticated animals; but 

 individual influences can only be noted in the two latter 

 classes, and especially in domestic mammals. 



It has been freely stated, originally by Aristotle and sub- 

 sequently by numerous biologists, that the sexual seasons 

 ai'e governed by the requirements of the newly born young; 

 in other words that the season for conception is regulated 

 by the length of gestation and the time of year which is 

 most favourable for the birth of the young; and it is argued, 

 tliat the different times of the year during which the sexual 

 seasons of similar animals occur is sufficient ground for that 

 view. 



I cannot agree with this opinion ; if it were so, why should 

 some bats experience a sexual season in the autumn and not 

 produce j^oung until about June (Beneden, 1880, and others, 

 see below), although not more that two months are required 

 for gestation and these animals are active for that length of 

 time in the spring, before parturition takes place ? Again, 

 why should roe deer, in Germany, have their sexual season 



