THE CESTROUS CYCLE IN THE MAMMALIA 73 



pletely disorganise the recurrence of the sexual season. In such 

 animals as the dog they do not do so, because the dog is mon- 

 oestrous, and has, as a rule, only two sexual seasons annually, 

 so that the ancBstrous period considerably exceeds in length 

 the period of gestation. In large animals such as the camel, 

 on the other hand, where the gestation period extends for 

 thirteen months, the recurrence of the sexual season is post- 

 poned by pregnancy for a whole year. Again, in small animals 

 like the rat, gestation only prevents the recurrence of oestrus, 

 reducing the number of dicestrous cycles, but not interfering 

 with the recurrence of the sexual season. " But whenever 

 gestation occurs it encroaches upon, if it does not entirely 

 absorb, the ancestrum ; that is to say, it reduces the period 

 during which the generative organs would he fallow if the sexual 

 season were a barren one. Thus in the case of a mare, a barren 

 sexual season may consist of a series of dicestrous cycles lasting 

 for as long as six months, in which case the ancestrum lasts 

 six months also, after which another sexual season begins. A 

 reproductive sexual season, however, results in a period of 

 eleven months' gestation, interfering not only with the di- 

 cestrous cycles which would have recurred if conception had 

 not taken place, but also absorbing practically the whole of the 

 ancestrum." ^ 



The duration of the gestation period is intimately connected 

 not only with the size of the body, but also with the stage of 

 development at which the young are born.^ It is longest in the 

 large terrestrial and gigantic aquatic Mammals (Ungulata and 

 Cetacea), which live amid favourable conditions of nourishment. 

 With these animals the young are so far advanced in develop- 

 ment at the time of birth that they are able to follow the mother 

 about, and to a certain extent shift for themselves. In Carnivores 

 and Rodents the period of pregnancy is relatively shorter, and the 

 young are often born naked, and with unopened eyes, and con- 

 sequently are absolutely helpless for a considerable time after 

 birth. The gestation period is shortest in the aplacental 

 Mammals (Monotremata and Marsupialia), in which the young 

 are born at a very early stage and transferred to a pouch 



' Heape, loc, cit. 



^ Sedgwick, Student's Text-look of Zoology, vol. ii., London, 1905. 



