THE CESTROUS CYCLE IN THE MAMMALIA 67 



polyoestrum which appears to depend largely upon the extent to 

 which domestication has been carried as well as upon food and the 

 influences of the surroundings. On the other hand, the existence 

 of the continuous polyo3strum in tropical climates among such 

 primitive Mammals as the Insectivores, and the common occurrence 

 of varying degrees of polyoestrum among the Eodents, not only in 

 captivity but also in the wild state, point to the possibility that 

 poly oestrum may be the more primitive condition, and one which can 

 easily be reverted to under the influence of a favourable environment. 



Hill and O'Donoghue l are of opinion that the momjestrous condi- 

 tion is the more primitive, basing their conclusion on what is known 

 of the breeding habits of the Marsupials, e.g. Dasyiirus, 7W/-// as- //?//*, 

 PhascolarctiLS, and Phascolomys^ and Monotremes, which are the 

 most primitive of all Mammals, as well as on considerations relative 

 to the sexual functions in reptiles, and this conclusion \vo may 

 tentatively accept as correct. It has already been mentioned that 

 in moiKPStrous animals like .Duxijurus and the dog osstrus, if not 

 succeeded by pregnancy, is followed by pseudo-pregnancy, and both 

 these conditions are associated with the persistence of the corpus 

 luteum or glandular structure formed from the discharged follicle 

 after the expulsion of the ovum. In acquiring the polyrpstrous habit 

 the duration of the corpus luteum has been much shortened down, 

 and the period of pseudo-pregnancy has almost or quite disappeared 

 so as to make way for a new ovulation and the associated changes 

 of oestrus. 



The main purpose of polycestrum (to use teleological language) 

 is no doubt, as already remarked, to provide increased opportunity 

 for coition, and so to promote the fecundity of the race. But it 

 must be remembered that cestrus is not necessarily associated with 

 ovulation, and consequently the explanation just given of the 

 polycestrous habit is not of universal application. This is a point 

 which will be referred to again in dealing with ovulation. It is 

 of course possible, however, that the polycpstrous condition, luivin^ 

 once been acquired, might in certain circumstances IK- perpetuated 

 in spite of its inutility. 



Before concluding the present chapter it remains I'm- me to allude 

 briefly to the effect of maternal influences on the instrous cycle. 

 These, as pointed out by Heape, may or may not completely dis- 

 organise the recurrence of the sexual season. In such animals as 

 the dog they do not do so, because the dog is mono-stroiis, and has, 

 as a rule, only two sexual seasons annually, so that the umestrous 

 period considerably exceeds in length the period of gestation. In 

 large animals such as the camel, on the other hand, where the 

 1 Hill and O'DoiiughiK-, /</. <>(. 



