72 THE PHYSIOLOGY OF REPRODUCTION 



Whether the monoestrous or the polyoestrous condition is the 

 more primitive is a question which cannot at present be decided. 

 The fact that polyoestrum is secondarily acquired among many 

 animals may perhaps be regarded as evidence that moncEstrum is 

 the more primitive of the two conditions ; for, as already shown, 

 there are numerous instances of Mammals which are almost 

 certainly monoestrous in their wild state, but which have inde- 

 pendently assumed a condition of polyoestrum under the more 

 luxurious influences of domestication. Thus, while the sheep, 

 the sow, and the cat are almost certainly monoestrous in a 

 state of nature, the domesticated breeds of these animals show 

 a varying degree of 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 polyoestrum 

 in tropical climates among such primitive Mammals as the 

 Insectivores, and the common occurrence of varying degrees of 

 polyoestrum among the Rodents, not only in captivity but also 

 in the wild state, point to the possibihty that polyoestrum may 

 in reaUty be the more primitive condition, and one which can 

 easily be reverted to under the influence of a favourable en- 

 vironment. 



The main purpose of polyoestrum (to use teleological lan- 

 guage) 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 oestrus is not necessarily 

 associated with ovulation, and consequently the explanation 

 just given of the polyoestrous habit is not of universal apphcation. 

 This is a point which will be referred to again in deahng with 

 ovulation. It is of course possible, however, that the poly- 

 oestrous condition, having once been acquired, might in certain 

 circumstances be perpetuated in spite of its inutihty. 



Before concluding the present chapter it remains for me to 

 allude briefly to the effect of maternal influences on the oestrous 

 cycle. These, as pointed out by Heape, may or may not com- 



discussed this question at some length, adducing evidence of a sexual rhythm 

 in men. See especially appendix to Ellis's work by Perry-Coste, who shows 

 that there may be a tendency towards rhythmic regularity in the sexual 

 functions as rnanifested especially in the recurrence of seminal emissions. 



