'I'HE " 8EXLJA1. SILVSON " OF MAMMALS, 65 



lu spite of the fact that the evidence I have brought 

 foi'vvard is fragmentary, and notwithstanding that only the 

 fringe of a vast subject has been touched upon, I venture to 

 hope enough has been said to show that the wide variations 

 in the sexual functions exhibited by various mammals are 

 variations in degree, not variations in kind; and I venture to 

 think that the evidence of the homology, not only of pro- 

 oestrum and menstruation, but of each of the various sexual 

 phenomena dealt with in the various types, is incontrovertible. 



One word with regard to the future development of the 

 subject. It is the cause of the sexual season which requires 

 determination. 



Much stress has been laid upon the rhythmical nature of all 

 breeding processes; this has been carried furthest by Lataste 

 (1887 and 1891), and by Beard in a very suggestive paper on 

 gestation (1897). So far as the sexual season is concerned, 

 its rhythm is no explanation of its origin. It may, I suppose, 

 be asserted that all forces are exerted rhythmically, that is a 

 condition ; whereas what is required here is knowledge of 

 the nature of the force itself, and the causes which govern or 

 limit its rhythm. 



These are questions for the comparative physiologist, in 

 whose hands, as it seems to me, lie so many of the great bio- 

 logical problems of the day. 



Speaking generally, the rhythm of the sexual season and 

 the power of breeding is seasonal, it is governed by external 

 forces which are exerted in consequence of seasonal change, 

 and by internal forces which are dependent upon individual 

 powers; further there is abundant evidence that nutriment, 

 and the capacity for storing nutriment, and the energy result- 

 ing therefrom are essential factors. 



I differ from those who, like Beard, consider the ovary the 

 seat of the governing power of the breeding function; ovula- 

 tion and the oestrus cycle are not necessarily coincident, the 

 stimulus sufficient to induce the one is apparently not suffi- 

 cient in all cases to induce the other, and it would appear 

 that the requisite initiative is independently produced. 



VOL. 44, FAKT 1. NEW SEEIES. E 



