60 WALTER HEAPE. 



the active changes due to pro-oestrum have taken place in the 

 uterus ; it is always present, under normal conditions, in the 

 lower mammals at that time, and is much more frequent then 

 in the Primates than is generally supposed. 



Conclusion. 



The conclusions I draw from the evidence detailed above 

 are then, very briefly, as follows : 



A sexual season is common to all female mammals; its 

 recurrence may be interfered with in consequence of climatic, 

 individual, or maternal influences, and it maybe modified by 

 the influences attending captivity, domestication, or civilisa- 

 tion. 



The modification brought about by one or other of these 

 various influences is not necessarily the same in different 

 species of the same genus, nor in different individuals of the 

 same species, nor even in the same individual at all times ; 

 but whatever differences there may be, they are merely 

 modifications of the same plan. 



The sexual season of all mammals is evidenced by a series 

 of phenomena which constitute, in the absence of the male, 

 one oestrous cycle (monoestrous mammals) or a series of 

 oestrous cycles (polycestrous mammals); animals usually 

 moncBstrous may, under certain circumstances, show a 

 tendency to poly oestrum ; in the same way animals usually 

 polycestrous may show a tendency to monoestrum. These two 

 conditions are very closely related, and the main difference 

 between them is the method by which the reproductive power 

 is increased. 



The various constituent parts of an oestrous cycle are in- 

 variably demonstrated in all mammals; there is in all of them 

 a period during which the generative organs are hypertro- 

 phied and congested (pro-oestrum), followed by a period of 

 desire for coition (oestrus), which, in the absence of the male, 

 gradually dies away (metoestrum), and results in a period of 

 rest (dioestrum or anoestrum). When this period of rest 

 merely separates two recurrent dicestrous cycles it is brief. 



