18 WALTEE HEAPE. 



I doubt if, in the present state of onr knowledge of the 

 subject, it is possible to determine which is the original of 

 these two conditions. The behaviour of animals in captivity 

 and under domestication inclines one to believe that monoes- 

 trum is the original form ; then, again, it is the simplest form, 

 and on that ground may be thought the more archaic. But, 

 on the other hand, it is the lower animals which are the most 

 prolific breeders, and, for many reasons, we may perhaps 

 expect the ancestral mammal to have been more prolific than 

 wild animals are now. 



If this should be true, the increased capacity for reproduc- 

 tion, shown by domesticated animals, would indicate rever- 

 sion to ancestral powers, due to the removal of such obstruc- 

 tions as must be inseparable from the struggle for existence. 

 Thus all we can be certain of is the close similarity between 

 these two forms of sexual season. 



A further complication is introduced by certain breeds of 

 domesticated sheep and by pigs; these are poly oestrous ani- 

 mals when domesticated, and they may also exhibit more than 

 one sexual season each year. Such a condition appears to be 

 exceptional, and I have not included this form of variation 

 in the foregoing account for that reason ; but I am quite 

 prepared to believe a more exact knowledge of what takes 

 place among domesticated animals will show a similar varia- 

 tion among individuals of other classes of animals. 



Variation in the periodicity of sexual seasons is brought 

 about by an increase or decrease in the persistence of the 

 anoestrum, and has nothing to do, necessarily, with variation 

 in the oestrus cycle itself; while, on the other hand, variation 

 in the duration of a sexual season is brought about by an 

 increase or decrease in the number of consecutive dioestrous 

 cycles (polyoestrous mammals), or by an increase or decrease 

 in the persistence of the oestrus (moncestrousand polyoestrous 

 mammals), the ancestrum being only secondarily affected in 

 consequence thereof. 



The effect of an increase in the periodicity of sexual 

 seasons maybe twofold ; it permits of reproduction at differ- 



