CHAPTER XIV 



FERTILITY 



' ' Nam multum harmonias veneris dif erre videntur. 

 Atque alias alii complent magis ex aliisque, 

 Succipiunt alise pondus magis inque gravesount. 



Atque in eo refert quo vietu vita colatur." — Lucretius. 



The rate of propagation in any species of animal depends not 

 only upon the average number of young born in each litter, but 

 also upon the frequency of recurrence of the sexual season and 

 the duration of the reproductive period in the animal's hfe. 

 The frequency of recurrence of the sexual season — that is to say, 

 the oestrous cycle — ^in different species of Mammals has been 

 discussed at some length in an early chapter of this work. In 

 the present chapter it remains to consider a Uttle more closely 

 some of the causes which control this periodicity and the factors 

 which afEect fertihty. 



The duration of the reproductive period of an animal's existence 

 extends in most cases from a time when that animal has almost 

 reached its full size until the beginning of senescence, so that the 

 normal period of generative actiArityin the individuals of any 

 given species beats a definite relation to their average length of 

 hfe. In the male the sexual maturity is usually reached later than 

 in the female. Moreover, in the male there is no definite ending 

 of the reproductive period, since in Man, for example, the power 

 of producing spermatozoa continues in a gradually diminishing 

 degree even in extreme old age, whereas in the female, on the 

 other hand, the chmacteric marks the cessation of generative 

 activity (see below, p. 672). 



Broadly speaking, the average number of young produced 

 in a htter in any species of Mammal is inversely proportional 

 to the average size of the animals belonging to that species. 

 Thus, in most species of Ungulates twins are the exception 

 rather than the rule ; and there are seldom more than two young 

 produced at a time even in sheep and goats, which show a greater 

 degree of fertihty than most Ungulates. The sow, however, is 



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