CHAPTEE XIV 

 FERTILITY 



" Nam multum harmonise veneris diff erre videntur. 

 Atque alias alii complent magis ex aliisque, 

 Succipiunt alise pondus magis inque gravescunt. 



Atque in eo refert quo victu vita colatur." 



— LuCEBTItfS. 



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 life. The frequency of 

 recurrence of the sexual season — that is to say, the cestrous 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 little more closely some of the causes which control this 

 periodicity and the factors which affect fertility. 



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 activity in the individuals of any given 

 species bears a definite relation to their average length of life. 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 

 climacteric marks the cessation of generative activity (see below, 

 p. 714). 



Broadly speaking, the average number of young produced in a 

 litter 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 fertility than 

 most Ungulates. The sow, however, is exceptional in having very 

 large litters, as many as seventeen young being sometimes born. 

 On the other hand, in small Mammals such as Kodents large litters 



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