FERTILITY 587 



exceptional in having very large litters, as many as seventeen 

 young being sometimes born. On the other hand, in small 

 Mammals such as Rodents large litters are the rule ; the rat, 

 for example, being known occasionally to bear as many as sixteen 

 or even twenty young ; but the Cheiroptera, or bats, are remark- 

 able for their relative infertility, only one young one ordinarily 

 being produced at a time, although the common bat is no larger 

 than the mouse. 



Generally speaking, only one young one is produced in those 

 animals in which the period of gestation exceeds six months. 

 The number of teats characteristic of the species also affords 

 an approximate indication of the average size of the litter. 



" Among women, the birth of twins occurs once in about 

 eighty deliveries. Triplets, quadruplets, quintuplets, and even 

 higher figures, are occasionally observed ; they are very un- 

 common, and the rarity is progressive with the number. The 

 normal or ordinary rule in woman is to bear one child at a time ; 

 and the next most frequent condition is temporary or persistent 

 sterility two points in which she signally differs from what 

 is generally believed [of animals]/' l Veit's statistics 2 for 

 13,000,000 births in Prussia showed that twins were produced 

 once in 89 cases, triplets once in 7910, and quadruplets only 

 once in 371,125 cases. There is some evidence also that the 

 frequency of occurrence of multiple pregnancy in women depends 

 upon the race or climate, and that it is commoner in cold than 

 in warm countries. 3 



Herbert Spencer 4 elaborated a theory whereby he explained 

 the relative degrees of fertility in the different races of men and 

 animals. According to this theory the power to sustain individual 

 life and the power to produce new individuals are inversely 

 proportional, a conclusion which is summarised in the generalisa- 

 tion that Individuation and Genesis vary inversely. When 

 there is an abundant food supply and a favourable environment, 



1 Matthews Duncan, Fecundity, Fertility, Sterility, and Allied Topics, 

 Edinburgh, 1866. 



2 Veit, "Beitrage zur geburtshulflichen Statistik," Monatsschr. f. Geb., 

 vol. vi., 1855. 



3 For further statistics and references see Williams, Obstetrics, New 

 York, 1904. 



4 Spencer, Principles of Biology, revised edition, vol. ii., London, 1899. 



