624 THE PHYSIOLOGY OF REPRODUCTION 



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 remarkable 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 uncommon, 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]." 1 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 generalisation that Individua- 

 tion and Genesis vary inversely. When there is an abundant food 

 supply and a favourable environment, and the necessary expenditure 

 of energy is relatively slight, the cost of Individuation is much 

 reduced, and the rate of Genesis is correspondingly increased ; in 

 other words, there is a high degree of fertility. Spencer cited 

 the Boers, the Kaffirs, and the French Canadians as examples of 

 fertile races in which the rate of increase is associated with a nutrition 

 that is greatly in excess of the expenditure. Conversely, he concluded 

 that a relative increase of expenditure leaving a diminished surplus 

 reduces the degree of fertility, and in support of this statement 

 adduced evidence that bodily labour tends to make women less 



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. 



