FERTILITY 625 



prolific, since the reproductive age is said to be reached a year later 

 by women of the labouring class than by middle-class women. 



Spencer applied his generalisation to animals as well as to man, 

 and attempted to explain thereby the average contrast between the 

 fertility of birds and Mammals. " Comparing the large with the 

 large and the small with the small, we see that creatures which 

 continually go through the muscular exertion of sustaining them- 

 selves in the air and propelling themselves rapidly through it, are 

 less prolific than creatures of equal weights which go through the 

 smaller exertion of moving about over solid surfaces. Predatory 

 birds have fewer young ones than predatory Mammals of approximately 

 the same sizes. If we compare rooks with rats, or finches with mice, 

 we find like differences. And these differences are greater than at 

 first appears. For whereas among Mammals a mother is able, 

 unaided, to bear and suckle and rear half-way to maturity a brood 

 that probably weighs more in proportion than does the brood of a 

 bird ; a bird, or at least a bird that flies much, is unable to do this. 

 Both parents have to help ; and this indicates that the margin for 

 reproduction in each adult individual is smaller." 



Spencer cites numerous instances from among both birds and 

 Mammals illustrating the effects of different degrees of activity upon 

 fertility. The hare and the rabbit, for example, are closely allied 

 species, "similar in their diet, but unlike in their expenditures for 

 locomotion. The relatively inert rabbit has six young ones in a litter, 

 and four litters a year; while the relatively active hare has but 

 two to five in a litter. That is not all. The rabbit begins to breed 

 at six months old ; but a year elapses before the hare begins to breed. 

 These two factors compounded result in a difference of fertility far 

 greater than can be ascribed to unlikeness of the two creatures 

 in size." 



Furthermore, Spencer refers to the case of the bat, which has 

 been already mentioned as being abnormally unprolific in proportion 

 to its size. The relatively low rate of multiplication is of course 

 ascribed to a relatively high rate of expenditure resulting from the 

 habit of flying. 



In a similar way Spencer explains such well-known facts as that 

 hens cease to lay when they begin to moult. " While they are 

 expending so much in producing new clothing, they have nothing to 

 expend for producing eggs." 



There can be little doubt that Spencer's generalisation is in the 

 main true, but it is equally certain that it cannot be applied 

 indiscriminately to explain the relative degrees of fertility in all 

 animals, and consequently it must not be pressed too far. Some of 

 the more special factors which control fertility are referred to below, 



