588 THE PHYSIOLOGY OF REPRODUCTION 



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 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 themselves 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 indi- 

 cates 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 fitter, 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 



