116 THE LAW OF BIRTHS AND DEATHS 



his concubine or wife, appears to become less fertile, 

 although she has more regular diet, comfort and covering." 1 

 She probably becomes less fertile because she receives 

 better diet, comfort and clothing. The principle provides 

 that in actual famine conditions or cases of excessive 

 debility the birthrate will diminish. But if such conditions 

 are very prolonged the race will become extinct. 



It is in comparisons between the birthrates of different 

 social classes of the same country that the operation of 

 the principle will be most clearly traced. The people 

 will live under the same climate and under much the 

 same laws and institutions, and the statistics . will be 

 compiled by the same methods. As it is probable that 

 the degree of fertility will be decided mainly by the 

 effects of the environment on the women of the com- 

 munity, it is to the condition of the women that we must 

 look for evidence. 



Criticising another theory Spencer says : " That absolute 

 or relative infertility is commonly caused by mental 

 labour carried to excess is more clearly shown. Though 

 the regimen of upper-class girls is not what it should be, 

 yet, considering that their feeding is better than that of 

 girls belonging to the poorer classes, while in most other 

 respects their physical treatment is not worse, the deficiency 

 in reproductive power among them may be reasonably 

 attributed to the over-taxing of their brains, an over- 

 taxing which produces a serious reaction on the physique. 

 The diminution of reproductive power is not shown only 

 by the frequency of absolute sterility, nor is it shown 

 only in the early cessation of child-bearing ; but it is also 

 shown in the very frequent inability of such women to 

 suckle their infants. In its full sense, the reproductive 

 power means the power to bear a well-developed infant 



1 The History of Human Marriage, Westermarck. 



