204 PROBLEMS OF HUMAN EVOLUTION 



ledge ; but here it would seem that there is no corresponding 

 evolution. How civilisation affects physical evolution there is 

 no need to explain again. 



Social It remains to point out another marked difference in the work- 

 m S f evolution in a civilised nation and in a species of wild 

 animals. Among the latter all the individuals lead the same kind 

 of life all are subject, speaking generally, to similar conditions. 

 But a nation is divided into classes. The poorest obtain neces- 

 sary food and clothing only by hard bodily labour I am speak- 

 ing now of northern nations. Above the lowest social stratum 

 come, in succession, strata, in each of which there is more wealth 

 and more comfort than in that which underlies it. And since 

 the higher classes are less productive, and do not maintain their 

 numbers, there is a perpetual moving of individuals to the stratum 

 next above that in which they were born, and, only to a much 

 less extent, a corresponding movement downwards. Those who 

 ascend to a higher social level have strength sufficient to cope 

 with the harder conditions that obtain among the class from which 

 they have sprung. Consequently, when they find themselves in 

 an easier environment, they have not only the bare minimum of 

 vigour required, but a superabundance of it. Thus the lower 

 strata of society are a recruiting ground for the upper, which 

 annex the very pick of the hardier stock, and so are continually 

 renewing their strength. Were it not for this, there would be 

 great enfeeblement and loss of energy through the survival amid 

 comfortable surroundings of the weak and unenergetic. 

 Vigour in But, in addition to the strength due to the influx of stronger 

 blood from below, there is invigoration of a sort traceable to 

 to the another cause. When medical and sanitary science make very 

 tTand ra P^ P ro g ress > so tnat men understand the laws of health better 

 science than their fathers and grandfathers and live under healthier con- 

 ditions, then a great apparent gain in strength may result. There 

 is, in fact, a race between the softening of the conditions of life 

 (through the progress of science and the increase of wealth) and 

 the physical deterioration that follows in their train. When 

 science and wealth make great and rapid strides, then there is 

 likely to be a feeling that the race is growing stronger. And 



