THE PRINCIPLES OF BIOLOGY. 
47 
though better fed, are harder worked. Mr. Spencer draws 
the conclusion that in the human race, as in all other races, 
such absolute or relative abundance of nutriment as leaves a 
large excess after defraying the cost of carrying on parental 
life, is accompanied by a high rate of genesis. 
The converse truth that relative increase of expenditure, 
leaving a diminished surplus, reduces the degree of fertility 
is next dwelt upon. Much bodily labour is said to render 
women less prolific, and there is some evidence of this. 
According to De Boismont, of France, and Dr. Szukits, of 
Austria, the reproductive age is reached a year later by women 
of the labouring classes than bv those of the middle class. 
The low rate of increase in France is probably due partly to 
the hard work thrown on the women by the abstraction of 
men for non-productive occupations, military and civil, while 
the higher rate of increase in England is probably furthered 
by the easier lives which English women lead. 
Mental labour is more easily shown to be the cause of 
absolute or relative infertility ; for instance, upper-class girls 
have a less productive power than poor girls, though their 
food is better; the greater tax on their brains reacts on their 
physique. This diminution of reproductive power is shown 
by absolute sterility, earlier cessation of child-bearing, and 
frequent inability to suckle their infants. The antagonism 
between Genesis and Individuation is not often shown in men 
by suppression of generative power consequent on unusual 
expenditure in bodily action, owing to the cost of reproduction 
being much less in men than in women, but the ancient 
Athleta are said to have rarely had children, while “ trainers ” 
insist on continence. Cerebral expenditure is believed to 
diminish generative power, and intense application to mathe¬ 
matics and the excitement of gambling are said to have had 
this result, while men of unusual mental activity often leave 
no offspring. 
Two objections have to be guarded against here. The 
first objection is that since civilised races are on the average 
larger, more complex, and more active than uncivilised, they 
ought, according to the alleged general law, to be less 
prolific, whereas there is no evidence to prove that they are 
so ; on the whole, they seem rather the reverse. The answer 
is that, were all other things equal, these superior varieties of 
men would have inferior rates of increase, but other things 
are not equal. Domesticated animals are more fertile than 
wild ones ; the causes are the same that render civilised more 
fertile than savage men. Then there are differences in the 
amount of food. Many races with low rates of mortality are 
