46 
THE PRINCIPLES OF BIOLOGY. 
than that of any mammal except the elephant—is the natural 
result, says Mr. Spencer, of his higher evolution, and changes 
of bulk, structure, or expenditure will be found to be the 
causes of changes in the degrees of his fertility. Where 
there is much expenditure, as in man and most of the higher 
animals, genesis commences only when growth and develop¬ 
ment are nearly completed, continues during the prime of 
life, and ceases when the vigour declines. There is, too, a 
period when fecundity culminates—in women it increases up 
to about the age of twenty-five years, but gradually wanes 
after the age of thirty; increase in weight and size of the 
offspring also accompany increase of fecundity, and vice-versa , 
both in cases of uniparous and pluriparous animals, while a 
too early bearing of young causes arrest of growth and 
enfeeblement of constitution. There is, in fact, evidence that 
what causes increase or decrease of genesis in other creatures 
does so in man, though only few unobjectionable comparisons 
can be made, because the human races differ considerably in 
size, and notably in their degrees of cerebral development; 
the quantities and qualities of their food are unlike, greatly 
on account of differences in climate, and their expenditures 
in bodily, and more particularly mental, action are extremely 
unequal. 
That increased fertility accompanies a nutrition in excess 
of expenditure is shown by contrasting populations of the 
same race, or of allied races, which are differently nourished, 
and Mr. Spencer cites three examples of this :— 
(1.) That of the idle, stupid, and self-indulgent Boers, 
who not uncommonly have from twelve to twenty children. 
(2.) That of the cruelly used Hottentots, who do all the 
work for the Boers, and who seldom have more than two or 
three children, while the well-fed Kaffirs are very prolific. 
(8.) That of the French Canadians, who live in a region 
where subsistence is easily obtained, and who pass a consider¬ 
able portion of the year in idleness, and have large families, 
in contrast with the Anglo-Saxon Canadians, who, leading- 
lives of excessive activity, have a low fertility. Mr. Spencer 
argues that though the case of the Irish peasantry may 
appear adverse to this view, their rapid multiplication may be 
due to earlier marriages, and consequent quicker succession 
of generations, to greater generality of marriage, and particu¬ 
larly to the fact that they obtain a return of food that is large 
in proportion to the amount of labour expended in procuring 
it. A good surplus is, therefore, left for genesis—a greater 
surplus probably than remains to the English peasantry, who, 
