-whether
it takes the form of greater strength or higher speed, facilitates some
habitual movement or utilises better the absorbed aliment--is a greater
surplus of vital capital; part of which goes to the aggrandisement of
the individual and part to the formation of new individuals. Hence every
type that is best adapted to its conditions has a rate of multiplication
that insures a tendency to predominate. Survival of the fittest, acting
alone, is ever replacing inferior species by superior species. But
beyond the longer survival, and therefore greater chance of leaving
offspring, which superiority gives, we see here another way in which the
spread of the superior is insured. Though the more evolved organism is
the less fertile absolutely, it is the more fertile relatively.

_Multiplication of the Human Race_

What causes increase or decrease of genesis in other creatures causes
increase or decrease of genesis in man. It is true that, even more than
hitherto, our reasonings are here beset with difficulties. So numerous
are the inequalities in the conditions that but few unobjectionable
comparisons can be made. The human races differ not only in their sizes
and foods, and in the climates they inhabit, but also their expenditures
in bodily and mental action are extremely unequal.

The increase of fertility caused by nutrition that is greatly in excess
of expenditure is to be detected by comparing populations of the same
race or of allied races one of which obtains good and abundant
sustenance much more easily than the other. On carrying out such
comparisons it is seen 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.

It is also apparent that relative increase of expenditure, leaving a
diminished surplus, reduces fertility. That infertility is generally
produced in women by mental labour carried to excess is shown in the
fact that most of the flat-chested girls who survive their high-pressure
education are incompetent to bear a well-developed infant and to supply
it with the natural food for the natural period. It is a matter of
common remark how frequently men of unusual mental activity leave no
offspring.

It is likely to be urged that since the civilised races are on the
average larger than many of the uncivilised races, and since they are
also so