plex brain; as are
also those more numerous, more varied, more general, and more abstract
ideas, which must also become increasingly requisite for successful
life as society advances. And the genesis of this larger quantity of
feeling and thought in a brain thus augmented in size and developed in
structure, is, other things equal, the correlative of a greater wear of
nervous tissue and greater consumption of materials to repair it. So
that both in original cost of construction and in subsequent cost of
working, the nervous system must become a heavier tax on the organism.
Already the brain of the civilised man is larger by nearly thirty
percent, than the brain of the savage. Already, too, it presents an
increased heterogeneity--especially in the distribution of its
convolutions. And further changes like these which have taken place
under the discipline of civilised life, we infer will continue to take
place.... But everywhere and always, evolution is antagonistic to
procreative dissolution. Whether it be in greater growth of the organs
which subserve self-maintenance, whether it be in their added
complexity of structure, or whether it be in their higher activity, the
abstraction of the required materials implies a diminished reserve of
materials for race-maintenance. And we have seen reason to believe that
this antagonism between Individuation and Genesis becomes unusually
marked where the nervous system is concerned, because of the costliness
of nervous structure and function. In Section 346 was pointed out the
apparent connection between high cerebral development and prolonged
delay of sexual maturity; and in Sections 366, 367, the evidence went
to show that where exceptional fertility exists there is sluggishness
of mind, and that where there has been during education excessive
expenditure in mental action, there frequently follows a complete or
partial infertility. Hence the particular kind of further evolution
which Man is hereafter to undergo, is one which, more than any other,
may be expected to cause a decline in his power of reproduction.'

This means that men who have to live an intellectual life, or who can
be induced to lead one, will be likely not to have so many children as
they would otherwise have had. In particular cases this may not be
true; such men may even have many children--they may be men in all ways
of unusual power and vigour. But they will not have their maximum of
posterity--will not have so