t whose social and psychic life no one knows
anything, and any theorizer can say what he chooses without fear of
shipwreck on incontrovertible facts. Whether the lowest races known
to-day are differentiated from the highest only by acquired social and
psychic characteristics, or also by differences of psychic nature, may
perhaps be an open question. However this may be, the case is fairly
clear in regard to the higher races inhabiting the earth. Their
differentiating psychic characteristics are, for the most part, not
due to diverse psychic nature, but to diverse social orders, while the
transmission of these characteristics takes place, as a matter of
observation, through social heredity.

The discussions of this work are exclusively concerned with the
evolution of society and of psychic characteristics. But even in this
limited field we have not attempted to cover the whole ground. We have
given our chief attention to the interdependence of social phenomena
and psychic characteristics. The causes of evolution in the social
order have not been the main subject under discussion.

Segregation is the essential condition on which divergent evolution is
dependent. Many forms of segregation may be specified, under each of
which evolution proceeds on a different principle. In brief, it may be
said that biological segregation prevents the swamping of incipient
organic divergences, by preventing the intermarriage of those
possessing such divergences, while social segregation prevents the
swamping of incipient social divergences and their corresponding
incipient psychic characteristics by preventing the inter-association
of those having such tendencies.

Biologically segregated groups undergo divergent biological evolution
through segregated marriage, producing distinct physiological unities
or racial types. These racial types are now relatively fixed and can
be appreciably modified only by the intermarriage of different races.

Socially segregated groups undergo divergent social evolution through
the segregated social intercourse of the members of each group,
producing distinct civilizational and psychic unities. The differences
between these social or psychic groups are relatively plastic and are
the subject of constant variation. The modification of the social and
psychic characteristics of a group takes place through a change in the
physical or social environment of the group, or through the rise of
strong personalities