within the limits of that city the population is segregated again and
again by racial and vocational interests. Neighborhood sentiment, deeply
rooted in local tradition and in local custom, exercises a decisive
selective influence upon city population and shows itself ultimately in
a marked way in the characteristics of the inhabitants.

2. Isolation as a Result of Segregation[111]

There is the observed tendency of mental defectives to congregate in
localized centers, with resulting inbreeding. Feeble-mindedness is a
social level and the members of this level, like those in other levels,
are affected by social and biological tendencies, such as the
congregation of like personalities and the natural selection in matings
of persons of similar mental capacities. These are general tendencies
and not subject to invariable laws. The feeble-minded are primarily
quantitatively different from normals in mental and social qualities,
and do not constitute a separate species. The borderline types of
high-grade feeble-minded and low-grade normals may therefore prove
exceptions to the general rule. But such studies as Davenport and
Danielson's "Hill Folk," Davenport and Estabrook's "Nams," Dugdale's
"Jukes," Kostir's "Sam Sixty," Goddard's "Kallikaks," Key's "Vennams"
and "Fale-Anwals," Kite's "Pineys," and many others emphatically prove
that mental defectives show a tendency to drift together, intermarry,
and isolate themselves from the rest of the community, just as the rich
live in exclusive suburbs. Consequently they preponderate in certain
localities, counties, and cities. In a large measure this segregation is
not so much an expression of voluntary desire as it is a situation
forced upon mental defectives through those natural intellectual and
social deficiencies which restrict them to environments economically and
otherwise less desirable to normal people. This phenomenon is most
conspicuous in rural communities where such migratory movements as the
modern city-drift have exercised a certain natural selection, but it is
also plainly evident in the slums and poorer sections of the cities,
both large and small, as any field worker will testify. Closely related
to this factor of isolation are the varying percentages of mental
defectives found in different states and in different sections of the
same state, city or community. It is therefore likely that the
percentages of mental defectives among different groups of juvenile
d