e subject abounds in concrete illustrations, and we seem
warranted in believing that conditions of isolation have been
and are of frequent occurrence.

Reibmayr has collected from human history a wealth of
illustrations of various forms of isolation, and there seems
much to be said for his thesis that the establishment of a
successful race or stock requires the alternation of periods of
inbreeding (endogamy) in which characters are fixed, and
periods of outbreeding (exogamy) in which, by the introduction
of fresh blood, new variations are promoted. Perhaps the Jews
may serve to illustrate the influence of isolation in promoting
stability of type and prepotency; perhaps the Americans may
serve to illustrate the variability which a mixture of
different stocks tends to bring about. In historical inquiry
into the difficult problem of the origin of distinct races, it
seems legitimate to think of periods of "mutation"--of
discontinuous sporting--which led to numerous offshoots from
the main stock, of the migration of these variants into new
environments where in relative isolation they became prepotent
and stable.[94]

The biological use of the term "isolation" introduces a new emphasis.
Separation may be spatial, but its effects are increasingly structural
and functional. Indeed, spatial isolation was a factor in the origin of
species because of specialized organic adaptation to varied geographic
conditions. In other words, the structure of the species, its habits of
life, and its original and acquired responses, tend to isolate it from
other species.

Man as an animal species in his historical development has attempted
with fair success to destroy the barriers separating him from other
animals. Through domestication and taming he has changed the original
nature and habits of life of many animals. The dog, the companion of
man, is the summit of human achievement in association with animals.
Nevertheless, the barriers that separate the dog and his master are
insurmountable. Even if "a candidate for humanity," the dog is forever
debarred from any share in human tradition and culture.

2. Isolation and Segregation

In geography, isolation denotes separation in space. In sociology, the
essential characteristic of isolation is found in exclusion from
communication.

Geographical forms of isolation are sociologically significan