for five or six years is necessary even for
the physiological survival of the offspring. Not only biologically but
sociologically complete isolation is a contradiction in terms.
Sociologists following Aristotle have agreed with him that human nature
develops within and decays outside of social relations. Isolation, then,
in the social as well as the biological sense is _relative_, not
_absolute_.

The term "isolation" was first employed in anthropogeography, the study
of the relation of man to his physical environment. To natural barriers,
as mountains, oceans, and deserts, was attributed an influence upon the
location of races and the movements of peoples and the kind and the
degree of cultural contact. The nature and the extent of separation of
persons and groups was considered by geographers as a reflex of the
physical environment.

In biology, isolation as a factor in the evolution and the life of the
species, is studied from the standpoint of the animal group more than
from that of the environment. Consequently, the separation of species
from each other is regarded as the outcome not only of a sheer physical
impossibility of contact, but even more of other factors as differences
in physical structure, in habits of life, and in the instincts of the
animal groups. J. Arthur Thomson in his work on "Heredity" presents the
following compact and illuminating statement of isolation as a factor in
inheritance.

The only other directive evolution-factor that biologists are
at all agreed about, besides selection, is isolation--a general
term for all the varied ways in which the radius of possible
intercrossing is narrowed. As expounded by Wagner, Weismann,
Romanes, Gulick, and others, isolation takes many
forms--spatial, structural, habitudinal, and psychical--and it
has various results.

It tends to the segregation of species into subspecies, it
makes it easier for new variations to establish themselves, it
promotes prepotency, or what the breeders call "transmitting
power," it fixes characters. One of the most successful breeds
of cattle (Polled Angus) seems to have had its source in one
farmsteading; its early history is one of close inbreeding, its
prepotency is remarkable, its success from our point of view
has been great. It is difficult to get secure data as to the
results of isolation in nature, but Gulick's recent volume on
th