THE MIGRATION OF POPULATIONS 77
The principles of the migration question may be
thus envisaged :—In a world in which, say, the
physical conditions of its surface were homogeneous,
inhabited by a race not differentiated in any way with
respect to language or economic, political and social
character, migrations would of course occur readily.
Great diversities of density of population over very
large areas would tend automatically to disappear.
Man’s social instincts lead, however, to concentrations
of population, and actual physical conditions are by
no means homogeneous. These two factors powerfully
 influence his movement and affect his choice of
location. A good climate and great fertility, or other
resources, offer inducements to aggregate densely, while
rigour of climate, and the absence of resources easy
of exploitation, tend to limit aggregation. Such tendencies,
 however, are greatly modified by differences
of language, differences of political and social tendency,
of racial characters and instincts, and also to'a considerable
 extent by the expense and difficulties of
transportation, the rupture of social ties, as well as by
communal or national prejudices, etc., the last sometimes
 fully justified, but also often with little real
foundation.
It is easily seen that, in the present constitution of
the world, collisions of interest are inevitable. And
owing partly to the factors mentioned, the world
to-day exhibits those great differences in populationdensity
 referred to in Chapter II. It may be noted
that even within populations which are sensibly homogeneous,
 and within areas of like physical nature,
differences of density can arise from mere diversity of
occupation. For example, as before stated, the conditions
 of agricultural life operate ordinarily to limit
the closeness of aggregation, and at best the aggregations
 can, in general, constitute but small villages.
Industrial life and commercial life, on the other hand,