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 power- 
fully 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 ten- 
dencies, however, are greatly modified by differences 
of language, differences of political and social tendency, 
of racial characters and instincts, and also to'a con- 
siderable extent by the expense and difficulties of 
transportation, the rupture of social ties, as well as by 
communal or national prejudices, etc., the last some- 
times 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 population- 
density referred to in Chapter II. It may be noted 
that even within populations which are sensibly homo- 
geneous, and within areas of like physical nature, 
differences of density can arise from mere diversity of 
occupation. For example, as before stated, the con- 
ditions of agricultural life operate ordinarily to limit 
the closeness of aggregation, and at best the aggrega- 
tions can, in general, constitute but small villages. 
Industrial life and commercial life, on the other hand,