THE MIGRATION OF POPULATIONS 75 
of any countries are, however, a source of danger to 
others in a world where wealth and power are regarded 
as supreme desiderata; for they confer advantages in 
the arbitrament of war, should it be resorted to. 
This, one is compelled to recognise, is a limiting 
factor, so long as the principle of nationality governs 
the human race and divides the interests of the world’s 
populations. Thus it is important for certain countries 
to add to their possible “ natural ” increase a further 
increase by immigration. A notable example is 
Australia, at the present time, with its average density 
of about two per square mile. 
The necessity for emigration and birth-control 
arises in the following way. Whenever a country 
develops its agriculture and its industries to the 
uttermost, and still finds its population increasing 
beyond the carrying-capacity of its territory, it is 
immediately faced with two alternatives. Either its 
excess of population must emigrate, or the excess 
must be made to vanish by birth-control. The 
latter is but a partial remedy. It runs counter to 
natural tendencies. There is no doubt, however, that 
the more rapidly a people multiply the sooner must 
come the appropriate measures of birth-control, which 
with civilised peoples are, in some form or another, 
always operative. 
As soon as, in any country, the condition of rela- 
tively dense population, or over-population, arrives, 
the impulse to emigrate therefrom is stimulated, and 
countries whose population-carrying capacity is un- 
exhausted tend to be invaded, the tendency—other 
things being equal—being measured by the differ- 
ence between their potential and actual populations. 
i If we denote the greatest population a country can carry by 2, and 
ts actual population by p, then the measure of the immigration- 
potential is a function of the quantity (P—p)/P. The function, how- 
ever, is not a simple one.