60 THE SHADOW OF THE WORLD’S FUTURE 
regions, a knowledge of methods of combating yellow- 
fever, sleeping sickness, hookworm, malaria, filariasis, 
etc., have made it possible to live fairly safely in 
almost any region. When, therefore, backward peoples 
advance, or when they are replaced by more capable 
and sturdy races, who know how to live in, and how 
to deal with the territories they occupy, and who, 
moreover, are in earnest about the general purposes of 
life, then the countries in which they dwell will be 
greatly improved, and will as a consequence carry 
many more people to the square mile. The data do 
not exist for evaluations of these possibilities in detail 
for given countries and with any given standards-of- 
living: for this reason any practical estimates have also 
to be based upon general considerations. 
Initially all countries depend upon primary pro- 
ductions, and rely upon the exchange of their com- 
modities, etc., for such secondary productions as they 
need. Primary production, however, in general, does 
not lead to dense populations, and it leaves a people 
largely at the mercy of others in respect of political 
control, and of the conditions of trade and commerce. 
It is not too much to say that, when one has regard to 
the risks of armed conflict, it is also evident that it 
leaves a people subject to the risk of national ruin. 
The significance of this matter was ably dealt with, as 
far back as 1841, by Friedrich List in his Das nationale 
System der politischen Ockonomie. It is because of the 
limitations and dangers of a dependence solely upon 
primary productions, that as nations advance they 
find themselves compelled more and more to become 
self-supporting, and therefore to promote secondary 
industries. This in its turn tends greatly to increase 
the population they can support, provided outlets for 
their manufactured goods, in return for the raw 
supplies needed, are found. In recent times the 
economic history of the United States of America and