326 THE TREND OF THE RACE 
industries which greatly affect the differential death rate of 
relatively large numbers of the population; (3) the growth of 
cities with the resulting subjection of their inhabitants to a 
changed and often deleterious environment and mode of life; 
(4) the effect of economic factors on the marriage and birth rates 
of different stocks; and (5) the possible influence of altered 
environmental factors on the trend of germinal variability. 
We shall consider briefly these different topics, although 
it should be borne in mind that they are closely interrelated. 
The striking increase of the populations of civilized countries 
during the 19th century is in large part due to the application of 
science to industry which has increased enormously the wealth 
with which nature has been compelled to reward the labors of 
man. To a large extent also this increase of population has 
resulted from the reduction of the death rate which has followed 
the advances made in medicine, surgery, and especially those 
branches of hygiene which are concerned with the control of 
infections and epidemics. But whatever progress is made in the 
art of saving life, the population of a country must obviously be 
limited by the resources furnished by nature for human subsist- 
ence. The yield of nature has been greatly increased by the 
application of scientific discovery. Improvements in mining, 
manufacturing, agriculture and transportation make it possible 
for the earth to support a greatly increased number of inhabi- 
tants, and human population even now comes sufficiently’near 
obeying the law of Malthus to respond to the opportunities thus 
created for its maintenance. 
Through the increase of numbers which industrial development 
has made possible those races and peoples among whom such 
development has reached a higher stage are enabled, by war or 
otherwise, to prevail over races and peoples on a lower industrial 
level. The Anglo-Saxon has doubtless been aided in extending 
his domain on account of the very rapid growth of the population 
of Great Britain which followed upon the unprecedented develop- 
ment of her industries. The great economic development of 
Germany, by creating opportunities for her people at home and 
