350 THE TREND OF THE RACE 
married, and it tends as a rule to be large where the wages of the 
husband are low. In many industrial towns and cities it is 
common for both husband and wife to be employed in the same 
industry. When the wife is employed outside the home, infant 
mortality is generally found to be higher than when she looks 
after her own household. The employment of married women 
thus has its effect upon the death rate and brings into play a form 
of selection whose racial effects may be good or ill as a number of 
attendant circumstances determine. 
Besides the influence of industrial development upon the birth 
rate and death rate of different hereditary classes, there is the 
possibility of important effects upon the production of variations 
in the germ plasm. If germinal variations arise in response to 
changes in the environment it is highly probable that the pro- 
found influence which industrial development has exerted upon 
the conditions under which people live and work may have 
produced some modifications in the inherited qualities of the race. 
Economic conditions not only have their effect upon the preva- 
lence of alcoholism, but they lead to an abnormal congestion of 
population under conditions unfavorable for healthy living and 
thereby increase the prevalence of many diseases which may 
possibly produce permanent changes in the germ plasm. Statis- 
tics on the causes of death in cities bring out clearly how different 
are the biological conditions to which the urban dweller is exposed 
as compared with those which surround his rural compatriot. As 
we have pointed out in a previous chapter, we are ignorant of 
how environmental changes affect the germ plasm of human 
beings. We can only say that since our industrial development 
has so greatly modified the environment of large masses of man- 
kind it is not improbable that more or less change has thereby 
been produced in the germ plasm of the race. 
The course of evolution in man has been influenced to no small 
degree by the migration of peoples, whether this has occurred as 
the result of conquest, or by the more orderly method of peaceful 
invasion. People ever tend to overflow their boundaries as 
a result of the pressure coming from their increase in numbers. 
