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. 



