448 HEREDITY AND ENVIRONMENT 



while increase at an amazing rate until the 

 pressure of population becomes sufficient to 

 reestablish an equilibrium between the birth 

 rate and the death rate. Thus when the 

 English sparrow was introduced into the 

 United States it increased at a phenomenal 

 rate for a number of years, but now the num- 

 ber of individuals in any given locality remains 

 about the same from year to year, the birth rate 

 merely compensating for the death rate. This 

 equilibrium is brought about in the main by 

 increased mortality, especially among the 

 young, though decreasing fecundity may play 

 a minor part. 



Essentially the same principles apply to 

 human populations. Up to two or three cen- 

 turies ago the populations of the older coun- 

 tries of the world were practically stationary. 

 Fecundity was relatively high but the death 

 rate was also very high, the excess of popula- 

 tion in each generation being carried off in 

 large numbers by war, pestilence and famine. 

 Then owing to the developments of science and 

 industry and to the opening up of new coun- 

 tries a period of remarkable expansion of pop- 



