43J HEREDITY AND ENVIRONMENT 



of Families. — Among animals and plants in a 

 state of nature the number of individuals in 

 each species remains fairly constant from year 

 to year; that is, only enough young are born 

 and survive to take the places of mature indi- 

 viduals that die. But when a species is placed 

 in new and favorable conditions it may for a 

 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. 



