3io Heredity and Environment 



5. The Declining Birth Rate. Stationary Population Normal. 

 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 individuals that die. But when a species is 

 placed in new and favorable conditions it may for a while in- 

 crease at an amazing rate until the pressure of population be- 

 comes 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 number of individuals in 

 any given locality remains about the same from year to year, the 

 birth rate merely compensating for the death rate. This equili- 

 brium is brought about in the main by increased mortality, espe- 

 cially among the young, though decreasing fecundity may play 

 a minor part. 



Essentially the same principles apply to human populations. Up 

 to two or three centuries 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 population in each generation being carried off in large num- 

 bers by war, pestilence and famine. Then owing to the develop- 

 ments of science and industry and to the opening up of new 

 countries a period of remarkable expansion of population began. 

 The population of Europe, which was about 175 millions in 1800, 

 increased to 420 millions in 1900, and this in spite of the fact that 

 about 35 millions migrated from Europe to new countries during 

 this period. This great increase in the population of Europe was 

 due primarily to reduction of the death rate since the birth rate 

 also declined slightly during this period, while in the newer coun- 

 tries there was both an increase in the birth rate and a decrease 

 in the death rate. 



It is perhaps an open question how long the advances of science 

 in rendering available the natural resources of the earth may be 



