2l8 THE SOCIAL LIFE OF ANIMALS 



million people, or less than fifty for every square 

 mile of our country. Others put the figure higher, 

 but I know of no expert who expects our Ameri- 

 can population to double itself again unless there is 

 a radical increase in available energy or in other 

 aspects of our living conditions. 



For the world as a whole. Dr. Pearl estimated in 

 1936 that, if present trends continue, as they may 

 not, the world population will reach a maximum of 

 about 2,650 millions by the year 2100. (95) This is 

 a density of about fifty persons per square mile of 

 land surface on the globe, counting good and bad 

 land alike. 



I must dissociate myself from any responsibility 

 for these and similar estimates. I fully realize, as do 

 their authors, the pitfalls inherent in such predic- 

 tions. Human trends being what they are and have 

 been in the last three hundred years, this is as good 

 an approximation as can be made at present, and 

 with all its faults it is worth considering. 



The important aspect to me is that we do not 

 have reason to expect in the United States or in the 

 world a continuation of the unprecedented rate of 

 increase of the last three centuries, or even a con- 

 tinuation of the present rate of increase. Unless 

 population experts are all at fault, the rate of re- 



