210 THE SOCIAL LIFE OF ANIMALS 



ing to a 1935 revision of the estimated world popula- 

 tion which was made by Professor Pearl, something 

 over two thousand millions of people. This is almost 

 exactly forty people per square mile of the whole 

 earth's surface, or about fifty people per square mile, 

 if the arid and semi-arid land is excluded. We can 

 better visualize the meaning of these figures when 

 we know that they are almost exactly the average 

 population density for the United States; forty per 

 square mile for the whole land area, and fifty per 

 square mile if on land with fair rainfall. 



A recent estimate of human population of three 

 hundred years ago, tentatively advanced by Profes- 

 sor Pearl, is that in 1630 there were probably about 

 445 million people on the whole earth, or about 

 eight per square mile of total land surface. (95) Dr. 

 Pearl thinks that this was probably the largest hu- 

 man population which the earth had supported up 

 to that time. Then came the opening of the Americas 

 for settlement, and the beginnings of modern use 

 of transport and manufacturing processes, and the 

 scattering of information by modern methods. The 

 result has been that in the last three centuries the 

 population of the world has increased almost five- 

 fold, from eight to forty per square mile, largely 

 because food and shelter and mechanical energy were 

 made available for five times as many people, and 



