178 THE EVOLUTION OF MAN 



one another, commit race suicide, or die at the top, they are 

 destined to furnish the principal contribution to the future 

 population of the globe. 



The Limits of Population. Finally, the human population 

 of the world is rapidly approaching its maximum for while 

 populations tend to increase continually the limits of the habi- 

 table globe remain fixed and the means of subsistence are sub- 

 ject to the law of diminishing returns. Pearl estimates that 

 within 200 years the population of the United States will have 

 reached its maximum of about 200,000,000. Alfred Marshall 

 calculates that the population of the entire world will have 

 reached its maximum of about 6,000,000,000 in 200 years. 

 These estimates assume that increase of population and of 

 means of subsistence will in the future conform to the same 

 mathematical formulae as hitherto, an assumption that is 

 probably justified. If these estimates are correct the grand- 

 children or great-grandchildren of persons now living may see 

 the maximum population of this country, and perhaps of the 

 world, attained. Even if by means of greatly improved agri- 

 culture or by revolutionary scientific discoveries this time 

 should be doubled it would still be a relatively short period 

 before the limits of the possible population of the globe would 

 be reached. Thereafter the population will remain stationary, 

 either through increase of the death-rate or decrease of the 

 birth-rate. Under these conditions it is probable that both 

 natural and artificial selection will be intensified. It is reason- 

 able to expect that on the whole natural selection will make 

 for progress and it does not seem probable that artificial means 

 of limiting population will continue to cut off the better stocks 

 and to favor the worse. 



There is ground to hope, therefore, that in a crowded globe 

 both natural and artificial selection will make for the improve- 

 ment of the race, but such improvement is likely to be slow and 

 painful. Natural selection, under various aspects, is still the 



