50 2 THE POPULAR SCIENCE MONTHLY 



lation is a mere bugbear and agricultural science is turning the law of 

 diminishing returns into a law of increasing returns. As regards the 

 exhaustion of our forests and mines and the impoverishment of our 

 soil, the conservation movement is already here to protect them. Our 

 forests may be renewed as they are in other countries and substitutes 

 may be found for our coal which will be as superior to it as the electric 

 light is superior to the old candle or lamp. Few will be sorry to see 

 the passing of the coal with its dirt and its smoke. As regards the 

 exhaustion of the combined nitrogen of our soils, science even now is 

 learning how to imprison the free nitrogen of the air. 



In an article by W J McGee in Science, October 6, 1911, on the 

 " Prospective Population of the United States," we have a painstaking 

 study of this subject, based on all kinds of data, including not only the 

 observed decrease in human productivity, but also the relation of our 

 natural resources to the increase of population. He finds the only real 

 limitation of our natural resources to be in the water supply, and talcing 

 this fully into account, he estimates the population of the United States 

 to be doubled in 1950, trebled in 1980, quadrupled in 2010, and so on 

 to the year 2210, when we shall be supporting over eleven times our 

 present population, or 1,017,000,000 people. His view is wholly opti- 

 mistic, showing how movements already initiated are likely to overcome 

 great apparent evils. 



As regards the action of tuberculosis and other diseases of this class 

 in purifying society by removing the unfit, it may be answered without 

 hesitation that sanitary science can provide methods of purification far 

 superior to these filthy diseases. An unsanitary medieval city might 

 perhaps need dogs as scavengers. A well-kept modern city needs none 

 such. So in regard to any possible racial deterioration as the result of 

 the participation of mothers in industrial and political occupations, it 

 is the business of society to consider just as much the conservation of 

 human health and human vitality as the conservation of our forests and 

 our soils. It is by no means impossible that society in the future will 

 find means of preventing the production of the unfit and providing 

 for the production of the best. The present movement in advancing 

 the position of woman may go farther than equality of rights. It may 

 give to the future mothers of the nation superior rights and superior 

 privileges. The notion, however, that work and motherhood are incom- 

 patible has no foundation in experience. . Small families and weak 

 children are more often found among the idle and luxurious than among 

 the workers. 



The fact is that pessimism finds its explanation not in objective, but 

 in subjective conditions. The psychological grounds of pessimism are 

 not obscure. It springs usually from one of three sources. The first 

 of these is lowered vitality. Optimism is the natural and necessary 

 accompaniment of health. It flows from it as naturally as light from 

 the sun. It is just the mental reflex of that normal physical activity 



