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