So far as science can reveal, there seems to be no principle limiting life. There are many good and bad reasons why men die, but no underlying necessary reason why they must die. The brilliant Carrel has kept tissue cells of animals alive outside of the body for the past three years. These cells are multiplying and growing, apparently unchanged by time, to all appearances immortal so long as they are periodically washed of poison and nourished in a proper medium. If we could at intervals thoroughly wash man free of his poisons and nourish him, there seems to be no reason why he should not live indefinitely. Section V--Hygiene and Civilization In view of the vast extent of human misery from ill health, the question naturally arises, How does it happen that the world is burdened with so colossal a load? Is it no more than is biologically normal? Is it true that in other organisms, animals and plants, ill health is the rule rather than the exception? Are all races of men subject to the same heavy load? [Sidenote: Natural Adjustments Upset] These questions have not yet received sufficient attention. The answer seems to be that man is suffering from his own mistakes made unconsciously and in ignorance. He has upset the equilibrium which Nature had established among the various powers and activities of his body, and between himself and the outside world. Man has done mischief for his own body similar to what he has done for the natural resources on which he lives. In Professor Shaler's epoch-making little book, "Man and the Earth," he shows, for instance, that the little layer of soil on the surface of the earth from which plants and animals derive their nutriment was, before the advent of man, replenished quite as fast as it was washed away, but that after man had put his plow into it and had taken off the protective mat of vegetation, he unconsciously despoiled the accumulation of ages. "In a plowed field, an hour's torrential rain may wash off to the sea more than would pass off in a thousand years in the slow process of erosion which the natural state of the earth permits." He also shows that the constant croppings of the soil rob it of nitrogen, phosphorus, and other elements faster than Nature restores them. The problem of conservation is to reestablish the balance which has been lost through the depredations of man, for instance, to lessen soil-wash by terracing, and to restore to the soil the lost elements by supplying nitrates and phosphates and by other methods of scientific farming.