Less than two centuries ago most men were working on the land, the sea, and the forests and mines. In the cities they worked in hand-operated workshops and the cities themselves were not so large; the countryside was close at hand. They worked hard and long, using the muscles of their bodies, and so did their wives. This involuntary exercise of the muscular system, this exposure to sunshine and fresh air, this limitation to fresh and unpreserved foods, kept most of them healthy and strong even if the lack of better housing and sanitation kept short the lives of some of them. Then came the industrial revolution, when the machine and the civilization it created changed their habits of living. Now they crowd into cities, enter sedentary occupations, sit in chairs for long hours, or stand at mechanical assembly lines. Their bodies become soft, flabby, and undeveloped. Their organs of digestion function imperfectly. Yet such is their hypnotized condition that they do not often realize the harm which modern ways have done them; indeed, they usually pity their ancestors! But those who do realize it and feel uneasy in their conscience about it, need to make a positive effort to eliminate the deterioration and the atrophy which are the price paid for straying away from Nature.
-- Notebooks Category 5: The Body > Chapter 5: Exercise > # 3