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 The Reaction between Man and his Environment.—The estimates by various geologists of the time man has existed as a species vary from 20,000 to 200,000 years. The active life out of doors which man led for ages (Fig. 4) has thoroughly adapted his body only for such a life. Now steam and other forces work for him, and his muscles dwindle; his lungs are seldom fully expanded, and the unused portions become unsound; he lives in tight houses, and the impure air makes his blood impure and his skin delicate; he eats soft concentrated food, and his teeth decay and his too roomy food tube becomes sluggish. His nerves and brain are fully active and they become unsound from overwork and impure blood.

, showing clothing and weapons of chase and war.

Degeneration of Unused Parts.—Several facts just stated illustrate the biological law that disuse causes degeneration.

Man's Modification of his Environment.—The energy of the world, whether of coal, waterfall, oil, forest, or rich soil, has the sun as its source. All of these are being destroyed by man, often with recklessness and wantonness. The promised land which "flowed with milk and honey" is now almost a desert. Other examples are Italy, Carthage, Spain. The destruction of forests causes floods which wash away the soil. It is estimated that there are only one fourth as many song birds in the United States as there were fifteen years ago. Insects and weeds or deserts replace rich soil, noble quadrupeds, singing birds, and stately trees. Many farmers, however, preserve the fertility of the soil.

To the erect posture is due man's free use of his hands and the coöperation of hands and senses. This has given man his intellectual