=Man’s Original Environment.=--Primitive man lived without the use of fire or weapons other than sticks or stones. His _first home was in the tropics_, where his needs were readily supplied, and probably in Asia. Many nations have a tradition of a home in a garden (Greek, _paradisos_). His food was chiefly _tree fruits and nuts_. When because of crowding he left nature’s garden, he acquired skill in _hunting and fishing_ and the use of fire that flesh might supplement the meager fruits of colder climates. His weapons were of rough (chipped) stone at first--_in the old stone age_. In this age the mammoth lived. He learned to polish implements in the _new stone age_. The Indians were in that stage when Columbus came to America (Figs. 2, 3). The cultivation of grain and the domestication of animals probably began in this age. The _bronze_ and _iron ages_ followed the stone age. [Illustration: FIG. 3.--INDIAN TOMAHAWK. Polished Stone. Keyport, N.J.] =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.[3] [3] It has been prophesied that the future man will be a brownie-like creature with near-sighted eyes, shrunken body, slim little legs and arms, large hairless head, toothless gums, a stomach using only predigested food, muscles suited only to push an electric button or pull a lever, and mind very active. But this disregards the indispensable need of a sound mind for a sound body. There cannot even be a play of emotion without a change in the circulation. [Illustration: FIG. 4.--PRIMITIVE MAN, showing clothing and weapons of chase and war.] =Degeneration of Unused Parts.=--Several facts just stated illustrate the biological law that _disuse causes degeneration_.