=What rate of adjustment to new environment= is possible for man? This has not been ascertained; _it is probably much slower than has been generally imagined_. The natives of Tasmania, New Zealand, and many of the Pacific Islands became _extinct in less than a century_ after adopting clothing and copying other habits from Europeans. Life in the country in civilized lands differs less from the environment of primitive man than does life in cities. Cities have been likened to the lion’s cave in the fable, to which many tracks led, but from which none led. _The care of health in cities_ is now making rapid strides along the biological basis of purer air, more open space, less noise, simple food, and pure water. Biology, by supplying as a standard the conditions which molded man’s body for ages, furnishes a simple and sure basis for hygiene. To mention one instance among many, man blundered for centuries in attempting the cure of consumption, and well-nigh gave up in despair. Yet it has recently been shown that if the sufferer returns only in a measure to the open-air habits of his remote ancestors, tuberculosis is one of the most curable of diseases. The biological guide to health is surer and simpler than tinkering with drugs, fussing with dietetics, and avoiding exposure. _Man is of all animals least thoroughly adjusted to his environment_, because of his continual and rapid progress. _Disease_ may be defined as the _process by which the body adapts_, or attempts to adapt, _itself to_ so sudden a _change of environment_ that some organ has failed to work in harmony with the others. By disease the body comes into adjustment with the new condition, or attempts to do so. =Protoplasm.=--The life and growth of man’s body, as the life and growth of all animals and plants, depend upon the activity of the living substance called _protoplasm_, as manifested in minute bodies called _cells_. In fact, protoplasm cannot exist outside of cells. The cells of the human body and their relation to the body as a whole will next be considered. [Illustration: FIG. 5.--AN AMEBA, highly magnified. _nu_, nucleus; _psd_, false foot.]