=Division of Labor.=--There is a great advantage in each cell of the human body _having its special work_, instead of having to do _everything for itself_, as each ameba cell must do. _Under this system each cell can do its own work better than a cell of any other kind can do it._ Among wild tribes there is very little division of labor. Each man makes his own weapons, each knows how to weave coarse cloth, how to cook, how to farm, etc. Savages do not have as good weapons as do people who leave the making of weapons to certain men whose special business it is. What kind of pocketknives or pencils do you think the boys of this country would have if each boy had to make his own pocketknife or pencil? What kind of scissors and thread would the girls have if each girl had to make them herself? Our muscle cells can contract better than the ameba; the cells in the lungs can absorb oxygen better than the ameba. We have just as great an advantage in digestion, feeling, and other processes; for the ameba eats without a mouth, digests without a stomach, feels without nerves, breathes without lungs, and moves without muscles. Division of labor between the sexes also occurs among the higher animals. Those who desire that man and woman should have the same education and work would violate the biological law of “progress by specialization,” which could only cause race degeneration. A part of the body which is somewhat distinct from surrounding parts, and has special work to do, is called an =organ=; the special work which the organ does is called its =function=. The eye is the organ of sight. The skin is an organ; its function is to protect the body. This book will treat of (1) the structure, appearance, and position of each organ, or =anatomy=; (2) the function of each organ, or =physiology=; (3) the conditions of health for each organ, or =hygiene=; (4) the conditions under which each organ worked in the primitive life of the race; (5) the effects of change of environment; (6) the anatomy of man compared with the lower animals. (5) belongs to the science of =Ecology=. These sciences are parts of the science of =Biology=. [Illustration: FIG. 10.--THREE MUSCLE FIBERS from the heart (showing the nuclei of six cells).] [Illustration: FIG. 11.--NERVE CELLS, showing their branches interlacing.]