That physiology is a thing which can take up a man’s whole energies will, I think, be admitted by anyone who realizes how wide is its scope. The physiologist himself must specialize, for the subject is too vast for one man to undertake the whole. The body is composed of the same elements as the rest of the world, and their arrangement is very important, so he must be a proficient chemist. It is composed of solids, liquids and gases, and diffusion, filtration, leverage, are frequent processes, and every motion is accompanied by an electric manifestation, so that mechanics and physics must have been part of his training. He can scarcely study organs if he does not know their shape, so he should know some anatomy. And, lastly, as his business is to study life and all its attendant phenomena—and the basis of life is the cell—he must be a histologist. To be all these things is a great deal to require of one man; but though he may specialize for the advancement of a particular branch of his science, he must be _au fait_ with the rest, as no vital function is dependent upon one alone of the factors enumerated. Hard work is required of him, though some people say he has done but little. What he has discovered is briefly, very briefly, set forth in the ensuing essays, with a hint or two at the knots he would like and is trying now to unravel. THE COMMONWEALTH OF CELLS. Some Popular Essays on Human Physiology. ESSAY I. LIVING MATTER. Physiology is the study of life, and the thing of all others which the physiologist would like to discover is what life really is. If this were fully known, all physiological processes could probably be deduced from it, and disease, which is an interference with one or other of them, could be scientifically treated. So far he has not got beyond describing the consequences of life, and his deductions carry him no further than this: life is a property of a substance, protoplasm, and protoplasm can only continue to exist in the form of a cell.