Yet such discoveries are but a beginning in the explanation of disease. It soon appeared that there is something vastly more important about a bacterium than its ability to grow in the body—viz., the kind of poison which it yields; else why the difference between typhoid fever and tuberculosis? Thus arises the search for such poisons or toxins, a fruitful and important department of medical investigation. But what of the fate of the toxin in the body—what of this effect upon the host? The result of researches upon this line has been the discovery of antitoxins and the science of immunity.

In another direction the progress of micro-biology has been quite as important. Evidently it is not with the help of toxins that yeast forms alcohol and carbonic acid from sugar; it is with the help of enzymes or soluble ferments. These are imprisoned within the cell, but otherwise they resemble pepsin and the other soluble ferments of digestion. But if the yeast cell performs its chemical functions with the help of soluble ferments, why not all other cells as well? Such is in truth the case. Hence the study of the chemical processes which make up the activity of unicellular organisms has explained much that takes place in every living thing. In short, our progress in the solution of the fundamental problem of physiology, the physico-chemical organization of protoplasm, depends in no small degree upon studies of those minute living things which have but a single cell within which to enclose all the activities of an individual being.

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