The early nineteenth century made two highly important new contributions to the old problem: the view that all living things are made up of cells as their ultimate structural elements; and, secondly, acquaintance with various digestive ferments contained in liquids like the gastric juice, which are now known to be cell free, yet are capable of bringing about processes resembling fermentation. The latter discovery led at a later date to the distinction between organized (living) and unorganized ferments.

Out of the cell theory have grown the wonderful modern sciences of embryology, largely through the efforts of K. E. von Baer, and pathology, in which Rudolf Virchow has a similar position. The study of ferments and fermentation, and of simple chemical agents which can produce like changes, has led to many new problems and to new methods of attacking old ones.

The chemical aspects of fermentation〖See Pasteur, “The Physiological Theory of Fermentation,” in H.C., xxxviii, 275ff.〗 have a special historical importance because they are especially associated with Pasteur’s discoveries. Trained as a chemist, he applied the exact methods of physical science to the biological problem, and solved what had been thought by many insoluble. The studies of Pasteur convinced the scientific world that life as we know it never originates spontaneously, that minute living organisms—microbes, germs, bacteria—are far more active agents in this world than had been guessed. Such organisms turned out to be the essential factors in fermentation of all kinds, save only those due to digestive ferments; it is such organisms which form alcohol, sour milk, make vinegar, etc. Thus in the organic cycle the rôle of the organisms formed of a single cell at length appeared to be a great one. Everywhere present, borne by the wind, they are the true scavengers; for nothing, no matter how small, can escape them. But they are more than this. Wherever they find organic matter, dead or alive, that can support life, they seize upon it; they transform many of the most important waste products of the animal into the food of the plant; they grow within larger living things, and by their growth cause disease, or do not, according to their nature. In short, it is their activity, invisible but omnipresent, fitting in at every point where gaps would otherwise occur, which completes the organic cycle.

All Directories