Page:The Harvard Classics Vol. 51; Lectures.djvu/128

 118 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.

At length the chemical processes of life upon the earth were unified. Living things were seen to make up a single community, the great laboratory through which alone matter flows in its everlasting cycle.

The results of Pasteur's discoveries and of the methods of investigation which he introduced are probably already greater than the results of Napoleon's life. The simple great man, who almost alone among the scientists of the nineteenth century equals the genius and virtue of Faraday, shares with the latter the first position among those who have revolutionized our twentieth-century world. Pasteur's discoveries explained at once such observations as those of Oliver Wendell Holmes. They gave a clue to such mysterious processes as vaccination. And one after another each great pest has yielded up its secret cause—a specific micro-organism—to the disciples of Pasteur.

Yet such discoveries are but a beginning in the explanation of disease. It soon appeared that there is something vastly more