were but different stages of one and the same plant. This view has long since been recognized as false. But even yet some botanists claim that all bacteria are but one species, appearing under different forms according to their surroundings, and that these forms are mutually convertible. The question is a difficult one to answer, since bacteria of widely differing powers may resemble each other in form. Hence, if a species cultivated in a flask be contaminated by other germs accidentally introduced, which is very likely to happen, the gravest errors may arise. But the more our methods gain in precision, and the more positive our experience becomes, the more do we drift toward the view that each variety of bacteria represents a species as distinct and characteristic as the separate species among the higher animals. From a medical stand-point this view, indeed, is the only acceptable one.
A disease remains the same in essence, no matter whom it attacks or what its severity be in the individual case. Each contagious disease breeds only its own kind, and no other. When we experiment with an isolated disease-producing, germ it causes always one and the same affection, if it takes hold at all.
But evidence is beginning to accumulate that, though we can not change one species into another, we can modify the power and activity, in short, the virulence, of parasites. Pasteur has shown that when the bacteria of chicken cholera are kept in an open vessel, exposed to the air for many months, their power to struggle with the animal cells is gradually enfeebled. Taken at any stage during their decline of virulence, and placed in a fresh soil in which they can grow, be it in the body of an animal or outside, they multiply as before. But the new breed has only the modified virulence of its parents, and transmits the same to its progeny. Though the form of the parasite has been unaltered, its physiological activity has been modified: it produces no longer the fatal form of chicken-cholera, but only a light attack, from which the animal recovers. By further enfeeblement of the parasite, the disease it gives to its host can be reduced in severity to almost any extent. These mild attacks, however, protect the animal against repetitions. By passing through the modified disease, the chicken obtains immunity from the fatal form. In the words of Pasteur, the parasite can be transformed into a "vaccine virus" by cultivation under conditions which enfeeble its power. The splendid view is thus opened to us of vaccinating, some day, against all diseases in which one attack grants immunity against another. Pasteur has succeeded in the same wav in another disease of much greater importance, namely, splenic fever. The parasite of this affection has also been modified by him, by special modes of cultivation, so as to produce a mild attack, protecting against the graver form of the disease. Pasteur's own accounts of his results, in vaccinating, against anthrax, the stock on French farms, are dazzling. But a repetition of his experiments in other countries, by his