584 THE POPULAR SCIENCE MONTHLY. 



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 ac- 

 cording to their surroundings, and that these forms are mutually con- 

 vertible. 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 acci- 

 dentally 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 char- 

 acteristic 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 dis- 

 ease 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 ac- 

 tivity, in short, the virulence, of parasites. Pasteur has shown that 

 when the bacteria of chicken cholera are kept in an open vessel, ex- 

 posed 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 en- 

 feeblement of the parasite, the disease it gives to its host can be re- 

 duced in severity to almost any extent. These mild attacks, however, 

 protect the animal against repetitions. By passing through the modi- 

 fied 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 daz- 

 zling. But a repetition of his experiments in other countries, by his 



