26 
PATHOLOGICAL PHYSIOLOGY. 
the vaccine virus of chicken cholera and of anthrax, and also of 
other diseases, with a virulency progressively increasing, and to 
bring them finally to their maximum degree of virulency, we 
have inoculated them to young animals and then successfully to 
older ones. 
I may observe, in passing, that these results restore the virus- 
microbes to their place under the general laws of life and its man¬ 
ifestations in the superior vegetable and animal species. These 
manifest their plasticity, if we may so call it, under the influence 
of the conditions of the media where their successive generations 
have taken place. The only difference between the microbes and 
the superior species would consist in the rapidity of the varia¬ 
tions in the virus, which is very slow in the large kinds. Each 
culture of a virus, had it existed but for twenty-four hours, would 
have represented immense numbers of successive generations, 
while in creatures of higher rank thousands and millions of years 
are required for the formation of a similar succession of genera¬ 
tions. 
If, however, changes in the virulency of our attenuated virus, or 
vaccine virus, may result from their passage in subjects of the 
same breed, is it not possible that a virus which has arrived at its 
completed state for one breed to be modified in its virulency by 
passing to another breed ? 
The answer of experience seems to be in the affirmative. 
The Academy will remember the virus microbe which we found 
some time ago in the saliva of hydrophobs. While very virulent 
for rabbits, it, on the contrary, showed itself harmless for adult 
guinea pigs, though it rapidly kills pigs only a few hours or a few 
days old. In carrying the inoculations from mature guinea pigs 
to younger ones, we have seen the virulency increase and easily 
acquire the power of killing old ones, and the lesions themselves 
have changed in a notable manner. We now return to the facts 
that I have just referred to, namely, an increase in the virulency 
by the successive passages through individuals of the same race. 
But the new and unexpected result that I desire to mention 
consists in this; that k the microbe, after having reached its viru¬ 
lency by successive passages through the bodies of guinea pigs, 
