1901] MICROSCOPICAL JOURNAL. eae 
facts indicate an obligatory parasitic existence. When or 
under what conditions in the prehistoric ages of the past 
it first became parasitic can never be known, nor can we 
determine at this late day how long a time was required 
to transform it from an organism which was only occa- 
sionally or accidentally parasitic into one which could live 
no other but a parasitic life. What appears certain is that 
for more than two thousand years rabies has been the 
same disease it is to-day ; that it has been propagated by 
the same species of animals, manifested itself by the same 
symptoms, and produced the same fatal results. 
It is not likely that other microscopic organisms will 
from time to time take up their habitat in the animal body 
and become obligatory parasites. There are a number 
of different bacilli now known which are capable of liv- 
ing in the flesh and causing fatal disease, but which only 
do this under accidental conditions. Among these are 
the anthrax bacillus, the bacillus of blackleg, the bacillus 
of malignant edema, and the bacillus of tetanus, all of 
which are deadly in their effects on animals inoculated 
with them, but all of which lack some quality required 
for their rapid dissemination or for the ready infection of 
susceptible animals. Consequently, they do not usually 
spread from animal to animal. With slight modification 
the anthrax bacillus might become the most terrible of the © 
known disease germs. But that such modifications re- 
quire time and conditions not often found, is proved by 
the fact that though this disease has been known since 
the beginning of medical knowledge, the bacillus has in 
the memory of man made no progress as a disease-pro- 
ducing organism, but on the contrary appears less capa- 
ble to-day of gaining entrance to the tissues than it was 
two or three centuries ago.— Ag. Depart. Year Book, 1900. 
