262 BACTERIOLOGY. 
of course, a possibility, but not a certainty. The one 
thing we can probably safely assert is that there is no 
probability that any saprophytic variety now existing 
can, under any possibility, develop into the now recog- 
nized varieties of pathogenic bacteria. It is almost 
impossible to conceive that any such variety should 
start with the same characteristics and then develop 
parasitic tendencies under exactly the same circum- 
stances as those varieties which now produce disease. 
Attenuation. It is now a well-established fact that 
the great majority of parasitic bacteria can be so altered 
by change of conditions, and especially by being sub- 
jected to unfavorable conditions, that they, while mor- 
phologically the same, lose their power of developing 
in the body and of producing specific poisons. When 
either or both these properties are partially destroyed 
they can usually be redeveloped; but when power to 
produce specific toxins is absolutely lost, it is, so far 
as we now know, lost forever. 
The recovery of toxin production is brought about 
by developing the micro-organism for a considerable 
length of time under the conditions best suited for it. 
The recovery of the ability to grow in the body of any 
animal species is brought about by causing the germ 
to develop in a series of such animals whose resistance 
has been overcome by reducing their vitality through 
poisons, heat, cold, ete. Another method is to ac- 
custom the micro-organism to the animal’s body by 
letting it remain surrounded by the animal fluids as it 
rests in a pervious capsule in the peritoneal cavity. 
