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, etc. 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. 



