IMMUNITY. 325 



The principle of specificity as exemplified in 

 the setting up of " true " species of bacteria 

 can evidently be carried to such an excess as 

 to lead eventually to scientific absurdities. 

 The error made by R. Pfeiffer is due to the 

 fact that in consequence of the post hoc, ergo 

 propter hoc fallacy, he conceived that immuni- 

 zation was brought about by the antitoxin. 



Ehrlich was the first to attempt to measure 

 the efficacy of the toxins. Such a calculation 

 appears somewhat more exact than it really is 

 because the outcome of an attempt to establish 

 the degree of toxicity of a substance depends 

 upon the animal subjected to experiment, and 

 the animal, for the reason that it is a living 

 organism, often reacts more delicately than a 

 balance to be sure, but never so precisely. 

 The process of determining the strength of a 

 dose is as follows. The minimal fatal dose of 

 the poison is first determined ; for instance 

 diphtheria cultures may be obtained, o.icc. of 

 which is sufficient to kill a guinea-pig of 500 

 grams weight inside of 24-48 hours, and teta- 

 nus cultures, o.ooacc, of which can kill a 

 guinea-pig, the solid poison being estimated to 

 be more potent than strychnine or even prussic 

 acid. Such a degree of immunity as may be 

 gained by an animal through protective inocu- 

 lation enables it to withstand many times the 



