ON SPECIFIC THERAPEUTICS. 59 



Correspondingly we may presume, that the 

 variola virus, when passing through the cow, 

 permanently loses certain atom-groups which 

 cannot be regained ; I believe, however, that in 

 cow-pox this loss is of a lesser degree than in 

 bird-pox. 



On the other hand, in bacteriology we fre- 

 quently find that such alterations produced by 

 certain animal passages are not of a permanent 

 nature. Thus one can, by successively passing 

 streptococci through animals of one species, 

 obtain a maximum of virulence for that species. 

 It is possible, therefore, at will to produce a 

 rabbit-strain, a mouse-strain, etc. But one can, 

 by repeatedly passing such a strain through 

 animals of a different species, transform it into a 

 different strain, and thus in course of time 

 obtain from a rabbit-strain a mouse-strain. 

 This is therefore a case of atrepsy of the 

 receptoral apparatus of the bacteria for the 

 animal species in question, but this atrepsy 

 differs from that of the variations of small-pox 

 by not possessing a permanent character. 



Probably the majority of so-called non-patho- 

 genic micro-organisms, if introduced into an 

 animal's body, perish by this mechanism. It is 

 not neccessary to assume the presence of special 



