EXPERIMENTAL EVOLUTION LECT. 



some variation in the pathogenetic organism. For 

 the present purpose this comes to exactly the same 

 thing, our only point being to show that variability 

 does exist in a marked manner. 



Later on I shall have something to say concerning 

 the degree of variability among pathogenetic organ- 

 isms under different modes of culture or treatment ; it 

 is enough here to allude to the general fact of the 

 attenuation of many sorts of virus which has led to 

 the humane although as yet unexplained l practice of 

 vaccination ; but something must now be said concern- 

 ing the external manifestation of this variability. 

 Many bacteriologists have thought at times that it 

 might be possible to transmute one micro-organism 

 into another under definite circumstances, and we have 

 all heard of Biichner's or other experiments concerning 

 the relationship between the common hay bacillus and 

 the typhoid fever bacillus, as well as of similar in- 

 vestigations. But investigators seem to think much 

 too highly of mere morphological transmutations, and 

 to have too much disregard for other transmutations 

 which are in fact of much greater importance. They 

 seem to be running after shadows while substantial 

 reality lies disregarded at their very feet. Let us take 



1 " Unexplained " refers of course to the process by which a bacillus 

 or bacterium, although in appearance unchanged, becomes incapacitated 

 for the production of disease of a virulent type. 



