CH. vi] VARIATIONS IN VIRULENCE 91 



We are forced, therefore, to the conclusion that if in some 

 cases the formation of substances which are toxic in their 

 action is purely a physiological process of excretion following 

 on altered assimilation, in other cases this result is due to a 

 special adaptation, the toxin being a secretion rather than an 

 excretion. 



The explanation of this and other kindred phenomena is, 

 however, unsatisfactory and the suggestion has been made in 

 many quarters that the property of virulence may be due 

 to the action of something more or less distinct from the 

 organism itself but grafted on or attached temporarily to it 

 something in the nature of a ferment or enzyme. This 

 theory we shall discuss later (vide chap, xi) but in connection 

 with it one observation will be made at this point, namely, 

 that if the liberation or acquisition of a ferment by the 

 organism is of advantage to it in its life struggle, it may still 

 be regarded as an adaptation to environment and natural 

 selection will in time cause the characteristic to predominate. 



THE VALUE OF VIRULENCE IN CLASSIFICATION. 



In concluding this section it only remains to say one word 

 as to the value of classification according to virulence. 

 Differences in a character so variable as we have shown 

 virulence to be cannot, alone, be regarded as sufficient to 

 justify a separation of bacteria into distinct "species." The 

 inconsistency to which such a classification gives rise can 

 best be demonstrated by examples. 



For instance, we find in the throats of some patients con- 

 valescent from diphtheria, the so-called "carriers," bacilli 

 indistinguishable from the Klebs-Loeffler bacillus, but in 

 many cases non-virulent. In such cases the organism is 

 almost certainly the lineal descendant of the original 

 virulent infecting organism and cannot be regarded as a 

 distinct species. 



In other cases, in which no history of diphtheria can be 

 elicited, bacilli again are found in the throat morphologi- 

 cally and culturally indistinguishable from the Klebs-Loeffler 



