86 VARIATIONS IN, VIRULENCE [ch. vi 



showed increased powers of resistance to chlor-acetic acid 

 failed to show any similar increase in their power of resisting 

 other antiseptics such as carbolic acid or formaldehyde. In 

 other words it was a case of adaptation and not merely 

 increased robustness. 



{d) The acquirement of virulence during the process of 

 passage is sometimes accompanied by other changes in the 

 character of the organisms which are only to be accounted for 

 by the theory of adaptation to a particular environment. For 

 example, the parasitic pneumococcus isolated from the human 

 lung in an acute lobar pneumonia (Eyi-e and Washbourn) was 

 characterised by inability to grow except at a temperature 

 near that of the human body. Again the avian tubercle 

 bacillus — the particular race of tubercle virulent to birds — 

 grows best at their body temperature which is higher than 

 that of man, namely 43"5° C, a temperature at which human 

 tubercle dies. 



Certain difficulties are nevertheless presented by this 

 theory of the development of virulence by natural selection. 



(a) The first to suggest itself is the fact that toxins are 

 "intracellular" as well as "extracellular" and, inasmuch as the 

 intracellular toxins are not liberated until the death of the 

 organism and its disintegration, their nature and potency can 

 obviously have no influence on that organism's survival or 

 perpetuation. We are not concerned, however, with one 

 isolated bacterium's struggle for existence so much as with 

 the fate of a host of bacteria invading the tissues, and it is no 

 less obvious in their case that if their intracellular toxins are 

 destructive of the vitality of the tissues the living bacteria 

 will receive assistance from their dead and disintegrated 

 comrades which they would not otherwise do, and this fact may 

 determine the success of their invasion and consequently their 

 perpetuation. 



It is possible that in the case of some bacteria the toxic 

 action of the substances set free on their disintegration is 

 a purely physiological one — comparable to the effects produced 

 by the absorption of extravasated blood — and not due to 

 a special adaptation. The protective action, however, of 



