84 VARIATIONS IN VIRULENCE [ch. vi 



advantage is taken of the same phenomenon to attenuate the 

 virus of smallpox. 



The Significance of Variation in Virulence. 



There is a mass of evidence, therefore, to show that the 

 virulence of bacteria is very variable. What is the explanation 

 and significance of these observations? 



Andrewes, in the Horace Dobell Lecture already quoted 

 {vide p. 3), described the evolution of bacteria from harmless 

 mineral feeders into animal saprophytes and finally into 

 parasitic organisms and showed that virulence was probably 

 the latest property to be acquired in the process. It is an axiom 

 in the study of evolution that the latest characteristic or 

 function to be acquired is the most unstable and the first to 

 be lost if retrogression occurs. Each new function acquired 

 indicates a higher degree of specialisation. The more highly 

 specialised the activity of an organism becomes the more 

 intricate are the processes upon which it depends and the 

 more readily is it — like a complicated mechanism — "put out 

 of gear." It is recognised, for example, by alienists that in cases 

 of mental degeneration the highest faculties, which, from their 

 absence or rudimentary character in lower animals and on 

 other grounds, are regarded as having been the last to be 

 evolved, are the first, and often the only ones, to show 

 signs of impairment. The loss of virulence by bacteria, often 

 unaccompanied by any other alteration in the character of 

 the organism is another illustration of the same phenomenon. 



The process by which this property of virulence is regained 

 by a strain of bacteria is exactly similar to the process by 

 which it was originally acquired by the race, but accelerated, 

 that is to say it is a case of the survival of the fittest. 



The fate however of a band of soldiers raiding an enemy's 

 country depends not only on their numbers and strength but 

 also on their resourcefulness, so that the "fittest" in this 

 connection must be interpreted to mean not necessarily the 

 strongest or most robust but those best able to protect them- 

 selves ; and since the most successful method of resistance 



