THE BACTERIOLOGICAL EVIDENCE 87 



If recovery occurs, the cells of the body gradually become accus- 

 tomed to the toxins, which are digested by them. 1 They obtain 

 the upper hand ; the individual acquires immunity ; the microbes 

 are destroyed ; and, as a consequence, the sufferer is no longer 

 infective to his fellows. 



139. A human race undergoes evolution against such a disease 

 as measles or diphtheria when it becomes more resistant. It 

 becomes more resistant when successive generations become, 

 through Natural Selection, more capable of reacting to the toxins 

 and so 'acquiring' immunity. The germ -plasm becomes such 

 that under the stimulus of nutriment individuals develop, who, if 

 they experience the disease, are able to develop further and 

 ' acquire ' actual immunity. Therefore, while the power of acquir- 

 ing immunity is an 'inborn' character, the immunity itself is a 

 ' use-acquirement.' That is to say, the individual who acquires 

 immunity becomes used\& the disease (develops under its stimulus), 

 just as the muscles of the athlete become used to endure fatigue, 

 or the skin of the palm to endure rough labour. The process of 

 evolution is a long and a slow one, extending over many genera- 

 tions. The final effect, as we see, is not the creation of a race 

 the individuals of which resist infection (i.e. are innately immune), 

 but the creation of a race the individuals of which tend to recover 

 from infection (i.e. acquire immunity). This is not true of all 

 diseases, but it is true of the majority. 2 We shall study the subject 

 at length in a subsequent chapter. Meanwhile the points on which 

 I wish to lay stress are (i) that human races become adapted 

 through Natural Selection to the presence of diseases, and (2) that 

 in the case of many diseases this adaptation consists in a swift 

 reaction to infection a reaction so swift that the cells of an affected 

 individual begin to produce their enzymes as soon (within a few 

 minutes) as the microbes make their presence felt. This produc- 

 tion of enzymes is a step on the road to immunity but only a 

 step, for after the enzymes disappear more or less completely from 

 the body, the immunity remains. Here we have an instance of 

 the high organism making a pre-eminently advantageous use- 

 acquirement in spite of the fact that its cells are continually bathed 

 in and injured by the toxins. 



140. By skilful management it is possible to alter artificially 

 the characters of unicellular species. Thus certain saprophytic 

 organisms (which normally obtain their nutriment from dead 

 organic matter) may, under favourable conditions, be gradually 



1 See 410 et seq. 2 See chapter xiii. 



