CHAPTER IV 



ADAPTATION TO DISEASE-PRODUCING AGENCIES IN THE 



HIGHER ANIMALS 



Before passing to the other end of the scale of living forms, 

 and studying the phenomena of infectious disease in the terms 

 of adaptation, and other adaptations to disease-producing 

 agencies as observed in man and the higher animals, let us pause 

 for a moment and endeavour to summarize the conclusions 

 which may reasonably be drawn from the data brought forward 

 up to this point. 



1. The evidence is abundant and conclusive that bacteria 



are capable of being modified by alterations of environ- 

 ment of certain orders, chemical and physical. 



2. The modifications in question that have been brought 



forward conform with Herbert Spencer's " direct equili- 

 brations " : a particular alteration of environment of 

 certain orders leading inevitably to a particular modifica- 

 tion whether of function or of form. 



3. By employing appropriate methods it can be shown that 



not some but all the microbes subjected to particular 

 orders of alteration of environment exhibit the particular 

 modification : the hypothesis of " chance variation " in 

 a particular direction with survival of the fittest is in- 

 capable of explaining the phenomena. 

 Yet other conclusions are to be drawn, but for the time being 

 these suffice for my immediate purpose. 



There are two interesting and, I think, basal observations 

 upon adaptation which, affecting protozoal forms higher in the 

 scale than the bacteria, deserve note before passing to consider 

 adaptation in multicellular organisms. The one we owe to the 



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