Chapter VII 

 NATURAL RESISTANCE AND FATIGUE 



All forms of life from the lowest bacteria to man 

 are so constituted that they will live only within cer- 

 tain definite limits of environmental changes. While 

 it is true in many instances that the interval between 

 such limits may be rather wide, it is well known that 

 organisms live normally and longest only under opti- 

 mal environmental conditions. This is a necessary con- 

 sequence of the operation of the mechanism of natu- 

 ral selection, which has permitted the survival of only 

 those individuals which are more or less adapted to 

 the particular environment wherein the development 

 of a given species has taken place. This has been due 

 to the fact that natural selection operates according 

 to the laws of chance, rather than purposefully, to 

 produce and maintain a race of organisms best fitted 

 to survive and reproduce its kind. 



The various races of animals and plants resulting 

 from such a process are therefore not ideally adapted 

 to their environment, but only better adapted to it 

 than their ancestors. By a similar process the consti- 

 tuent body cells of the metazoans and higher plants 

 have become adapted to a definite internal environ- 

 ment which is comparatively constant for a given 



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