ADAPTATION EXPLAINED BY NUTRITION. 221 



individual species the number of variations which differ 

 more or less from the prevailing or typical form of the 

 species. Indeed, in every careful systematic special treatise 

 one finds, in the case of most species, mention of a number of 

 such variations, which are described sometimes as individual 

 deviations, and sometimes as so-called races, varieties, de- 

 generate species, or subordinate species, and which often 

 differ exceedingly from the original species, solely in con- 

 sequence of the adaptation of the organism to the external 

 conditions of life. 



If we now endeavour to fathom the general causes of these 

 phenomena of Adaptation, we arrive at the conclusion that 

 in reality they are as simple as the causes of the phenomena 

 of Inheritance. We have shown that the nature of the 

 process of propagation furnishes the real explanation of 

 the facts of Transmission by Inheritance, that is, the trans- 

 mission of parental matter to the body of the offspring; 

 and in like manner we can show that the physiological 

 function of nutrition, or change of substance, affords a 

 general explanation of Adaptation or Variation. When I 

 here point to "nutrition" as the fundamental cause of 

 variation and adaptation, I take this word in its widest sense, 

 and I understand by it the whole of the material changes 

 which the organism undergoes in all its parts through the 

 influences of the surrounding outer world. Nutrition thus 

 comprises not only the reception of actual nutritive sub- 

 stances and the influence of different kinds of food, but 

 also, for example, the action upon the organism of water 

 and of the atmosphere, the influence of sunlight, of tem- 

 perature, and of all those meteorological phenomena which 



are implied in the term "climate." The indirect and 

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