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 
