DIFFERENTIATION OF INDIVIDUALS AND ADAPTATION lOQ 



external condition and by use are definite and naturally in the 

 direction to meet the necessities of the case. For example, 

 cold stimulates the surface cells of the body of an animal. The 

 immediate response of the nervous and nutritive processes in 

 the organism are such that the surface cells take on greater 

 activity and produce materials at the surface of the body which 

 tend to protect the animal from the ill effects of the cold. This 

 is an individual variation. To become effective in making the 

 species better adapted to the environment these results must 

 be handed down by inheritance to the next generation. If this 

 can take place this theory would go a long way toward explain- 

 ing how adaptations arise. There is, however as we have seen, 

 very great doubt whether such adaptations acquired in the life 

 of an individual can be transmitted to offspring. If this cannot 

 occur we are thrown back upon natural selection of mutations 

 as the principal explanation thus far offered to account for the 

 progressive adaptation of animals to the environment. While 

 natural selection does not cause variations, there is no reason- 

 able doubt that it is a part of the explanation of progress. 

 So far as we know either small or large variations may be 

 selected and increased by breeding if they are only inherited 

 variations and useful ones. To what extent natural selection 

 is assisted by other factors is at present uncertain. It will be 

 assumed in the following pages that it is one of the most impor- 

 tant known factors in producing adaptation. 



141. Classification. — Since the environment is not the same 

 at any two places on the earth and there is usually an accumula^ 

 tion, from generation to generation, in animals, of those features 

 which tend to bring them into harmony with their different 

 environments, it is inevitable that the animals themselves come 

 to be very diverse, no matter how similar they were at the 

 outset. In the discussion of them it therefore becomes neces- 

 sary to devise some means of expressing the degree of likeness 

 and unlikeness among the great number of individual animals 

 existing on the earth. This may be done by means of an 

 appropriate classification. The differences of structure and func- 

 tion may be superficial or fundamental, but it must be remenir 



