CHAPTER XIII 

 THE BASIS OF ADAPTATION 



Variation. We have already noted that organisms vary and 

 that variation itself is the most invariable thing in nature. The 

 possibility of change in organisms is directly associated with this 

 phenomenon. 



If an individual were incapable of varied responses change of 

 environmental conditions beyond its power to respond could only 

 result in death. In the latitude of response within the individual 

 we find the foundation of individual success. Individuals differ 

 in their power of response, however, even within the same species 

 because of their variation, so that conditions which make life 

 impossible for one may be met easily bj^ another. 



Since species are merely aggregations of individuals these things 

 are also the basis for success of species in meeting varied conditions 

 and in becoming fitted for life in a limited environment. Extinc- 

 tion may occur and has occurred in many known cases, but it 

 can result only from a change so extreme that no individuals can 

 meet it. Any change less extreme could be met by some; these 

 survivors would continue to perpetuate the species which would 

 differ from its former state through the loss of the unsuccessful 

 individuals. The responses of the species as a whole would then 

 be those of the surviving individuals; it would be changed to 

 meet the changed environment. 



Environment. Although change of external environmental 

 relationships must always accompany the evolution of organisms, 

 change of the external environment need not always be the 

 inciting cause. Non-adaptive changes in organisms produce struc- 

 tures correlated wholly with the internal environment, although 

 their establishment may later exact adjustments to the external 

 environment as the price of existence. It is probable, at least, 

 that any change in an organism will modify its ability to avail 

 itself of surrounding conditions in some degree. 



Whatever may be the source of change in organisms, it involves 

 changed response to conditions in the complex environment. 



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