CHAPTER VIII 



DIFFERENTIATION OF INDIVIDUALS AND ADAPTATION 



129. The Individual and its Environment. We have thus 

 far considered the mature individual as the end for which the 

 various developmental processes exist. It has been seen that 

 the individual becomes complex as its parts grow and become 

 differentiated to do the work necessary to the well-being of the 

 animal. In this differentiation the parts become dependent 

 upon each other, and in the healthy state they work harmo- 

 niously among themselves. It is now necessary to pass from the 

 consideration of these internal structures and relations in order 

 to consider the individual animal as a unit in its relations to 

 everything about it, that is, to its environment. This term 

 includes not merely the inanimate materials and conditions 

 surrounding an organism, but in addition all living things both 

 plant and animal which directly or indirectly influence it. The 

 environment of no two animals is the same, nor is it the same for 

 any given animal for two moments in succession. This con- 

 tinual change in the environment leaves its impress on the struc- 

 ture and habits of all organisms. Every individual is thus re- 

 lated to its own environment from day to day; in addition to 

 this, all the individuals of any generation, owing to the facts of 

 reproduction, are also to be considered in the light of the con- 

 ditions to which their parents have been subjected from the 

 remotest time. The study of the individual in its relations to 

 its environment brings the student face to face with many very 

 important problems. No department of zoology is more 

 interesting. 



130. Heredity. One does not study organisms very long 

 without being impressed with two things: first, that there are 

 remarkable similarities among them, even among those little 

 related ; and second, that there are interesting differences among 



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