CHAPTER VIII. 

 INDIVIDUAL DIFFERENTIATION AND ADAPTATION. 



125. 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 

 harmoniously among themselves. It is now necessary to pass 

 from the consideration of these internal structures and rela- 

 tions 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 condi- 

 tions 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 continual change in the environment leaves its impress 

 on the structure and habits of all organisms. Every individual 

 is thus related 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 conditions to which their parents have been sub- 

 jected 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. 



126. 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 



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