CHAPTER X 



THE ORIGIN OF THE DIFFERENT KINDS OF 

 ADAPTATIONS 



In the present chapter we may first consider, from the 

 point of view of discontinuous variations as contrasted with 

 the theory of the selection of individual variations, the 

 structural adaptations of animals and plants, i.e. those 

 cases in which the organism has a definite form that adapts 

 it to live in a particular environment. In the second place, 

 we may consider those adaptations that are the result of the 

 adjustment of each individual to its surroundings. In sub- 

 sequent chapters the adaptations connected with the 

 responses of the nervous system and with the process of 

 sexual reproduction will be considered. 



It should be stated here, at the outset, that the term 

 mutation will be used in the following chapters in a very 

 general way, and it is not intended that the word shall 

 convey only the idea which De Vries attaches to it ; it is 

 used rather as synonymous with discontinuous and also defi- 

 nite variation of all kinds. The term will be used to include 

 "the single variations" of Darwin, "sports," and even ortho- 

 genic variation, if this has been definite or discontinuous. 



Form and Symmetry 



Almost without exception, animals and plants have defi- 

 nite and characteristic forms. In other words, they are 

 not amorphous masses of substance. The members of each 

 species conform, more or less, to a sort of ideal type. Our 



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