EVOLUTION 105 



ency to become somatic cells. In a similar 

 way, somatic cells are never derived from cells 

 which have begun to assume the character of 

 germ cells, but are also directly derived from 

 the segmented egg. The two classes of cells 

 are thus fundamentally distinct, and it is not 

 inappropriate to describe the body of one of 

 the higher animals as composed of an enor- 

 mous aggregation of somatic cells in which is 

 harbored and protected a certain number of 

 germ cells. 



What has been defined as acquired charac- 

 ters, namely, those changes which are not the 

 products of inheritance, but which are the di- 

 rect results of some change in the environ- 

 ment or in the activities of an animal, are 

 necessarily alterations in its somatic cells. The 

 change in a muscle in consequence of a new 

 form of exercise is a change in somatic cells ; 

 the alteration that the skin exhibits under a 

 new exposure is of the same kind ; the change 

 in the nervous mechanism by which a new 

 habit is established is also somatic. If acquired 

 characters, then, are modifications in the so- 

 matic cells, and these cells are distinct from 

 the germ cells, it is difficult to see how a change 

 which may affect a group of somatic cells, 



