186 THE STORY OF LIFE'S MECHANISM. 



although we do not know what sort of influences 

 can produce them. If the germ plasm is wholly 

 stored within the reproductive gland, it is cer- 

 tainly in a position to be only slightly affected 

 by surrounding conditions which affect the 

 animal. We can readily understand that the 

 use of an organ like the arm will affect it in 

 such a way as to produce changes in its proto- 

 plasm, but we can hardly imagine that such use 

 of the arm would produce any change in the 

 hereditary substance which is stored in the re- 

 productive organs. External conditions may 

 thus readily affect the body, but not so readily 

 the germ material. Even if such material is dis- 

 tributed more or less over the body instead of 

 being confined to the reproductive glands, as 

 some believe, the difficulty is hardly lessened. 

 This difficulty of understanding how the germ 

 plasm can be affected by external conditions has 

 led one school of biologists to deny that it is 

 subject to any variation by external conditions, 

 and hence that all modification of the germ plasm 

 must come from some other source. Probably 

 no one, however, holds this position to-day, and 

 it is the general belief that the germ plasm may 

 be to some slight extent modified by external 

 conditions. Of course, if such variations do 

 occur in the germ plasm they will become con- 

 genital variations of the next generation, since 

 the next generation is the unfolding of the germ 

 plasm. 



The second method by which the variations of 

 germ plasm may arise is apparently of more im- 

 portance. It is based upon the fact that, with 



