1 62 THE STORY OF THE LIVING MACHINE. 



occur appears to be hardly doubtful, 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 certainly in a position 

 to be only slightly affected by surrounding con- 

 ditions 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 protoplasm, but we can hardly imagine that 

 such use of the arm would produce any change 

 in the hereditary substance which is stored in the 

 reproductive 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 sub- 

 ject 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 congenital 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 

 all higher animals and plants at least, each indi- 

 vidual has two parents instead of one. In our 

 study of cells we have seen that the machinery 



