1 64 THE STORY OF THE LIVING MACHINE. 



variations can have no effect upon this machine, 

 for they would disappear with the individual in 

 which they appeared. In order that they should 

 have any influence in the process of machine 

 building they must be permanent ones ; or, in 

 other words, they must be inherited from genera- 

 tion to generation. Only as such variations are 

 transmitted by heredity can they be added to the 

 structure of the developing machine. Therefore 

 we must ask whether the variations are inherited. 



In regard to the congenital variations there 

 can be no difficulty. The very fact that they are 

 congenital shows us that they have been produced 

 by variations in the germ plasm, and as such they 

 must be transmitted, not only to the next genera- 

 tion, but to all following generations, until the 

 germ plasm becomes again modified. This germ 

 plasm is handed on from generation to genera- 

 tion with all its variations, and hence the varia- 

 tions will be added permanently to the machine. 

 Congenital variations are thus a means for per- 

 manently modifying the organism, and by their 

 agency must we in large measure believe that 

 evolution through the ages has taken place. 



With the acquired variations the matter stands 

 quite differently. We can readily understand 

 how influences surrounding an animal may affect 

 its organs. The increase in the size of the 

 muscles of the blacksmith's arm by use we under- 

 stand readily enough. But with our understand- 

 ing of the machinery of heredity we can not see 

 how such an effect can extend to the next gener- 

 ation. It is only the organ directly affected 

 that is modified by external conditions. Ac- 

 quired variations will appear in the part of the 

 body influenced by the changed conditions. But 



