THE LIVING MACHINE BUILDING FACTORS. 189 



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 gen- 

 eration. 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 

 the germ plasm within the reproductive glands 

 is not, so far as we can see, subject to the influ- 

 ence of an increased use, for example, in the arm 

 muscles. The germ material is derived from the 

 parents, and, if it is simply stored in the indivi- 

 dual, how could an acquired variation affect it ? If 

 an individual lose a limb, his offspring will not be 

 without a corresponding limb, for the hereditary 

 material is in the reproductive organs, and it is 

 impossible to believe that the loss of the limb can 

 remove from the hereditary material in the repro- 

 ductive glands just that part of the germ plasm 

 which was designed for the production of the 

 limb. So, too, if the germ plasm is simply stored 

 in the individual, it is impossible to conceive any 

 way that it can be affected by the conditions 

 around the individual in such a way as to ex- 

 plain the inheritance of acquired variations. If 

 acquired variations do not affect the germ plasm 

 they cannot be inherited, and if the germ plasm 

 is only a bit of protoplasmic substance handed 

 down from generation to generation, we can not 

 believe that acquired variations can influence it. 

 From such considerations as these have arisen 



