160 THE STORY OF THE LIVING MACHINE. 



skin which otherwise would have been dark. In 

 other words, a conclusive answer can not here be 

 given. It is not our purpose, however, to attempt 

 to distinguish between these two kinds of varia- 

 tions, but*simply to recognize that they occur. 



Our next problem must be to search for an ex- 

 planation of these variations. With the acquired 

 variations we have no particular trouble, for they 

 are easily explained as due to the direct action of 

 the environment upon animals. One of the fun- 

 damental characters of the living protoplasm 

 (using the word now in its widest sense) is its 

 extreme instability, So unstable is it that any 

 disturbing influence will affect it. If two similar 

 unicellular organisms are placed under different 

 conditions they become unlike, since their un- 

 stable protoplasm is directly affected by the sur- 

 rounding conditions. With higher animals the 

 process is naturally a little more complicated ; 

 but here, too, they are easily understood as part 

 of the function of the machine. One of the ad- 

 justments of the machine is such that when any 

 organ is used more than usual the whole machine 

 reacts in such a way as to send more blood to 

 this special organ. The result is a change in the 

 nutrition of the organ and a corresponding varia- 

 tion in the individual. Thus acquired variations 

 are simply functions of the action of the machine. 



Congenital variations, however, can not re- 

 ceive such an explanation. Being born with the 

 individual, they can not be produced by condi- 

 tions affecting him, but rather to something affect- 

 ing the germ plasm from which he sprung. The 

 nature of the germ plasm controls the nature of 

 the individual, and congenital variations must 

 consequently be due to its variations. But it is 



