EVOLUTION AS A NATURAL PROCESS 113 



A blacksmith uses his arm muscles more constantly 

 than do most other men, and his prolonged exercise 

 leads to an increase of his muscular capacity. All 

 of the several organic systems are capable of consider- 

 able development by judicious exercise, as every one 

 knows. If the functional modifications through use 

 were unreal, then the routine of the gymnasium and 

 the schoolroom would leave the body and the mind as 

 they were before. Furthermore, we are all familiar 

 with the opposite effects of disuse. Paralysis of an 

 arm results in the cessation of its growth. When a 

 fall has injured the muscles and nerves of a child's 

 limb, that structure may fail to keep pace with the 

 growth of the other parts of the body as a result 

 of its disuse. These are simple examples of a wide 

 range of phenomena exhibited everywhere by an- 

 imals and even by the human organism, demonstrat- 

 ing the plasticity of the organic mechanism and 

 its modification by functional primary factors of 

 variation. 



But by far the greater number of variations seem 

 to be due to the so-called congenital causes, which are 

 sharply contrasted with the influences of the first 

 and second classes. It is quite true that the influences 

 of the third class cannot be surely and directly demon- 

 strated like the others, but however remote and vague 

 they themselves may appear to be, their effects are 

 obvious and real, while at the same time their effects 

 are to be clearly distinguished from the products of 

 the other two kinds. Congenital factors reside in 

 the physical heritage of an organism, and their results 

 are often evident before an individual is subjected 



