160 HEREDITARY CHARACTERS 



tinuous growth throughout the life of the animal. In rats, 

 for instance, in whom the capacity for gnawing hard sub- 

 stances is very necessary, the teeth appear to grow through- 

 out the animal's life. If one of the upper eye teeth be lost 

 through accident, the corresponding lower tooth grows to 

 such an extent that it prevents the animal feeding properly, 

 and sometimes even grows into its skull and causes death. 

 The two opposing teeth wear each other away under normal 

 conditions, so that when this check is removed in one jaw, 

 the tooth in the other grows until it kills its possessor, either 

 through preventing its feeding or by penetrating some vital 

 part. Now the capacity for indefinite growth in this case 

 actually meets and renders ineffective the acquirement 

 made by the individual. The acquirement is the wearing 

 out of the teeth, the inborn character is the capacity to 

 grow indefinitely. The power to gnaw though hard sub- 

 stances and the possession of sharp teeth is a necessary 

 adaptation to the environment in the case of the rat. If 

 the teeth did not grow throughout the animal's life they 

 would soon be worn out, and the evolution of this character 

 is explained very simply by natural selection acting upon 

 inborn variations. If it is explained by the transmission of 

 acquired characters, we must suppose that the wearing out 

 of the teeth in the parent has been transmuted into an 

 inborn capacity for indefinite growth in the offspring. 



The origin of joints has been explained in a somewhat 

 similar manner. By continual friction, the end of one bone 

 is supposed to have become rounded, the end of the other 

 to have become hollowed out. At first this shaping of the 

 end of one bone by the other was very slight, but was 

 increased at each generation as an acquirement made by 

 the individuals. This explanation fails to show us why it is 

 that this process of reciprocal wear, which means destruction 

 of a certain amount of bone, does not continue indefinitely. 

 If this interpretation were true, joints should be getting 

 deeper and deeper and the long bones should be becoming 

 shorter as they are worn out in each generation. Observa- 



