ORIGIN' OF VARIATIONS. 163 



molecular arrangement, and may become crystallised 

 in structure. A bone presents this difference : 

 its particles are undergoing a gradual change all 

 the time, and this mobility of its particles must, from 

 its nature, take away from the elasticity of the bone. 

 When a bone is compressed by a blow, its imperfect 

 elasticity would prevent it from returning to its 

 original shape. We must suppose, therefore, that 

 those vital restorative processes in the bone 

 which prevent it from crystallising under the 

 many shocks it receives, at the same time cause 

 a repair in such directions that the bone resumes 

 its former shape. It is well known that in many 

 cases such shocks, repeated blows, or friction 

 cause the repairs to exceed the waste, and excres- 

 cences of bone are the result, as on the hands or 

 feet. The bony excrescences on the heads of 

 horned animals have generally been explained 

 as having arisen in this way under the intensifying 

 effects of inheritance and selection. The action 

 of shocks would naturally cause the restoration 

 and increase to take place in the direction of the 

 compression caused by the blow ; nor must it be 

 supposed that the effect of the shock and its conse- 

 quent increase is merely on the surface of the 

 bone, but it takes place wherever the shock causes 

 a molecular displacement. The structure of the 



