THE INNER TISSUES OP ANIMALS. 355 



Of course it is not alleged that osseous structures arise in 

 this way alone. The bones of the skull and various dermal 

 bones cannot be thus interpreted. Here the natural selec- 

 tion of favourable variations appears the only assignable 

 cause — the equilibration is indirect. We know that ossific 

 deposits now and then occur in tissues where they are not 

 usually found; and such deposits, originally abnormal, if 

 they occurred in places where advantages arose from them, 

 might readily be established and increased by survival of the 

 fittest. Especially might we expect this to happen when a 

 constitutional tendency to form bone had been established by 

 actions of the kind described; for it is a familiar fact that 

 differentiated types of tissue, having once become elements 

 of an organism, are apt occasionally to arise in unusual 

 places, and there to repeat all their peculiar histological 

 characters. And this may possibly be the reason why the 

 bones of the skull, though not exposed to forces such as 

 those which produce, in other bones, dense outer layers in- 

 cluding less dense interiors, nevertheless repeat this general 

 trait of bony structure. While, however, it is beyond doubt 

 that some bones are not due to the direct influence of 

 mechanical stress, we may, I think, conclude that mechanical 

 stress initiates bone-formation. 



§ 302. What is the origin of nerve ? In what way do its 

 properties stand related to the properties of that protoplasm 

 whence the tissues in general arise? and in what way is it 

 differentiated from protoplasm simultaneously with the other 

 tissues? These are profoundly interesting questions; but 

 questions to which positive answer's cannot be expected. 

 All that can be done is to indicate answers which seem 

 feasible. 



That the property specially displayed by nerve, is a pro- 

 nourishment of the cell declines so that the waste is insufficiently replaced (or 

 otherwise that the bone-substance gradually loses its power of resistance to 

 the osteoblasts formed as a result of inactivity "). 



