290 PHYSIOLOGICAL DEVELOPMENT. 



that at this place will lie the unstable protoplasm that 

 develops outwardly into a substance which is stable in face 

 of outer forces, and inwardly into a substance which is stable 

 in face of inner forces ; so in animals, we may regard this 

 universally-present layer whence epidermis grows outwardly 

 and connective tissue inwardly, as similarly the place of 

 equilibrium between these antagonist forces. And for this 

 d priori interpretation we may indeed, among animals, find 

 d posteriori warrant. We have but to increase the mechanical 

 action or chemical irritation at some part of an animal's 

 surface, to make this plane of indifferent tissue retreat in- 

 wardly ; for to say that the epidermis becomes thicker, is, in 

 mechanical terms, to say that the place of equilibrium between 

 outer and inner forces is further from the surface. 



