250 PHYSIOLOGICAL DEVELOPMENT, 



another tissue lying between it and the wood. What is the 

 physical interpretation of these facts ? 



When a mass of something we distinguish as inert matter 

 is exposed to external agencies capable of working changes in 

 it — when it is chemically acted upon, or when, being dry, it 

 is allowed to soak, or when, being wet, it is allowed to dry — 

 the changes set up progress in an equable way from the 

 surface towards the centre. At any time during the process 

 (supposing no other action supervenes) the modification 

 wrought, first completed at the outside, either gradually 

 diminishes as we approach the centre, or ceases suddenly at 

 a certain distance from the centre. But now suppose that 

 the mass, instead of being inert, is the seat of active changes 

 ^suppose that it is a portion of complex colloidal substance, 

 permeable by light and by fluids capable of affecting its 

 unstable molecules — suppose that its interior is a source of 

 forces continually liberated and diffusing themselves out- 

 wards. Is it not likely that while at the centre the action 

 of the internally-liberated forces will dominate, and while at 

 the surface the action of the environing forces will dominate, 

 there will be between the two a certain place at which their 

 actions balance? May we not expect that this will be the 

 place where the most unstable matter exists — the place out- 

 side of which the matter becomes relatively stable in the 

 face of external forces, and inside of which the matter be- 

 comes relatively stable in the face of internal forces? And 

 must we not conclude that though part of the adjustment is 

 due to indirect equilibration, the initiation of it is due to 

 direct equilibration ? 



But we are here chiefly concerned with the more general 

 interpretation, which is independent of any such speculation 

 as the foregoing. These contrasted tissues and the contrasted 

 functions they severally perform are, beyond question, sub- 

 ordinated to the relations of outside and inside. And the 

 evidence makes it tolerably clear that the unlike actions or 

 forces involved by the relations of outside and inside, deter- 

 mine these contrasts — partly directly and partly indirectly. 



