THE EVOLUTION OF MIND 



nutrition are such as to aid in the setting-up of 

 the most perfect transit lines. In an inorganic 

 mass the molecules have comparatively little 

 mobility, and they do not leave their connec- 

 tions from moment to moment, to be instantly 

 replaced by new molecules. But the complex 

 clusters of molecules which make up living tis- 

 sue possess immense mobility, and they are con- 

 tinually falling to pieces and getting built up 

 again. Consequently the repeated passage of 

 waves either of fluid matter or of molecular 

 motion along a definite line of least resistance, 

 not only changes the positions of the molecular 

 clusters, but also modifies the nutritive changes 

 by which the temporary equilibrium of the 

 clusters is restored. Instead of a set of rela- 

 tively homogeneous molecules, which are simply 

 pushed aside and then tend to oscillate back 

 again, the advancing wave encounters a heter- 

 ogeneous edifice of molecules, which tumbles 

 to pieces and is instantly rebuilt. But in the 

 rebuilding the force exerted by the advancing 

 wave has to be expended ; and the result is that 

 in the rebuilt cluster there is a surplus tension 

 exerted in the very direction in which the waves 

 are travelling. The transit lines thus become 

 far more permeable than any which can be es- 

 tablished in inorganic bodies. The energy given 

 out by the decomposing cluster of molecules 

 adds to the momentum of the wave ; so that 



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