CHAPTEE X. 



INTERNAL FACTORS. 



153. WE saw at the outset (1016), that organic 

 matter is built up of molecules so unstable, that the slightest 

 variation in their conditions destroys their equilibrium, and 

 causes them either to assume altered structures or to decom- 

 pose. But a substance which is beyond all others changeable 

 by the actions and reactions of the forces liberated from 

 instant to instant within its own mass, must be a substance 

 which is beyond all others changeable by the forces acting on 

 it from without. If their composition fits organic aggregates 

 for undergoing with special facility and rapidity those re-dis- 

 tributions of matter and motion whence result individual 

 organization and life ; then their composition must make them 

 similarly apt to undergo those permanent re-distributions of 

 matter and motion which are expressed by changes of struc- 

 ture, in correspondence with permanent re-distributions of 

 matter and motion in their environments. 



In First Principles,, when considering the phenomena of 

 Evolution at large, the leading characters and causes of those 

 changes which constitute organic evolution were briefly traced. 

 Under each of the derivative laws of force to which the 

 passage from an incoherent, indefinite homogeneity to a 

 coherent, definite heterogeneity, conforms, were given illustra- 

 tions drawn from the metamorphoses of living bodies. Here 

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