130 THE EVOLUTION OF LIFE. 



distinct complexity. Unless we deny the persistence of 

 force, we must admit that the gravitation of an organism's 

 structure from an indefinitely homogeneous to a definitely 

 heterogeneous state, must be cumulative in successive genera- 

 tions, if the forces causing it continue to act. And for the 

 like reasons, the increasing assemblage of individuals arising 

 from, a common stock, is also liable to lose its original 

 uniformity; and, in successive generations, to grow more 

 pronounced in its multiformity. 



These changes, which would go on to but a comparatively 

 small extent were organisms exposed to constant external 

 conditions, are kept up by the continual changes in external 

 conditions, produced by astronomic, geologic, meteorologic, 

 and organic agencies : the average result being, that on 

 previous complications of structure wrought by previous 

 incident forces, new complications are continually superposed 

 by new incident forces. And hence simultaneously arises 

 increasing heterogeneity in the structures of individuals, in 

 the structures of species, and in the structures of the Earth's 

 Flora and Fauna. 



But while, in very many or in most cases, the ever- 

 changing incidence of forces is ever adding to the complexity 

 of organisms, and to the complexity of the organic world as a 

 whole ; it does this only where its action cannot be eluded. 

 And since, by migration, it is possible for species to keep 

 themselves under conditions that are tolerably constant ; 

 there must be a proportion of cases in which greater hetero- 

 geneity of structure is not produced. 



Uniting these three propositions, we are brought to a con 

 elusion which, so far as it goes, appears to be in harmony 

 with the facts. We find progression to result, not from a 

 special, inherent tendency of living bodies, but from a general 

 average effect of- their relations to surrounding agencies. 

 While we are not called on to suppose that there exists in 

 organisms any primordial impulse which makes them con- 

 tinually unfold into more heterogeneous forms ; we see 



