INTERNAL FACTORS. 515 



such as are congruous with the fundamental characters of its 

 organization such as subject its essential organs to actions 

 substantially the same as before. Great changes must kill it. 

 Hence, it can continuously expose itself and its descendants, 

 only to those moderate changes which do not destroy the 

 general harmony between the aggregate of incident forces 

 and the aggregate of its functions. That is, it must remain 

 under influences calculated to make greater the definiteness 

 of the chief differentiations already produced. If, for ex- 

 ample, we set out with an animal in which a rudimentary 

 vertebral column with its attached muscular system has 

 been established; it is clear that the mechanical arrange- 

 ments have become thereby so far determined, that subse- 

 quent modifications are extremely likely, if not certain, to 

 be consistent with the production of movement by the actions 

 of muscles on a flexible central axis. Hence, there will con- 

 tinue a general similarity in the play of forces to which the 

 flexible central axis is subject; and so, notwithstanding the 

 metamorphoses which the vertebrate type undergoes, there 

 will be a maintenance of conditions favourable to increasing 

 definiteness and integration of the vertebral column. More- 

 over, this maintenance of such conditions becomes secure in 

 proportion as organization advances. Each further com- 

 plexity of structure, implying some further complexity in 

 the relations between an organism and its environment, must 

 tend to specialize the actions and reactions between it and 

 its environment must tend to increase the stringency with 

 which it is restrained within such environments as admit of 

 those special actions and reactions for which its structure fits 

 it; that is, must further guarantee the continuance of those 

 actions and reactions to which its essential organs respond, 

 and therefore the continuance of the segregating process. 



How in each species, considered as an aggregate of indi- 

 viduals, there must arise stronger and stronger contrasts 

 among those divergent varieties which result from the in- 

 stability of the homogeneous and the multiplication of effects, 



