INTERNAL .FACTORS. 427 



stant forces; and doubtless, continual change of circumstances 

 interferes with this. But the interference can never be con- 

 siderable. For the pre-existing structure of an organism pre- 

 vents it from living under any new conditions except such as 

 are congruous with the fundamental characters of its organiza- 

 tion — such as subject its essential organs to actions substan- 

 tially 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 har- 

 mony between the aggregate of incident forces and the ag- 

 gregate 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 sub- 

 sequent modifications are extremely likely, if not certain, to 

 be consistent with the production of movement by the action 

 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. 



