INTERNAL FACTORS. 517 



are the physical conditions to which the sea exposes its in- 

 habitants, it becomes possible for such of them as live on 

 widely-diffused food, to be widely distributed; and wide dis- 

 tribution generally prevents the members of a species from 

 being all subject to the same cause. Our commonest cirr- 

 iped, for instance, subsisting on minute creatures every- 

 where dispersed through the water; needing only to have 

 some firm surface on which to build up its shell; and in 

 scarcely any danger from surrounding animals; is able to 

 exist on shores so widely remote from one another, that 

 nearly every change in the incident forces must fall within 

 narrower areas than that which the species occupies. Nearly 

 always, therefore, a portion of the species will survive un- 

 modified. Its easily-transported germs will take possession 

 of such new habitats as have been rendered fitter by the 

 change that has unfitted some parts of its original habitat. 

 Hence, on successive occasions, while some parts of the 

 species are slightly transformed, another part may continu- 

 ally escape transformation by migrating hither and thither, 

 where the simple conditions needed for its existence recur in 

 nearly the same combinations as before. And it will so 

 become possible for it to survive, with insignificant structural 

 changes, throughout long geologic periods. 



158. The results to which we find ourselves led, are 

 these. 



In subordination to the different amounts and kinds of 

 forces to which its different parts are exposed, every indi- 

 vidual organic aggregate, like all other aggregates, tends to 

 pass from its original indistinct simplicity towards a more 

 distinct complexity. Unless we deny the persistence of 

 force, we must admit that the lapse of an organism's struc- 

 ture from an indefinitely homogeneous to a definitely hetero- 

 geneous state, must be cumulative in successive generations, 

 if the forces causing it continue to act. And for the like 

 reasons, the increasing assemblage of individuals arising froiji 

 34 



