INTERNAL FACTORS. 429 



kind of plant or animal, it follows that since the intruders 

 will probably not spread throughout its whole habitat, the 

 species tvtlU, in one or more localities, remain unaffected by 

 them. Especially among marine creatures, must there fre- 

 quently occur cases in which modifjdng causes are con- 

 tinually eluded. Much more uniform as are the physical 

 conditions to which the sea exposes its inhabitants, it becomes 

 possible for such of them as live on widely-diffused food, to 

 be widely distributed ; and wide distribution generally pre- 

 vents the members of a species from being all subject to the 

 same cause. Our commonest cirrhiped, for instance, subsisting 

 on minute creatures that are everywhere dispersed through 

 the sea ; needing only to have some firm surface on which 

 to build up its shell ; and in scarcely any danger from sur- 

 rounding animals ; is able to exist on shores so widely remote 

 from one another, that nearly every change in the actions of 

 incident forces, must fall within narrower areas than that 

 which the species occupies. In nearly every case, therefore, 

 a portion of the species will suridve immoclified. 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 trans- 

 formed, another part may continually escape transformation 

 by migrating hither and thither, where the simple condi- 

 tions needed for its existence recur in nearly the same com- 

 binations as before. And it will so become possible for it 

 to survive, with comparatively trifling 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 in- 

 di\ddual organic aggregate, like all other aggregates, tend?. 

 to pass from its original indistinct simplicityrtowards a more 



