180 THE LIVING WORLD. 



rapid expansion, yet nevertheless on the whole 

 decreasing. If this is so, it plainly implies an end 

 to the process. 



The Organic World Approaching a Limit. 



That the organic world is approaching a limit in 

 its development is a conclusion which does not, how- 

 ever, depend upon any vague idea of growth for its 

 support ; for a little thought upon discovered laws 

 will clearly show us that there must be a limit to 

 advance. The best definition which has ever been 

 given to the grade of structure of animals and plants 

 is the degree to which differentiation of organs is 

 carried. Evolution as it tends to raise the grade of 

 animals is constantly increasing the amount of differ- 

 entiation and specialization. We have already seen 

 that such differentiation and specialization becomes 

 self-limited. A low undifferentiated and unspecial- 

 ized organism has an indefinite possibility in its 

 lines of specialization. A simple spherical cup of 

 cells, the supposed common ancestor of the animal 

 kingdom, may be modified in a very great variety of 

 directions, each of which may give rise to a different 

 type of animal. This possibility lies in the fact that 

 it is as yet undifferentiated and unspecialized. But 

 just as soon as it does become modified in any one 

 direction the possibilities decrease. Some of the 

 descendants of this ancestor becoming vertebrates 

 are forever precluded from becoming anything else ; 

 others becoming mollusks must remain mollusks 

 forever, with all of their descendants. And as later 

 descendants become further modified in any direc- 



