1 84 THE LIVING WORLD. 



enough certainly for a great amount of change, if 

 the limit had not been practically reached. It has 

 been sufficient for the entire development of the 

 class of mammals, with its great profusion ; but the 

 plant world has remained nearly stationary, and this 

 suggests to us that we may look for no farther de- 

 velopment along this line. 



With the animal kingdom, however, it is different. 

 Even until the present era we can see that develop- 

 ment of new and higher forms has continued. Is 

 there any indication that it has reached its end ? 



That there is a theoretical possibility of the origin 

 of new types cannot be denied. New types, i. e., new 

 lines for specialization, can arise only from undiffer- 

 entiated forms. But such undifferentiated forms 

 still exist in great numbers. Even the most un- 

 specialized forms of all, the unicellular animals, are 

 abundant enough, and in all groups we are acquainted 

 with more or less generalized types. Theoretically, 

 then, there is no reason w r hy any one of these forms 

 should not expand itself, and thus form an eternal 

 source of new world forms. So long as the un- 

 specialized forms do not become extinct, we cannot 

 deny the possibility of an infinite number of future 

 sub-kingdoms, which would of course make the 

 animal kingdom an example of never-ending evolu- 

 tion. But all of our evidence indicates that such 

 a future is probably not a practical possibility, 

 even though, so far as we can see, it may be 

 a theoretical one. All biological studies point 

 strongly to the conclusion that, instead of several 

 points of origin the animal kingdom has had only 



