THE FUTURE OF THE LIVING WORLD. 183 



here there has been the same story of limitation. 

 The ganoids culminated in the Devonian (3), and 

 have advanced no farther. One great line of reptiles 

 reached its limit in the Jurassic (7). And so every- 

 where. The study of every group teaches that the 

 past history has been a gradual specialization which 

 approaches a limit. In many cases in the past this 

 limit has been reached, and advance has ceased ; in 

 others, the animals are still on the road toward it. 



It is plain, then, that unless there is some way for 

 new lines of specialization to arise, the evolution of 

 the animal world is inevitably approaching its end. 

 With every advance in differentiation, the possible 

 lines of development decrease, and since the actual 

 lines followed are tending to run themselves out, the 

 whole must eventually stop. Is it, however, possible 

 that new lines of differentiation may arise, and thus 

 the development of the living world go on in- 

 definitely? 



New Lines of Specialization Not Now Appearing. 



First, we must notice that the development of the 

 vegetable kingdom practically ceased long ago. As 

 already noticed, the Cretaceous (8) rocks show us 

 representatives of the highest orders of plants. At 

 that time the plants in existence do not seem to have 

 differed materially from those of to-day, since many 

 species are identical so far as can be determined. 

 The vegetable kingdom thus practically reached its 

 culmination at this time ; for although many new 

 species have appeared since then, there has been no 

 advance in type. The time since then has been long 



