78 THE DOCTRINE OF DESCENT. 



often difficult, nay, impossible, to state which is the more 

 perfect. Progression is only to be shown collectively ; 

 in the concrete case it can rarely be demonstrated." 



The Echinae still predominate in the chalk. Recent 

 discoveries of analogous animals, with soft and flexible 

 persistent integuments, confirm what was theoretically 

 extremely probable, that from them proceeded the high- 

 est existing order of the Holothuria or Sea-cucumbers ; 

 and thus the division of Echinoderms conforms to the 

 universal experience of the ascent from the lower and 

 undifferentiated to the higher forms. 



With the Tertiary period dawns the state of things 

 now existing. Palms and arboraceous plants charac- 

 terize the vegetation. The animal world has likewise 

 remained essentially the same from the earliest sections 

 of the Tertiary period until now, as we shall more 

 elaborately set forth in the chapter on Geographical Dis- 

 tribution. In the most ancient formations the Fishes, in 

 the middle the Reptiles, were conspicuous in the world 

 of life as the representatives of the highest development; 

 now V\4ien the continents, not indeed without sundry 

 local oscillations, are approximating to their present 

 configuration, the impress of the Mammalia becomes 

 predominant. Under the influence of elevations and 

 depressions, of several glacial periods, and the more 

 sharply defined limits of the climatic zones, frequent 

 displacements occurred in the vegetal and animal world, 

 accompanied by differentiation and further develop- 

 ment. As we have already mentioned, the course of our 

 inquiries will bring us back to this subject. 



At the time when geologists believed in the rigid parti- 

 tion of the earth's periods of development and the sharply 



