78 THE DOCTRINE OF DESCENT. 



discoveries of analogous animals, with soft and still flexi- 

 ble integuments, confirm what was theoretically extremely 

 probable, that from them proceeded the highest existing 

 order of the Holothuria or Sea-cucumbers; and thus the 

 division of Echinoderms conforms to the universal experi- 

 ence 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 character- 

 ize the vegetation. The animal world has likewise re- 

 mained essentially the same from the earliest sections of 

 the Tertiary period until now, as we shall more elab- 

 orately set forth in the chapter on Geographical Distri- 

 bution. 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 when the continents, not indeed without sundry 

 local oscillations, are approximating to their present con- 

 figuration, the impress of the Mammalia becomes pre- 

 dominant. Under the influence of elevations and depres- 

 sions, of several glacial periods, and the more sharply 

 defined limits of the climatic zones, frequent displace- 

 ments occurred in the vegetal and animal world, ac- 

 companied by differentiation and further development. 

 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 

 separated succession of the evidence in its favour, that is 

 to say, of the systems of stratification, the fixed concep- 

 tion of a fossil was, that whatever had lived before the 

 appearance of man on the threshold of the Alluvial 



