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 Echinz 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 when 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 
