Geology and Natural History. 153 
of living things is a slow and gradual evolution of new types by 
descent with modification, or whether, according to the law so 
ably illustrated by Barrande in the case of the Cephalopods and 
Trilobites, pew forms are introduced abundantly and in perfection 
at once. The physical change was apparently of the most gradual 
character. Was it so with the organic change? That it was not 
is apparent from the fact that both Dr. Asa Gray and Mr. Cope, 
who try to press this transition into the service of evolution, are 
obliged in the last resort to admit that the new flora and fauna 
us. Neither seems to consider that if giant Sequoias and Dicoty- 
ledonous trees and large herbivorous Mammalia arose in the Cre- 
taceous or early Tertiary, and have continued substantially unim- 
proved ever since, they must have existed somewhere for period 
Species; 2. that the temperate-climate vegetation which once 
flourished beyond the trier! gene? was slowly driven southward by 
the cold. His moderate theoretical inference is, that the vegeta- 
tion among which we live has, as a bl escen ed orate! 
arctic Miocene vegetation, most of it with modification, sor 
It without, This ia what Dr. Dawson calls being “ obliged in the 
