140 ESSAYS. 
ferent species, Elks, Horses, Megalonyx, the Lion, ete. ; and, 
from the relations between this fauna and that of Europe, 
there is little doubt that the climate was as. much milder 
than the present on this as on the other side of the ocean. 
All the facts known to us in the tertiary and post-tertiary, 
even to the limiting line of the drift, conspire to show that 
the difference between the two continents as to temperature 
was very nearly the same then as now, and that the isother- 
mal lines of the northern hemisphere curved in the directions 
they now do. 
A climate such as these facts demonstrate for the fluvial 
epoch would again commingle the temperate floras of the 
two continents at Behring’s Straits, and earlier — probably 
through more land than now — by way of the Aleutian and 
Kurile Islands. I cannot imagine a state of circumstances 
under which the Siberian Elephant could migrate, and tem- 
perate plants could not. 
The fluvial was succeeded by the “ terrace epoch,” as Dana 
names it, “a time of transition towards the present condition, 
bringing the northern part of the continent up to its present 
level and down to its present cool temperature,” !— giving 
the arctic flora its present range, and again separating the 
temperate floras of the New and of the Old World to the 
extent they are now separated. 
Under the light which these geological considerations throw 
upon the question, 1 cannot resist the conclusion, that the ex- 
tant vegetable kingdom has a long and eventful history, and 
that the explanation of apparent anomalies in the geograph- 
ical distribution of species may be found in the various and 
prolonged climatic or other physical vicissitudes to which they 
have been subject in earlier times ; that the occurrence of cer- 
tain species, formerly supposed to be peculiar to North Amer- 
ica, in a remote or antipodal region affords itself no presump- 
tion that they were originated there, and that the interchange 
of plants between eastern North America and eastern Asia is 
explicable upon the most natural and generally received hy- 
1 For the collocation and communication of the geological data here 
presented, I am indebted to the kindness of my friend, Professor Dana. 
