136 ESSAYS. 
shown, that we cannot think it inapplicable to the organic 
world also, and especially to the creation of beings endowed 
with such enormous multiplying power, and such means and 
facilities for dissemination, as most plants and animals. 
Why then should we suppose the Creator to do that super- 
naturally which would be naturally effected by the very in- 
strumentalities which he has set in operation ? 
Viewed, however, simply in its scientific applications to the 
question under consideration (the distribution of plants in 
the temperate zone of the northern hemisphere), the autoch- 
thonal hypothesis might be tested by inquiring whether the 
primitive or earliest range of our species could possibly have 
remained unaffected by the serious and prolonged climatic 
vicissitudes to which they must needs have been subject; and 
whether these vicissitudes, and their natural consequences, 
may not suffice to explain the partial intermingling of the 
floras of North America and northern Asia, upon the supposi- 
tion of the local origin of each species. Let us bring to the 
inquiry the considerations which Mr. Darwin first brought to 
bear upon such questions, and which have been systematically 
developed and applied by the late Edward Forbes, by Dr. 
Hooker, and by Alphonse De Candolle. 
No one now supposes that the existing species of plants are 
of recent creation, or that their present distribution is the re- 
sult of a few thousand years. Various lines of evidence con- 
spire to show that the time which has elapsed since the close 
of the tertiary period covers an immense number of years ; 
and that our existing flora may in part date from the tertiary 
period itself. It is now generally admitted that about twenty 
per cent. of the Mollusea of the middle tertiary (miocene 
epoch), and forty per cent. of the pliocene species on the At- 
lantic coast still exist ; and it is altogether probable that as 
large a portion of the vegetation may be of equal antiquity. 
From the nature of the case, the direct evidence as respects 
the flora could not be expected to be equally abundant. Still, 
although the fossil plants of the tertiary and the post-tertiary 
of North America have only now begun to be studied, the 
needful evidence is not wanting. 
