compared with the New World. 259 
seas, appears to have reached a state of remarkable fixity, a charac- 
ter belonging to all the phenomena of the present era. 
Nore 2. The atmosphere is now richer in oxygen than in geolo- 
gical times ; this excess corresponds to the volume of carbonic acid 
which served for the nutrition of the plants of primitive ages. It 
corresponds also to the quantity of carbon and hydrogen contained in 
the carbonaceous remains left by ancient vegetables. The quantity 
of carbonic acid now existing in the atmosphere is not more than 
from four to six ten-thousand parts. To compensate for this trifling 
proportion of carbonic acid, some of it is continually escaping from 
the interior of the earth, as well as from volcanoes. Continental 
waters carry some of it along with them, and mineral and thermal 
waters exhale more or less considerable quantities of it. It is pro- 
duced also at the expense of the carbonated hydrogen existing in the 
air, the decomposition of which takes place from the electrical dis- 
charges of the clouds, so frequent under the tropics. 
Nore 3. The greater extent of seas, at different geological epochs, 
is demonstrated by the marine organic remains which they have left 
in sedimentary formations. These remains are in general more re- 
mote from present seas as they belong to more ancient periods ; it is 
the same with regard to their height above the level of the sea. 
Marine organic remains are referable principally to Zoophytes and 
Mollusks. They are distinguished into—1st, pelagic species, or such 
as have lived in deep seas; 2dly, others which have lived at a little 
distance from the shores; and lastly, such as lived only near the 
shore, and which on that account have been named lUitéoral. 
The different zones relative to the depth of water at which the 
ancient Zoophytes and Mollusks lived, likewise exist in modern seas ; 
each species is confined, by the conditions of its existence, to a de- 
terminate region. 
Nore 4. Large continents are so recent, that they only date from 
the most ancient tertiary formations, that is, from the period when 
interior seas became separated from the ocean, Continents were 
formed by degrees during a long series of successive upheavals and 
sinkings. Other facts confirm their tardy appearance, and prove 
that they were originally islands, and these islands of small: extent. 
Primitive vegetation has been essentially composed of acrogenous 
cryptogamic vegetables, which now flourish only in the small islands 
of the equatorial zone. They are particularly abundant in those 
where the maritime climate has reached its maximum ; accordingly 
these vegetables are characteristic of the transition and coal periods. 
The flora of the latter formations contained 250 species of ferns, 
while the whole of Europe affords scarcely 50 of them, that 
is 50 of the fossil species of this family. On the other hand, 
the gymnosperms do not exceed 25 species in Europe, while 
the flora of the coal formations has upwards of 120 of them, or nearly 
__ five times the amount. This flora, like that of the equatorial islands 
R 2 
