56 
SUBDIVISIONS OF 
[Ch.V. 
Eocene species still flourish in the same latitudes where they 
are found fossil, they are species which, like Lucina divaricata, 
are now found in many seas, even those of different quarters of 
the globe, and this wide geographical range indicates a capacity of 
enduring a variety of external circumstances, which may enable 
a species to survive considerable changes of climate and other re- 
volutions of the earth's surface. One fluviatile species (Melania 
inquinata), fossil in the Paris basin, is now only known in the 
Philippine islands, and during the lowering of the temperature 
of the earth's surface, may perhaps have escaped destruction by 
transportation to the south. We have pointed out in the second 
volume (chap, vii.), how rapidly the eggs of freshwater species 
might, by the instrumentality of water-fowl, be transported 
from one region to another. Other Eocene species, which still 
survive and range from the temperate zone to the equator, may 
formerly have extended from the pole to the temperate zone, 
and what was once the southern limit of their range may now 
be the most northern. 
Even if we had not established several remarkable facts in 
attestation of the longevity of certain tertiary species, we might 
still have anticipated that the duration of the living species of 
aquatic and terrestrial testacea would be very unequal. For 
it is clear that those which now inhabit many different regions 
and climates, may survive the influence of destroying causes, 
which might extirpate the greater part of the species now living. 
We might expect, therefore, some species to survive several 
successive states of the organic world, just as Nestor was said 
to have outlived three generations of men. 
The distinctness of periods may indicate our imperfect infor- 
mation. — In regard to distinct zoological periods, the reader 
will understand, from our observations in the third chapter, 
that we consider the wide lines of demarcation that sometimes 
separate different tertiary epochs, as quite unconnected with 
extraordinary revolutions of the surface of the globe, and as 
arising, partly, like chasms in the history of nations, out of the 
present imperfect state of our information, and partly from 
