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 

 separaledifferent tertiary epochs, Us 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 



