Ch. V.] IN DIFFERENT TERTIARY PERIODS. 59 



those now living, and that, as we approach the superior and 

 newer sets of strata, we find the remains of existing animals 

 and plants in greater abundance. It is almost as difficult to 

 find an unknown species in some of the newer Pliocene de- 

 posits, although very ancient and elevated at great heights 

 above the level of the sea, as to meet with recent species in 

 the Eocene strata. 



This increase of existing species, and gradual disappearance 

 of the extinct, as we trace the series of formations from the 

 older to the newer, is strictly analogous, as we before observed, 

 to the fluctuations of a population such as might be recorded 

 at successive periods, from the time when the oldest of the 

 individuals now living was born to the present moment. The 

 disappearance of persons who never were contemporaries of the 

 greater part of the present generation, would be seen to have 

 kept pace with the birth of those who now rank amongst the 

 oldest men living, just as the Eocene and Miocene species are 

 observed to have given place to those Pliocene testacea which 

 are now contemporary with man. 



In reference to the organic remains of the different groups 

 which we have named, we may say that about a thirtieth part 

 of the Eocene shells are of recent species, about one-fifth of 

 the Miocene, more than a third, and often more than half, of 

 the older Pliocene, and nine-tenths of the newer Pliocene. 



Mammiferous remains of the successive tertiary eras. But 

 although a thirtieth part of the Eocene testacea have been 

 identified with species now living, none of the associated mam- 

 tniferous remains belong to species which now exist, either in 

 Europe or elsewhere. Some of these equalled the horse, and 

 others the rhinoceros, in size, and they could not possibly have 

 escaped observation, had they survived down to our time. 

 More than forty of these Eocene mammifers are referrible to a 

 division of the order Pachydermata, which has now only four 

 living representatives on the globe. Of these, not only the 

 species but the genera are distinct from any of those which 

 have been established for the classification of living animals. 



