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- 
miferous 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. 
