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Show 58 NUMERICAL PROPORTION oF RECENT siiELLS (Ch. V. f th Miocene nnd Pliocene three simi 1a r su bd.L v·i si·O n s' both o . e f the middle period h the formatiOns o epochs. In t at case, 1 • h the assemblaae of 'd d ~ the types from w uc o must be consi ere a:s . d' tely antecedent or sub~ organic remam. s m. tl1 e oa roups Immc La sequent will diverge. · t oif denadure in all t ta form a common potn r . The Recent s .. ra . reat advantage from beginning our t· ies _We derive one g . . eoun r · . b a comparison of the fossils of the classification of formatwns y . . 1 1 . h tl pecies nowhvmg, name y, t1e ac~ m re recent strata wit 1e s . · 0 • t of departure 1 n every reg10n of the quisition of a common pom ' b d' · .c ple if strata should e 1scovered tn globe Thus, lOr exam ' . . · 1 A .· a containing the same small proportion India or Sout 1 meLIC ' . . . h 11 .c0und in the Parts basm, they also m1ght of recent s e s as are l' • • be terme d E~ ocene, and ' on analogous data, an app. roximatiO.n m·w 11 t b emad e t o the relative dates of strata placed m the. arctic dan o tropt· cal regw· ns , or the comparative age ascertamed of European d epos its ' and those which are trodden by our anti-podes. There might be no species common :o. the two groups; yet we might infer their synchronous origm from the common relation which they bear to the existing state of the animate crea tI. on. We may afterwards avail ourselves of the dates thus established, as eras to which the monuments of preceding periods may be referred. . . Numerical proportion of recent shells m the different Ter-t w· ry perw· ds .-There are seventeen species of shells discovered, which are common to all the tertiary periods, thirteen of which are still living, while four are extinct, or only known as fossil*· These seventeen species show a connexion between all these geological epochs, whilst we have seen ~hat a mu~h greater number are common to the Eocene and Miocene perwds, and a still greater to the Miocene and Pliocene. . . We have already stated, that in the older tertiary formatiOns, we find a very small proportion of fossil species identical with • See the Tables ~f M. De1>ha!es in Appsndi~ I, {:h, 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 deposits, 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 Mioc~ne 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. Mammife·rous 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 mammiferous 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. |