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Show 56 • SUBDIVISIONS OF ,[Ch.V. . . . the same latitudes where they Eocene species sttll flourish 1.n 1 . h like Lttcina divaricata, d .£' •1 t1 are species w nc ' are foun J.OSSI ' ley 1 f different quarters of d . y seas even t lose o are now foun 10 man ' h'cali·anue indicates a capacity of b d t1 · wide geourap 1 o the glo e,an .us f 0 1 circumstances, which may enable d . variety o externa en urmg a . .d . ble changes of climate and other re-ccies to survive consi eta . . . . av oslpu u.o ns o f t 1 ·tl, surface. One fluviatile species (Melama le eat 1 s . . . . 1. 1 Paris basm IS now only known m the inquinata)' fossi m t 1e ' Plu.l l.p pm. e . 1 d d during the lowering of the temperature IS an s, an . of the earth's surf :a ce, rna Y Perhaps have e.s caped de.s tructiOn by transportatiO· n to tl1 e so uth · We have pomted out m the secon. d volume ( c1 1 ap. vu·· . ) ' 11 o w rapidly the eggs of freshwater species ,m1· u 11 t, b y th e m· s trumentality of water-fowl, be transported 5 from one regi•O n t o another · Other Eocene species, which still survi·v e an d 1· ange ft·om the temperate zone to the equator, may formerly have extended from the. p~le to tl~e temperate zone, and what was once the southern hm1t 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 a uatic and terrestrial testacea would be very unequal. For itq is clear that those which now 1.1 1 h a bI' t many d'I.tnre rent regw. ns 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 pe·riods may indicate our imperfect information.- 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 Ch. v.] TilE TERTIARY EPOCII. 57 the irregular manner in which geological memorials are preserved, as already explained. We have little doubt that it will be necessary hereafter to intercalate other periods, and that many of the deposits, now referred to a single era, will be found to have been formed at very distinct periods of time, so that, notwithstanding our separation of tertiary strata into four groups, we shall continue to use the term contemporaneous with a great deal of latitude. We throw out these hints, because we are apprehensive lest zoological periods in Geology, like artificial divisions in other branches of Natural History, should acquire too much importance, from being supposed to be founded on some great interruptions in the regular series of events in the organic world, whereas, like the genera and orders in zoology and botany, we ought to regard them as invented for the convenience of systematic arrangement, always expecting to discover intermediate gradations between the boundary lines that we have first drawn. In Natural History we select a certain species as a generic type, and then arrange all its congeners in a series, according to the degrees of their deviation from that type, or according as they approach to the characters of the genus which precedes or follows. In like manner, we may select certain Geological formations as typical of particular epochs ; and having accomplished this step, we may then arrange the groups referred to the same period in chronological order, according as they deviate in their organic contents from the normal groups, or according as they approximate to the type of an antecedent or subsequent epoch. If intermediate formations shall hereafter be found between the Eocene and Miocene, and between those of the last period and the Pliocene, we may still find an appropriate place for alJ, by forming subdivisions on the same principle as that which has determined us to separate the lower from the upper Plio~ cene groups. Thus, for example, we might have three divisions of the Eocene epoch,-the older, middle, and newer; and |