. Water reptiles of the past and present . knowledge of living and extinct organisms, and of therocks which contain fossils, has made the problems of classificationmuch more complex than they seemed to be formerly. It is mucheasier to classify organisms or rocks, or anything else, when weknow only a few isolated kinds—much easier to draw divisionallines. Geological history is like a volume in which pages, leaves,and even whole chapters either are missing or are printed in lan-guages which we understand only imperfectly. Where the lost orunknown parts belong, the largest divisions may be made,
. Water reptiles of the past and present . knowledge of living and extinct organisms, and of therocks which contain fossils, has made the problems of classificationmuch more complex than they seemed to be formerly. It is mucheasier to classify organisms or rocks, or anything else, when weknow only a few isolated kinds—much easier to draw divisionallines. Geological history is like a volume in which pages, leaves,and even whole chapters either are missing or are printed in lan-guages which we understand only imperfectly. Where the lost orunknown parts belong, the largest divisions may be made, andpossibly such may have been epochs of unusual activity, of dias-trophic changes which greatly accelerated organic evolution. Noone can say just where the dividing line should be drawn betweenthe rocks of Paleozoic and Mesozoic age, or between the Mesozoicand Cenozoic, for there is none; the most that we can hope for is tomake the divisions everywhere in the world conform to those firstmade for local reasons. 44 THE AGE OF REPTILES 45. 46 WATER REPTILES OF THE PAST AND PRESENT The periods of the Paleozoic era are the Cambrian, Ordovician,Silurian, Devonian, Carboniferous, and Permian, in the order asgiven; those of the Mesozoic era are the Triassic, Jurassic, andCretaceous; those of the Cenozoic era, the Eocene, Oligocene,Miocene, Pliocene, Pleistocene, and Recent. As a relic of an oldclassification we still often divide the Cenozoic into two quitearbitrary divisions, the Tertiary and the Quaternary, the latterincluding the Pleistocene and- Recent only. The same may besaid regarding the limits of each of these periods as of the eras;the sole problem is to make each period contemporaneous through-out the world, an exceedingly difficult problem, because no faunasor floras have ever been the same over the whole earth. Indeed,with the exception of some of the lowliest and most generalizedforms, or man himself, no species are the same throughout theearth today. Inasmuch as we mu
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