----------------------------+-------+--------+----+----+-----+---+------+---------+-------+--------+------+------- FIG. 23_a_.—Range of the Reptilia. Heavy lines indicate occurrence in North America. The periods of the Paleozoic era are the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian, in the order as given; those of the Mesozoic era are the Triassic, Jurassic, and Cretaceous; those of the Cenozoic era, the Eocene, Oligocene, Miocene, Pliocene, Pleistocene, and Recent. As a relic of an old classification we still often divide the Cenozoic into two quite arbitrary divisions, the Tertiary and the Quaternary, the latter including the Pleistocene and Recent only. The same may be said regarding the limits of each of these periods as of the eras; the sole problem is to make each period contemporaneous throughout the world, an exceedingly difficult problem, because no faunas or floras have ever been the same over the whole earth. Indeed, with the exception of some of the lowliest and most generalized forms, or man himself, no species are the same throughout the earth today. Inasmuch as we must depend upon the fossils in the rocks for the determination of the ages, where none is quite the same in strata of remote localities the identification becomes very difficult or even impossible. Nor are the periods, as accepted, of equal or even approximately equal duration; the Cretaceous period, for instance, was longer than all the remainder of the Mesozoic, longer perhaps than all the time which has elapsed since its close. The earliest animals with a backbone, or rather the earliest that we call vertebrates—for some vertebrates have no vertebrae—began their existence, so far as we know, in late Ordovician times, as attested by fish bones in Ordovician rocks of Colorado and Utah. The first evidences of the existence of air-breathing vertebrates in geological history are footprints preserved in the uppermost Devonian rocks of Pennsylvania. We call them amphibian because they resemble footprints associated with amphibian skeletons in later formations, and because the foot itself is still the most important difference we know between fishes and the higher animals. [Illustration: FIG. 24.—Permocarboniferous landscape (adapted from Neumayr) with restoration of _Eryops_, a stegocephalian amphibian ancestrally allied to the reptiles; and _Limnoscalis_, a cotylosaur (in water).]