368 Revieics — A. Smith WoochvarcVs Vertebrate PalcBontology. 



they appear latest and become dominant latest in the geological 

 series. The evolution of all, except perhaps the larger groups, is 

 thus contemporaneous with the deposition of the series of rocks 

 which yield the most numei'ous cind satisfactory fossils. Moreover, 

 the skeleton of the Vertebrata is more intimately related to the 

 soft parts than that of any of the lower forms of life ; hence the 

 greater value of such remains as can be fossilized in determining 

 the precise nature of the original animals to which they belonged." 



'•■ The order in which the various divisions of the Vertebrata appear 

 in geological time, according to present knowledge, depends entirely 

 upon their degree of specialization — the simplest first, the more 

 complex afterwards. The earliest organisms, which seem to have 

 possessed a notochord, occur in the Upper Silurian ; and none of 

 these ancient types hitherto discovered exhibit either a lower jaw 

 or true paired limbs. Typical fishes appear first in the j)assage beds 

 between the Upper Silurian and Lower Devonian, and become 

 abundant in the latter formation. Batrachia begin to occur in the 

 Lower Carboniferous, and are dominant in the Permian. Undoubted 

 reptiles are found in the Lower Permian, but do not become 

 dominant until the Triassic and Jurassic. Fragments either of 

 mammals or of reptiles, which approach the latter extremely closely, 

 are met with in the Triassic, and there are undoubted small 

 mammals in the Jurassic ; but these are insignificant before the 

 Tertiary period. Birds occur first in the Upper Jurassic, but both 

 on this horizon and in the Cretaceous they retain conspicuous 

 characters of their ancestry which have subsequently disappeared ; 

 they seem to have become dominant contemporaneously with the 

 mammals at the beginning of the Tertiary period. 



" Bange in Time. — Gradual evolution — whether in the form of 

 progression, retrogression, or differentiation — is usually observable, 

 even in the minor divisions, when their range can be traced through 

 the geological formations ; and characters change more or less 

 slowly in proportion to their magnitude. In all satisfactorily 

 known instances, an order exhibits a longer geological range than 

 any of its contained families ; its family-types persist for a longer 

 time than any of the genera grouped under them ; whilst the genera 

 themselves remain for a more extended period than the species. 

 A highly specialized member of any division is also more liable 

 to early extinction than its more generalized congeners, probably 

 from its less adaptability to changes in the environment. An illus- 

 trative case may be cited. The order Ungulata (hoofed-mammals) 

 is known to range from the very earliest Eocene strata to the 

 present day. The family Equidee (horses), as commonly understood, 

 arises in the Upper Miocene ; the typical species of the surviving 

 genus Equus appears first in the Lower Pliocene. One genus of 

 this family (Hipparion), with highly complex teeth, was restricted 

 in its range to the Upper Miocene and Lower Pliocene periods, 

 while another (Hippidiim) , with much specialized nasal region, 

 had only a brief existence in South America ; whereas Eqmis itself, 

 with more normal teeth and rostrum, has survived from the Lower 



