INTRODUCTION. 



THE past history of the vertebrated animals as revealed by 

 fossils is not merely a subject of absorbing interest in 

 itself: it is also of prime importance in Biology from the 

 possibilities it affords for elucidating some of the most funda- 

 mental principles in the evolution of life. Since these organisms 

 represent the highest phase of development to which the animal 

 kingdom has attained, 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 numerous 

 and satisfactory fossils. Moreover, the skeleton of the Verte- 

 brata 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. 



Order of Succession. 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 noto- 

 chord, 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 passage 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. Frag- 

 ments either of mammals or of reptiles which approach the 



