380 W. B. DAWKINS ON THE CLASSIFICATION OF THE 



Mammalia, by applying the same method by which the Pleistocene 

 and the Prehistoric periods have already been defined*. 



2. The Value of Invertebrates and Vertebrates in Classification. 



The Eocene, Miocene, and Pliocene divisions of the Tertiary period 

 are based npon the varying percentages of living MoUusca in a com- 

 parison of three thousand fossil with five thousand living forms ; and 

 the term Pleistocene t was subsequently invented to imply a nearer 

 approximation to existing nature. Since that time the number of 

 living MoUusca in the various groups has been materially altered by 

 a wider area of observation, and it has been found impossible to map 

 off the Pleistocene from the Pliocene on the one hand, and from the 

 Prehistoric on the other, by their means, since by far the larger 

 majority of the Mollusca now living in our seas date from the Pli- 

 ocene age. In other words, the Mollusca have not changed with 

 sufficient swiftness to allow of their being used to classify the later 

 Tertiary divisions. Nor have the invertebrate faunas of Europe 

 generally changed fast enough to mark the later Tertiary divisions. 

 They arrived at their equilibrium towards the close of the Secondary 

 period. The lower vertebrates also had passed through their most 

 important biological changes before the beginning of the Tertiary, 

 and in the Eocene age we find ourselves confronted by fishes, am- 

 phibians, and reptiles belonging, for the most part, to living genera. 

 It is only when we appeal to the highest of all, the Placental Mam- 

 malia, that we are able to note specific changes which are sufiiciently 

 rapid for the purposes of classification. They appear in the Eocene 

 age, as Prof. Gaudry happily terms it, '^ en pleine evolution "J, and 

 were in the same transitory condition throughout the greater part 

 of the Tertiary that is seen in the lower animals of the Primary and 

 Secondary periods. 



3. The Principle of Classification. 



The fossil Mammalia of Europe, and (so far as I am able to judge 

 from the works of Marsh, Leidy, and Cope) of America also, pre- 

 sent stages of specialization which coincide with the geological 

 divisions, and enable us to attach new definitions to the old names, 

 as follows : — 



* International Congress of Prehistoric Archgeology. Norwich volume, 

 t Lyell, ' Principles,' 1st edit. iii. 1833 ; ' Antiquity of Man,' 1st edit. p. 3. 

 J Les Enchainements. 



