CHAP, i.] SPECIALISATION OF THE MAMMALIA EXPLAINED. 11 



The idea of percentages of living among extinct 

 species implied by the etymology of the first four terms 

 has been so modified by modern discoveries that I have 

 substituted for the more usual definitions those which 

 are in harmony with our present knowledge, taking the 

 most highly specialised animals as my guides, which 

 alone have changed swiftly enough to be used to classify 

 the subdivisions of the Tertiary period. The Prehistoric 

 and Historic stages constitute the Eecent division of 

 Lyell and most other authors, and the evidence on which 

 they, as well as the Pleistocene, are included in the 

 Tertiary period will be placed before the reader in the 

 course of this work. 



The Specialisation of the Mammalia explained by the 

 Theory of Evolution. 



The argument in favour of the theory of evolution, 

 founded on the specialisation of mammalian life, in its 

 progress from the Eocene times down to the present 

 day, seems to me so strong as to be almost irresistible. 

 The facts are put into a tangible shape in the accompany- 

 ing diagram (Fig. 1), in which it will be observed that the 

 orders, families, genera, and species fall into the shape 

 of a genealogical tree, with its trunk hidden in the 

 Secondary period, and its branchlets, the living species, 

 passing upwards from the Pleiocene, Pleistocene, and 

 Prehistoric stages to the present time a tree of life with 

 living mammalia for its fruit and foliage. Were the 

 extinct species taken into account, it would be seen that 

 they fill in the intervals separating one living form from 

 another, and that they grow more and more like the 

 living forms as they approach nearer to the present day. 



