INTRODUCTORY 



classes and orders. It is obvious that a class which consists 

 of 10,000 recent species may call for more sub-dividing than 

 one which comprises scarcely one-third of that number. 



After all, the practical aim of our classifications is sorting 

 and grouping ; the ideal aim is that the system should be a 

 condensed expression of the phylogeny of the creatures dealt 

 with. There are many, and there will be still more classifica- 

 tions, all artificial and dependent upon the taxonomic value 

 which we happen to attribute to the various organs. But 

 there can be only one true or natural system, namely, that 

 which expresses every degree of affinity or descent of every 

 creature which has ever lived or is still living. To that 

 gigantic system, however, no classification will be applicable. 

 Each horizon will require its own classification, with its 

 necessarily arbitrary boundaries. 



The living forms are like the growing plants in a peat 

 bog. The latter are more or less separated by intervening 

 stretches of water into patches, islands, and little continents. 

 A foot or two lower down, or if the water-level sinks, the 

 patches change in extent and in numbers, some still remaining 

 apparently separate (" very old, generalised, isolated groups "), 

 but after all connected by the peat, the entangled mass of 

 countless generations. 



The sequence of the groups, although arranged as much as 

 possible in ascending order, is of necessity as unnatural as that 

 of the maps in an atlas. 



Concerning the generic names, I have been as conservative 

 as possible, using those which we are familiar with in treatises 

 of general zoology and comparative anatomy. The book which 

 speaks of Molge, Tiliqua, Procavia, and Morunga, but does not 

 know Triton, Cyclodus, Hyrax, and Trichechus, has fortunately 



I 



