84 ORIGIN AND CLASSIFICATION 



The pes of the Amphibia and Anomodontia agree in having a 

 distinct intermedium, tibiale, fibulare, and centrale, whereas in 

 other Reptiles these bones are not generally distinct ; in Mammals 

 the intermedium, fibulare, and centrale are distinct, and according 

 to Cope's interpretation there may be a distinct tibiale. 



Classification. — In the present condition of the world, mammals 

 have become so broken up into distinct groups by the extinction of 

 intermediate forms, that a systematic classification is perfectly 

 practicable. Most of the associations of species, which Ave call 

 " orders," and even the " suborders " and " families," are natural 

 groups. In isolating, defining, and naming them, we are really 

 dealing with facts of nature of a totally different order from the 

 artificial and fanciful divisions formed in the infancy of zoological 

 science. 



When, however, we pass to the extinct world, all is changed. 

 In many cases the boundaries of our groups become enlarged until 

 they touch those of others. New forms are discovered which 

 cannot be placed within any of the existing divisions. As the 

 horizon of our vision is thus expanded, the principles upon which a 

 scheme of classification is constructed must be altogether changed. 

 Our present divisions and terminology are no longer sufficient for 

 the purpose ; and some other method will have to be invented to 

 show the complex relationships existing between different animal 

 forms when viewed as a whole. The present time, pre-eminently 

 distinguished by the rapidly changing and advancing knowledge of 

 extinct forms, is scarcely one in which this can be done with any 

 satisfactory result ; so that all attempts to form a classification 

 embracing even the already known extinct species must be only 

 of a provisional and temporary nature. 



In systematic descriptions in books, in lists, and catalogues, and 

 in arranging collections, the objects dealt with must be placed in a 

 single linear series. But by no means whatever can such a series 

 be made to coincide "with natural affinities. The artificial character 

 of such an arrangement, the constant violation of all true relation- 

 ships, are the more painfully evident the greater the knowledge of 

 the real structure and affinities. But the necessity is obvious ; and 

 all that can be done is to make such an arrangement as little as 

 possible discordant with facts. 



The following table contains a list of the orders, suborders, and 

 families of existing mammals as recognised by the authors, and placed 

 in the order in which they will be treated of in this work. The 

 more important of the groups containing only extinct forms are 

 added in a different type, being interpolated, as near as may be, 

 among those that appear to be their existing relatives. 



A few explanatory remarks upon the mutual relations of some 

 of the principal groups mentioned in the table may be useful here, 



