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 we 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, 
