42 LAND MAMMALS IN THE WESTERN HEMISPHERE 



of one and the same animal have been assigned to entirely 

 different groups, just because no one could have ventured, in 

 advance of experience, to suppose that such a skull and teeth 

 could belong to a creature with such feet. In all these cases 

 (and they are few) the error has been finally corrected by 

 the discovery of the skeleton with all its essential parts in 

 their natural connection. 



While the number of complete skeletons of Tertiary mam- 

 mals as yet collected is comparatively small, it is often possible 

 to construct a nearly complete specimen from several imperfect 

 ones, all of which can be positively shown to belong to the same 

 species. Such composite skeletons are almost as useful as 

 those in which all the parts pertain to a single individual, 

 though in making the drawings it is not easy to avoid slight 

 errors of proportion. It must not be supposed that no success- 

 ful restoration of missing bones is practicable ; on the contrary, 

 this can often be done very easily, but only when all the essential 

 parts of the skeleton are known. 



Even if an unlimited number of perfect skeletons were 

 available, of what use would they be ? A skeleton is a very 

 different looking object from a living animal, and how is it 

 possible to infer the latter from the former ? Do the many 

 restorations of extinct mammals which this book owes to the 

 skill of Mr. Horsfall and Mr. Knight deserve any other con- 

 sideration than that due to pleasing, graceful or grotesque 

 fancies, with no foundation of solid fact ? To answer these 

 questions, it is necessary first to consider the relations of the 

 bony structure to the entire organism and then to discuss the 

 principles in accordance with which the restorations have been 

 made. 



The skeleton is far from being merely the mechanical frame- 

 work of the animal. Such a frame-work it is, of course, but 

 it is much more than that ; it is the living and growing expres- 

 sion of the entire organism and is modified, not only by age, 

 but by the conditions of the environment and accidental cir- 



