38 LAND MAMMALS IN THE WESTERN HEMISPHERE 



the different processes. In whatever manner the preservation 

 may have been effected, there is great difference in the rel- 

 ative abundance and completeness among the fossils of the 

 various kinds of mammals which were living at the same time 

 and in the same area. It need hardly be said, that the more 

 abundant any species was, the better was the chance of its 

 being represented among the fossils ; hence, gregarious species, 

 living in large herds, were more likely to be preserved than 

 those which led a solitary existence, or were individually rare. 

 Most of the hoofed mammals are and apparently always have 

 been gregarious, and are therefore much better represented 

 among the fossils, and are, in consequence, better known than 

 the beasts of prey, which, of necessity, were individually less 

 numerous and generally solitary in habits. Not only this, 

 but large and medium-sized mammals, with strong and heavy 

 bones, were better fitted to withstand the accidents of entomb- 

 ment and subsequent preservation than small creatures with 

 delicate and fragile skeletons. The mere dead weight of over- 

 lying sediments often crushes and distorts the bones, and the 

 movements of uplift, compression, folding and fracture, to which 

 so many strata have been subjected, did still further damage 

 to the fossils. The percolating waters, which for ages have 

 traversed the porous rocks, often attack and dissolve the bones, 

 completely destroying the minute ones and greatly injuring 

 those which are massive and strong. In consequence of all 

 those accidents it frequently happens that only the teeth, 

 the hardest and most resistant of animal structures, and it 

 may be the dense and solid jaw-bones, are all that remain 

 to testify of the former existence of some creature that long 

 ago vanished from the earth. Very many fossil mammals are 

 known exclusively from the teeth, and it is this fact which makes 

 the exact study of teeth so peculiarly important to the palae- 

 ontologist. 



In view of all these facts, it is not surprising that con- 

 cerning the history of many mammalian groups we have but 



