258 



BIOLOGICAL LECTURES. 



White River" 5 . 



Uinta 



Bridger 



?Homacodonts 



Wasatch 



?Trigonolestes 



members of the suborder, which are in many ways exceedingly 

 peculiar. When, however, the extinct members of the group 

 are taken into account, we find that every item of almost any 

 definition that can be framed will be transgressed by one or 

 other genus. This difficulty of definition is, of course, no objec- 

 tion to the classification proposed ; on the contrary, it is the 

 inevitable result of greater completeness in phylogerietic his- 

 tory. Definition is easy in exact proportion to the isolation of 

 the group defined, but with a knowledge of the history and 

 ancestry of a group its isolation disappears. As the bounda- 

 ries between connected groups grow hazy, definition grows 

 more and more difficult, until at last it becomes impossible. 



Another necessary consequence of these conclusions is that 

 the Tylopoda and Pecora are but very remotely connected and 

 can have no common ancestor later than the middle or lower 

 Eocene. Hence the many characters which these two sub- 



