THE CHAIN OF LIFE TRACED BACKWARD 7 



we may shortly notice the grades of animals at 

 present existing, and then the evidence which we 

 have of their successive appearance in different 

 periods of geological time, in order that we may 

 eliminate all those of more recent origin, in so far 

 as the knowledge at present available will permit, 

 and restrict our consideration to forms which seem 

 to have been the earliest. In attempting this, we 

 may use for reference the table of geological periods 

 and animal types presented in the diagram facing 

 this chapter, which is based on one prepared by 

 Prof. Charles A. White, of the United States 

 Geological Survey, with modifications to adapt it 

 to our present purpose. In this table the leading 

 groups of animals are represented by lines stretch- 

 ing downward in the geological column of formations 

 as far as they have yet been traced. Such a table, 

 it must be observed, is always liable to the possibility 

 of one or more of its lines being extended farther 

 downward by new discoveries. 



The broadest general division of the Animal King- 

 dom is into back-boned animals (Vertebrates) and 

 those which have no back-bone or equivalent struc- 

 ture (Invertebrates).^ The former includes, besides 



* The twofold primary division now sometimes used, into 



