22 VEETEBRATA OF THE TERTIARY. 



disappears without leaving representatives in later ones. With this basis of 

 fact, which naturally enough has been furnished by the longest explored 

 and best-known portion of the earth, Europe, we turn to other lands, with 

 the hope of obtaining further light upon a subject so full of mystery. 

 These types of life, did they originate in a single centre, from which they 

 disseminated themselves; and, if so, did each form originate in a region of 

 its own, or not? Or, did the same types of generic structure appear at 

 diflferent points on the earth's surface independently ; and, if so, whether 

 cotemporaneously or at different times ? 



For a solution of these and similar questions, we naturally look to a 

 comparison of the facts first established, with those obtained more recently 

 by exploration in other regions. In this quest, no portion of the earth 

 offers greater promise of results than America. As the second great con- 

 tinent, separated from the other by the greatest possible water surface, we 

 anticipate the widest diversity in the character of its life-history. If the 

 types of life have originated independently, we will find evidence of it by 

 studying American paleontology ; if their origin has been through gradual 

 modification, America should furnish us with many intermediate faunee. 



Let us first consider the nature of the evidence on which we should 

 rely in classifying faunse and the deposits which contain them. We are 

 accustomed, at present, to rely for our definitions upon all the faunal peculi- 

 arities upon which we can seize: the period of appearance of certain types; 

 the duration of certain types; and the disappearance of certain types, 

 depending on orders, families, and genera for the major divisions, and species 

 at a given locality for the lesser. It is, of course, evident that either of the 

 above-mentioned three criteria are variable quantities, since discovery is 

 constantly extending our knowledge of the distribution of types. Hence 

 the definitions are empirical and temporary. We must, then, if we desire a 

 stable system, examine the principles involved, and endeavor to discover 

 definitions which stand on stronger foundations than those which we now 

 possess. 



As a matter of fact, the old definitions of epochs and periods are con- 

 tinually invalidated by new discoveries. As a matter of theory, this should 

 be the case. 



