40 



BULLETIN UNITED STATES GEOLOGICAL SURVEY. 



[Yol.Y. 



that eitlier of the above-mentioned threfc criteria are variable quantities, 

 since discovery is constantly extending our knowledge of the distribu- 

 tion 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 founda- 

 tions 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. 



To the believers in the doctrine of derivation, the obliteration of fau- 

 nal distinctions is not a cause of sui-prise. Such await with confidence 

 the day when complete phylogenies will be possible, and at present 

 regard the interruptions in the succession of life as local only. Will the 

 result then be, that palseontology will cease to be available in the defini- 

 tion of ages and of deposits? I answer no, on various grounds. In- 

 terruptions in the succession of life in any given locality due to various 

 causes have doubtless often occurred, and have left traces in the crust 

 of the earth which are ineffaceable by discovery. But apart from this, 

 one fact in this history is patent both to the friends and to the opponents 

 of the doctrine of derivation. It is known that the world has witnessed, 

 at every stage of its history, the extinction of some important type of 

 life. Familiar examples are the Placodermi of palaeozoic time, the vari- 

 ous reptilian groups of Mesozoic time, and the Amblypoda of the Tertiary. 

 Each minor subdivision of time offers its own record of persistences and 

 extinctions of particular families and genera. 



l^ow, all departments of biology compel us to recognize the law of 

 classification, that the order of forms is from the less to the more gen- 

 eralized, from the simple to the more complex, and vice versa, whether 

 the lines of succession be those of descent or of creative order ; and 

 this law is true in time as well as in classification. It follows from this, 

 that all types of life are, at the time of theii' appearance, less distinct 

 and more general in their characters than they are later in their his- 

 tory. 



It also follows, as a consequence of the principle of descent, which 

 states that the types of one age have taken their origiu from general- 

 ized types of preceding ages, that there is no descent from the most 

 specialized types ; which is to say, conversely, that the genera, families, 

 and orders whose extinction has been a marked featui-e of every geo- 

 logic age have been the specialized types of those ages. 



We now have a clue to a basis of a definition for faunse, and hence for 

 epochs, which discovery can safely build upon. The successive incre- 

 ments of structure by which an important modification of animal type 

 is introduced preclude the possibility of exact determination of the 

 time at which such type may be said to have appeared. Even where 

 such a point may be arbitrarily fixed, the type must then be less char- 



