DISCEIMINATION OF EPOCHS. 23 



To the believers in the doctrine of derivation, the obliteration of faunal 

 distinctions is not a cause of surprise. Such await with confidence the day 

 when complete phylogenies will be possible, and at present regard the inter- 

 ruptions in the succession of life as local only. Will the result then, be, 

 that paleontology will cease to be available in the definition of ages and of 

 deposits ? I answer no, on various grounds. Interruptions in the succes- 

 sion 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 extinc- 

 tion of some important type of life. Familiar examples are the Placodermi 

 of Paleozoic time, various reptilian groups of Mesozoic time, and the Amhly- 

 poda of the Tertiary. Each minor subdivision of time ofi'ers its own record 

 of persistences and extinctions of particular families and genera. 



Now, all departments of biology compel us to recognize the law of 

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

 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 their appearance, less distinct and more general in their 

 characters than they are later in their history. . 



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

 states that the types of one age have taken their origin from generalized 

 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 feature of every geologic age have been 

 the specialized types of those ages. 



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

 epochs, which discovery can safely build upon. The successive increments 

 of structure by which an important modification of animal type is intro- 

 duced, 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 characteristically represented 



