108 NEW YORK STATE MUSEUM 



eous, and these authors began to perceive the difference between 

 persistent primitive types (Huxley) and progressive or advancing 

 types. 



The causes underlying the variation of species have been, since 

 Darwin's time, and are still the storm center of discussions of 

 evolution; the negative side of the question or the continuous 

 failure to vary has been thought hardly worthy of notice, partly 

 because its frequency impresses itself principally upon the paleon- 

 tologist only, and partly because it seems to promise little of impor- 

 tance to that discussion. Various facts, however, have come to 

 our notice which bear on this problem and appear to invite a brief 

 investigation of the causes of persistence, principally as it appears 

 among invertebrate fossils. 



Persistence of types, generic and specific, presupposes the avoid- 

 ance of two antithetic causes of extinction ; to be persistent a 

 type must not be variable, for if it develops into another type it 

 becomes extinct as the former type and, though living in its 

 descendants, it is relatively extinct; further, it must not become 

 absolutely extinct through death of the race. Variability is the 

 general means to adaptation and progressive development, and so 

 persistent types must, generally speaking, be in the peculiar posi- 

 tion of being not so highly vitalized or alive as the developing and 

 climacteric forms, and yet still too vigorous to become extinct or 

 permit the death of the race. 



It seems to us that the generic units permit the clearest survey 

 of the persistence of types. We shall therefore make this unit the 

 principal subject of our investigation. 



The ranges of the persistent genera, as here given, are extracted 

 from Zittel-Eastman's Textbook of Paleontology (1913), and I 

 have here defined as persistent all genera which pass through more 

 than two periods. 



Tables of Persistent Types 



The most striking cases of persistence are among the Foram- 

 inifera, as follows : 



Ordovician — Recent Saccamina 



Silurian — Recent Lagena, Nodosaria 



Carboniferous — Recent Ammodiscus, Dentalina, Endothyra, Lituola, 



Rheophax, Textularia, Valvulina, 



Ammobaculites 



Carboniferous — Tertiary .... Nummulites 



Triassic — Recent Bulimina, Cristellaria, Frondicularia, Glandu- 



lina, Globigerina, Lingulina, Marginulina, 

 Orbulina, Polymorphina, Vaginulina, Mili- 

 ola, Placopsilina, Trochamminoides 



