THE PALEONTOLOGY OF ARRESTED EVOLUTION 



BY RUDOLF RUEDEMANN 



Address by the President of The Paleontological Society, Albany, December 



1916 



When last year your society honored me in electing me your 

 president, it was my first desire to present in my address a general 

 survey of the work in which I have been most interested, namely, 

 the analysis of the faunas of the graptolite shales and their corre- 

 lation. Fear that this theme might not be of sufficiently general 

 interest, and still more the discovery of new facts in the last two 

 years, calling for further investigations of the Ordovician shales, 

 have influenced me to select another problem for presentation in 

 which I have also been interested for some years, This is the 

 Paleontology of Arrested Evolution, as seen in the persistent types 

 and relicts. Unfortunately, this problem, also, as here presented, is 

 still far from being solved, and I have to beg your indulgence for 

 obvious shortcomings. 



INTRODUCTION 



The student of fossil faunas, both for stratigraphic and biologic 

 ends, can not fail to be struck by the remarkable differences in the 

 ranges of the various species, genera, famihes and higher groups. 

 As a rule, the broad stream of organisms that passes our view in 

 geologic history changes quite regularly from formation to fomia- 

 tion, but there are certain species and genera which reappear again 

 and again without any or with but slight variation, often in totally 

 different faunal associations. These types which thus remain 

 unchanged throughout a number of formations have been desig- 

 nated as persistent or conservative types, in contradistinction to the 

 variable types. As pointed out by Osborn {The Age of Mammals, 

 p. 7), it was early observed by Huxley,^ Cope and others, that 

 Cuvier's broad belief in a universal law of perfection was erron- 



^ Huxley, in his anniversary address to the Geological Society of 1862 on 

 "Geological Contemporaneity and Persistent Types of Life" (reprinted in 

 " Lay Sermons, Addresses and Reviews," 1906, p. 176) has, in a general way, 

 established the existence of persistent types of structure, and from a general 

 survey of the development of the organic classes reached the conclusion that 

 the common doctrines of progressive modification which suppose that modi- 

 fication to have taken place by a necessary progress from more or less 

 embryonic forms, or from more or less generalized types within the limits 

 of the period represented by the fossiliferous rocks, are negatived by the 

 evidence of the persistent types, for it either shows no evidence of any 

 such modification, or demonstrates it to have been very slight. 



Our purpose and scope of investigation in the present paper is different 

 from that pursued bv Huxley. 



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