The Principles of Palaeontology. 183 



to the distance gained from that form ; this is readily under- 

 stood. The radicle form being itself regressive, finds itself 

 situated by its characters near a form much more primitive; 

 matters are conducted thus as though nature, seeking to econo- 

 mize time and force, avoided the circuitous route through forms 

 which, though higher and more complex, are not enduring. But 

 the earlier types of this new series still may show, in the history 

 of their development, some traces of this complicated evolution. 



The histor}^ of the various branches of tht3 Ammonitidse pre- 

 sents a certain number of these radicle forms with simplified 

 characters ; thus Psilooeras plaaorhe is an Ammonite destitute 

 of ornaments, with rounded volutions not embracing. This is 

 the primitive stock of the long series of Arietidse. Eut the 

 suture lines of the adult are less complicated than those of the 

 young (Neumayr). 



If these phenomena allow us to separate, in certain cases, 

 relationships within the limits of a given series, and to connect 

 the series one with another, they also leave us confronted by new 

 difficulties in the solution of the problem. The apparent resem- 

 blance between regressive geratologic forms and the simple 

 primitive forms sometimes gives rise to a perplexity in the deter- 

 mination of radicle forms, the point of departure for new series. 

 This is the condition at the present day in regard to the Ammon- 

 ites of the chalk, which only recently have been studied in 

 their entirety with as profound analysis as have the Ammonites 

 of the Jurassic. 



§ 4. Method of Geologic Continuity. 



Application of the principle of continuity. — The third method 

 applies perhaps less than the other two to objections of a specu- 

 lative order, but in practice it also presents numerous difficulties. 

 It consists in the investigation of the correlation of the series of 

 fossil forms with the order of successi m of the layers containing 

 them ; in other words, it is the historic and chronologic study of 

 the group. If we have sufficient material at our disposal, and 

 if, on the other side, the chronologic order of the deposits is well 

 established, we ought to be able to follow the transformations 

 of all the types through the epochs, to determine whether any 

 one form is derived by progression or regression from a more 



