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phenomena of evolution in the Paleozoic were distinct from those of 

 later periods, having taken place with a rapidity paralleled only in 

 later times in unoccupied fields, like Steinheim. 



The hypothesis of Wagner, that an unoccupied field is essential 

 for the evolution of new forms, gains immensely in importance, if 

 it is practicable to apply it to the explanation of the morphic phe- 

 nomena that have been observed. Every naturalist must see at once, 

 by his own special studies, that this is a reasonable explanation of 

 the rapid development of types in new formations and of the sud- 

 den appearance of so many of the different types of invertebrates in 

 the Paleozoic. 



Newberry's theory of cycles of sedimentation shows that the sud- 

 den appearance of types is inexplicable, except upon the supposition 

 that their ancestors retired with the sea between each period of de- 

 posit, and again returning after long intervals of absence made their 

 appearance for the first time in a given littoral fauna bearing 

 changed characteristics and different structures acquired by the 

 migrations of their own stock in unknown seas. 



With this explanation and that of Wagner the facts that have 

 been observed fully coincide, and amply explain the phenomena, 

 both of sudden appearance in the first deposits of formations, and 

 subsequent quick development in the necessarily unoccupied 

 habitats. The researches of Barrande, Alexander Agassiz, Bigsby, 

 Gaudry and many others, show us that this must have been especially 

 true of the Paleozoic as compared with subsequent periods. 



In order to make a logical and generalized picture of correspond- 

 ence between all the changes in the life of a nautilian close-coiled 

 shell and the life of its own group accord exactly with the facts, care 

 must be taken to limit it to groups quickly evolved, and these ex- 

 clusively Paleozoic. Among Nautiloidea there are no series trace- 

 able directly to arcuate forms after the expiration of the Carboni- 

 ferous. This is the common story, and we can see that the series 

 must have risen very rapidly during the Paleozoic, branching out 

 on every side from the common ascending trunk of the straight and 

 arcuate forms. The same is true of the Ammonoidea in the 

 Silurian, but only one short series, the Nautilinidae, arises from the 

 common trunk of straight cones. The close-coiled shells of this 

 one family became the stock form for the whole of the Ammonoidea. 



The Nautiloidea of the Mesozoic are all nautilian forms, and 

 their genetic series do not present the rapid changes of form 



