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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 Nautilinide, 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 
