146 
If we admit such possibilities, and then find 
similar phenomena in the paleozoic epoch, we 
shall no longer need our first picture, but can 
construct a far more natural one. 
The Nautiloidea will not then present them- 
selves as a simple chain of being, but as they 
really were, eral distinct stocks, or grand 
series, and each of these grand series divisible 
into many smaller lines of genetically connect- 
ed forms. In the Cambrian, or perhaps ear- 
lier, some of these do not have close-coiled 
forms at all; some of them have: but all, ex- 
cept the most primitive series, which are com- 
posed wholly of straight or arcuate forms, have 
some close-coiled species. These we can often 
trace directly with the greatest exactness, both 
by their development and by the gradations 
of the adult forms, to corresponding species 
among the straight Orthoceratites. 
The series we have described above, from 
Orthoceras to Goniatites, compares closely 
with any single genetic series of the Nautiloi- 
dea, and shows that this ordinal type arose 
very suddenly in the protozoic, and evolved 
true nautilian shells in the Cambrian or earlier. 
The Ammonoidea evolved from the nau- 
tilian forms of the Cambrian into series, which 
are structurally much more distinct from each 
other in the paleozoic than any groups of the 
same value (i.e. genera) in the succeeding 
formations, and thus, in different but equally 
plain characters, teach us that they also had 
a quicker evolution within that period itself 
than in the later formations. Either this was 
the case, or else the Ammonoidea must have 
been created in full possession of an organiza- 
tion only attained by similar parallel series of 
congeneric, close-coiled nautiloids, after pass- 
ing through all the intermediate transforma- 
tions above described. Here is a curious fact: 
though taxonomically equal, we cannot com- 
pare the order of the Ammonoidea with the 
whole of the Nautiloidea, but only with a more 
or less perfect single series of that order. ‘This 
phenomenon fully accords with the true picture 
of the genetic relations. The remarkably sud- 
den appearance and fully developed structures 
of these earlier ammonoids finely illustrates 
the fan-like character of the evolution of forms 
from chronological centres of distribution, and 
the quickness with which they must have spread 
and filled up the unoccupied habitats. 
After the paleozoic, no absolutely new struc- 
tural modifications are produced ; though the 
complication of the structures is carried so 
much farther that we are at first apt to im- 
agine that there are several new types of struc- 
ture in the trias and Jura. We can carry out 
SCIENCE. 
~ 
this assertion, even into some minute structural 
characters. Thus the mesozoic ammonoids 
have, in all forms, a curious little short collar, 
which arises from the septa, and surrounds 
the siphon. It seems to be useful simply to 
close the joint, and perhajs make the connec- 
tions of this tube more perfect, and exists in no 
nautiloid at present known. It was supposed 
from its development, ete., to be confined to the ~ 
Ammonoidea of periods later than the paleo- 
zoic, but has recently been noted by Beyrich 
in a Goniatites of the carboniferous. We have 
found in a similar way every distinctive struc- 
tural peculiarity of the mesozoic Ammonites 
appearing in some form among the Goniatites 
of the carboniferous. 
The contemplation of the wonderful phe- 
nomena presented by these series has finally 
led the author, not without reluctance, to the 
conclusion that the 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 unoccu- 
pied field is essential for the evolution of new 
forms, gains immensely in importance, if, as we 
suppose, it is practical to apply it to the ex- 
planation of the phenomena we have observed. 
Every naturalist must see at once, by his own 
special studies, that this is the only reasonable 
explanation of the frequent rapid development 
of types in new formations, as well as the sud- 
den appearance of so many of the different 
types of invertebrates in the paleozoic. New- 
berry’s theory of cycles of sedimentation shows 
that the sudden appearance of types is inex- 
plicable, except upon the supposition that they 
retired with the sea between each period of de- 
posit, and again returned after long intervals 
of absence, or perhaps made their appearance 
for the first time in a given fauna. 
With this explanation and that of Wagner 
the facts we have observed fully coincide, and, 
we think, 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 or of the protozoic, if this sup- 
posed period is admitted, as compa with 
subsequent periods. 
We find, bee that, in order to make our 
1 Another statement of these facts in the form of a law of 
evolution is given in the author’s ‘Genera of fossil cephalopods’ 
(Proc. Bost. soc. nat. hist., xxii. 1884). 
. 
(VoL. IIL, No. 53. 
ee 
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