THE THREE MODES OF DEVELOPMENT. 39 
this shows conclusively the comparatively small influence of natural selection 
except during the acme of groups. 
Another series of facts in favor of the views here advocated is to be found 
in the cycle formed by the modes of development. The more ancient and sim- 
pler forms of Cephalopoda, like Orthoceras and most of the earlier forms of 
Nautiloidea, had a direct mode of development, during which the individual 
passed through certain well marked changes; but these were less numerous and 
not so crowded together as in types with more complicated development. The 
young and adults of the same individual among straight radical shells differed 
comparatively little in form and ornamentation. The siphon in Piloceras, and 
in some species of Endoceras, for example, was comparatively little changed in 
adults from what it had been in the young, and Cyrtocerina as described above, 
probably remained completely macrosiphonitic throughout life. 
When closer coiling was introduced and more concentrated development 
occurred, as in the higher Nautiloidea and Ammonoidea, we found, as one of 
the results, the omission of some hereditary stages. As we have said above, 
the first air-chamber disappeared in Ammonoidea, having become fused with 
the protoconch, which stage acquired through this earlier inheritance a ten- 
dency to secrete a calcareous shell, and in consequence of this fusion the 
excosiphonula was also carried back and appeared earlier in the life of the shell 
and animal, 
The neepionic or true larval stages, as we have said above, became more 
accelerated in the angustisellate young of the Lytoceratine and Ammonitine, 
and the succeeding or nealogic stages were introduced with a profusion of orna- 
ments and increased complications in the outlines of the sutures, curves of the 
septa, structure of siphon, increased involution, and changes of structure and 
form in the whorls, which multiplied the metamorphoses and made the different 
stages of growth more distinct than in the Goniatitinse and Nautiloidea. 
The complex mode of development of the normal acmic forms, due to the 
introduction of these new characteristics, is easily perceived in most of the 
Ammonitine, even without breaking down the whorls to examine the young. 
The ornaments and pile on the exposed sides of the whorls show this in most 
species of the Jura with sufficiently discoidal shells. 
The paracmic forms, and especially the degenerate uncoiled species of 
every group, exhibit a return to the direct mode of development, and a lessen- 
ing of the variety and number of structural changes. These suppressions be- 
came, as we have described them above, so well marked in all the straight 
baculites-like forms, that a tyro cannot fail to notice the fact. The smooth, 
slightly compressed whorl retained nearly the same form throughout life, the 
sutures retained the primitive number of nepionic lobes, and the. marginal 
digitations were comparatively simple even in adults. 
Similar phenomena occurred in every group. Thus in the Arietide the 
radical Psiloceras planorbe had the anaplastic or comparatively direct mode of 
development, while the descendent species in both these genera had more com- 
plicated, metaplastic transformations, due to the introduction of a quadragonal 
