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, daring 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 

 crecosiphonula was also carried back and appeared earlier in the life of the shell 

 and animal. 



The nsepionic or true larval stages, as we have said above, became more 

 accelerated in the angustisellate young of the Lytoceratina? and Ammonitina?, 

 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 

 Ammonitinse, even without breaking down the whorls to examine the young. 

 The ornaments and pilse 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 nsepionic lobes, and the marginal 

 dictations were comparatively simple even in adults. 



Similar phenomena occurred in every group. Thus in the Arietidse 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 



