48 GENESIS OF THE ARIETIDjE. 



There is also no ground for assuming exceptional protection in the cases of 

 dwarfs, which have accelerated development of retrogressive characters, as, for 

 example, in those of Ast. obhtsum, which we shall describe in detail farther on. 



Origin of Differentials. 



Differentials are essential characteristics, which distinguish one group from 

 another, and, unlike morphological equivalents, are apparently directly inherited 

 in successive generations of the series from the distal or proximate radicals. Thus 

 the distinct stocks of the Nautiloids, Ammonoids, Belemnoids, and Sepioids, may 

 be followed without difficulty. The Goniatitinse, Arcestinae, Lytoceratinae, Ammo- 

 nitinre, and Ceratitinse, among Ammonoids, are examples of suborders less easily 

 separated. Vermiceras, Coroniceras, Amaltheus, Oxynoticeras, and Asteroceras are 

 examples of genera affording still greater diagnostic difficulties. In all of these 

 the differentials can with certainty be considered hereditary, since after their 

 introduction in the earlier members of a group they are perpetuated, not only 

 in the earlier species or forms, but more or less even among the most aberrant 

 and geratologous members of the group. Differentials are often described as in- 

 variable, but this is an inaccurate expression, which is not in practice trusted by 

 any naturalist of the present time. They are perhaps, when compared with other 

 characters, relatively constant, but in all complete series necessarily pass through 

 stages or phases of evolution. On first appearance, they are apt to be more or 

 less variable within the limits of the species in which they originate, then they 

 become constant in descendent forms of the same series, and finally in extremely 

 geratologous (nostologic) forms they may be in large part or wholly obsolete. 



The close-coiled character of the young was certainly a differential among 

 Ammonoidea, but this became constant only in the mesozoic forms. The con- 

 traction which marked the tendency to reduce the size of the siphon was not very 

 important at first, and was variable in position among the Endoceratidae. It was, 

 as has been said, probably fixed at the first septum in Sannionites, and- became 

 perhaps invariably fixed at this stage in the Orthoceratidse, assuming in them 

 the aspect which subsequently also characterized the close-coiled nautilian shells, 

 and the entire stock of the Ammonoidea and Belemnoidea. The contraction in 

 these orders defined or cut off the primitive caecum from the parts of the siphon 

 formed subsequently, and its invariable occurrence at one place in the naepionic 

 history of such a vast number of forms is very important in its bearing upon the 

 mode of origin of other and less important differentials. 



There is no explanation of the introduction of these characters which permits 

 us to separate them, as belonging to a distinct category, from characters which 

 are adaptive. Nor can we say that any of them more deeply affected the ovum 

 than any other characters. They were simply the earliest in time, and made 

 their appearance in adults first as ephebolic characters, and then as suitable 

 characters were inherited, and, being replaced in due course of time by newer 

 modifications, were gradually forced back through the nealogic stages until they 

 secured representation in the naepionic stages. This seems to be a reasonable 



