48 GENESIS OF THE ARIETIDA. 
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 As¢. ob¢usum, 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 Goniatitinee, Arcestinee, Lytoceratinse, Ammo- 
nitins, and Ceratitine, 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 imaccurate 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 Endoceratida. It was, 
as has been said, probably fixed at the first septum in Sannionites, and became 
perhaps invariably fixed at this stage in the Orthoceratide, 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 neepionic 
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 neepionic stages. This seems to be a reasonable 
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