50 GENESIS OF THE ARIETIDA. 
by several naturalists since his time, the complication of these characteristics 
increases in each group of Ammonoidea, in strict accord with the amount of invo- 
lution of the whorls of the shell. The reason for this correlation is easily given. 
Involute shells have broader sides and must necessarily have a larger num- 
ber of lobes and saddles, or, if these do not increase in number, then they must 
necessarily deepen and have more profuse marginal digitations. Origin through 
fortuitous variation is consequently inadmissible, since one can predict the 
changes which are to occur under certain specified conditions. 
On the other hand, the assumption that these characteristics are advanta- 
geous differences, acquired through the struggle for existence, seems to be ruled 
out by the same facts. Characters and differences which were shared by many 
series, whether living and contending in the same horizons and localities, or 
occupying distinct horizons and widely separated localities, must have been due 
to causes which modified the forms by acting from within the organism. The 
external causes, as pointed out by several authors mentioned above, could not 
have had such similar effects, since they were assuredly diverse on distinct hori- 
zons and in different localities. The only cause of modification which could 
have produced similar change in different groups must have been the efforts — 
either as voluntary or involuntary mechanical reactions, or both'— of the ani- 
mals in response to the requirements of the surroundings in the same habitat. 
As has been said above, all external marks of similar reactions in the animals 
themselves, such as parallel forms and characters, tend to disappear when the 
habitat has become changed. That the differentials we have been treating of 
are of the same nature as parallelisms, in so far as those appearing in one series 
resembled those appearing in other series, or in so far as they correlate with such 
characters, will not, we think, be doubted by an experienced observer. 
While it is extremely difficult to account for the lengthening of the lobes, or 
the multiplication of marginal digitations, by means of natural selection, it is not 
difficult to understand that these complications might have been the result of the 
habit of holding the comparatively large shell high above the arms. The branch- 
ing of the posterior ends of the lobes would tend to give greater steadiness of 
carriage to the shell, and the efforts of the animal to use these organs while 
crawling would probably tend to increase them in size and length, and in the com- 
plication of the outlines. The length of the rostrum was not great in most forms 
of Ammonoidea, but in some groups it was quite prominent, as in the Amaltheidee. 
Its length in all groups, together with its position, was, however, sufficient to 
show that the shell must have been carried while crawling more elevated above 
the arms than in Nautiloids, and therefore in a position bringing greater strain 
upon the parts of the mantle most used in balancing this organ. 
Waagen?’ with his usual keenness has observed, that the annular muscle could 
not have served solely for holding the Nautilus within the shell, but must 
1 Henslow in his interesting book, ‘‘ The Origin of Floral Structures,’’ (Appleton’s Intern. Sci. Series, 
1888, p. 88,) takes somewhat similar ground, and says that “the forms and structures of flowers are the 
direct outcome of the responsive power of protoplasm to external stimuli.” Also pp. 123, 147, 333-337. See 
also quotation from Packard’s ‘* Cave Faunas of North America,” p. 52, note 1, of this memoir. 
2 Ansatz d. Haftsmusk. b. Naut. u. d. Amm., Paleontogr., XVII. p. 190. 
