50 GENESIS OF THE ARIETID.E. 



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 

 chano'es 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 1 — 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 Amaltheidas. 

 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 2 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, 

 188S, 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. 



