94 



THE QUARTERLY REVIEW OF BIOLOGY 



dominated by a sort of theological 

 philosophy, and particularly by one of 

 the outstanding tenets of the American 

 school of Neo-Lamarckianism, namely: 

 that of racial senescence as expressed in 

 seemingly bizarre appendages or orna- 

 mentation, and supposed atavistic simpli- 

 fication of the shell architecture. What 

 could seemingly be more conclusive evi- 

 dence of senescence than Baculites, 

 appearing near the close of the geological 

 record of the ammonites, with its tiny 

 coiled baby shell like that of its ancestors, 

 early forsaking the ancestral plan of 

 rectitude and straightening out into a 

 large orthocone? It would even have an 

 immoral quality in the mind of the late 

 John M. Clarke. 



However, since racial senescence, in the 

 sense that the protoplasm, vital force, 

 nucleic control, or whatever you choose 

 to call it, had suffered an old-age devitali- 

 zation which was responsible for the 

 observed changes in form, is non-existent, 

 some other explanation must be sought, and 

 my answer would be that this was adapta- 

 tion to new environments or habits. 

 Racial senescence is only permissible as a 

 descriptive phrase for a race, not necessa- 

 rily old, which is not overly successful in 

 competition with its contemporaries and 

 is therefore dwindling. Such a one, and 

 there are many throughout the history of 

 all fossil groups, is more appropriately 

 compared with a backward human race 

 than with a senile individual. In any case 

 the use of the phrase is not the explanation 

 of the observed changes of form or orna- 

 mentation that the fossil record discloses. 



THE EVOLUTION OF THE CEPHALOPODS 



In the following pages I shall endeavor 

 to give a very much abridged account of 

 the evolution and adaptation of the group, 

 and then discuss a few selected examples 

 in somewhat more detail. To attempt 



a complete survey of a group containing 

 as many described species as all the known 

 mammals — both recent and fossil — would 

 be to write a book, and probably to ob- 

 scure the subject. The general phylo- 

 genetic relations of the different groups 

 and a simplified epitome of their geologi- 

 cal history are shown on Plate i. 



Malacologists are rather generally 

 agreed that the archetypal stock of all of 

 the Mollusca could not have differed 

 greatly from the synthetic form which 

 E. Ray Lankester constructed many years 

 ago, and such a form is shown in longi- 

 tudinal section in Plate z, figure i, in 

 which the shell, visceral cone, course of 

 the alimentary canal, and crawling foot 

 are differentiated. Such a form might 

 readily give origin to gastropod, scapho- 

 pod, lamellibranch, or amphineuron. To 

 become an incipient cephalopod it would 

 have merely to narrow the aperture of the 

 shell, heighten the cone, and cut off its 

 apex by septa. How this came about I 

 shall consider presently. 



At this point it is necessary to correct 

 another misconception of the soft parts. 

 When the short body of the pearly Nautilus 

 is compared with that of a more active 

 squid the proportions are in striking 

 contrast, and it has been tacitly assumed 

 that the extinct shelled species had bodily 

 proportions comparable to those of 

 the Nautilus. Many had, or were even 

 shorter; others had not, and were quite 

 as elongated as a squid or even longer. 

 Examples can be cited at almost any time 

 during geological history. For instance, 

 the cigar-shaped body of the immature 

 Proterocameroceras brainerdi (Whitfield) of 

 the Ordovician was eight or nine times as 

 long as it was wide. 



Other early nautiloids with elongated bodies were 

 the genera Ophidioceras (fig. 13), Schrocderoceras (fig. 

 12.), Deltoceras, Piloceras, Vaginoceras, Lituites, etc. 

 Among the later ammonoids the genus Macro- 



