﻿588 



parallelisms in the evolution of different and diverging genetic 

 series. Thus the Belemonoids and Sepoids, both preeminently 

 swimming types* and with organizations obviously derived from an 

 Orthoceran radical, have straight internal shells. 



There is an obvious correlation between coiling of the shell and 

 the habit of crawling. Thus all univalve crawling mollusca have 

 this general tendency. Among Gasteropoda, this is well known 

 and those shells which degenerate and tend to loose the spiral 

 mode of growth and become irregularly straightened out in these 

 older stages of growth, are forms which become attached or lead sed- 

 entary lives, i. e., Vermetus attached late in life and Magilus buried 

 in coral. The most significant case, however, is that of Fissurella, 

 which has a coiled shell in the nepionic stage and becomes similar 

 to Patella, a depressed, straight cone in the neanic and ephebic 

 stages, the habitat being like that of Patella and the approximate 

 forms of Haliotis and others, comparatively sedentary upon littoral 

 rock ledges. 



A habit of crawling could be considered as sufficiently general in 

 application and sufficiently persistent in an organization like that 

 of the Nautiloids and Ammonoids, which are covered by shell and 

 possess only the hyponome as a motor organ to affect entire orders 

 and continue constant through time and geologic changes in the 

 majority of forms. 



With such a habit the tendency to become more exclusively 

 crawling and to depend upon that mode of life, might, as has been 

 explained in the introduction, produce in each series the same ten- 

 dency, but it seems impracticable, so far as my experience goes, to 

 find any other cause sufficiently general and likely to be undis- 

 turbed by geologic and climatic changes. 



It is certainly not inherent in the organism to coil up. If the con- 

 verse be assumed one must account for the continuance and persist- 

 ence of the absolutely straight Orthoceras from the earliest times 

 to the Trias and why these were unaltered and did not become 

 arcuate or coiled as a whole. Inherent tendencies must, if the 

 term has any meaning at all, work out their own evolution to some 

 degree. They must sensibly affect the organization in all series 

 having a common embryo unless held back or kept in abeyance by 

 interfering causes. It is difficult to imagine any interfering cause 

 acting so constantly through long geologic periods that it could 



* See Introduction, p. 356. 



