588 
parallelisms in the evolution of different and diverging genetic 
series. Thus the Belemonoids and Sepoids, both preéminently 
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, z. c., 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, 
