KINETOGENESIS. 255 



2. KINETOGENESIS IN MOLLUSCA. 



a. The Origin of the Plaits in the Columella of the 

 Gastropoda. 



Mr. W. H. Dall has developed the mechanics of 

 evolution in the Gastropoda, and I quote extracts from 

 one of his papers to show the harmony of hia views 

 with those of other Neo-Lamarckians.^ "The question 

 which first arises is as to the origin of the columellar 

 plications and their function. In considering the dy- 

 namic relations of the animal to its shell we may ob- 

 tain satisfaction on this point. In the fusiform Rha- 

 chiglossa an anatomical difference exists to which I 

 believe attention has not hitherto been called. In- 

 deed, unless the principles of dynamic evolution are 

 granted, it is a difference which would appear to have 

 little or no significance. These principles, however, 

 afford a key which seems to unlock this and many 

 other mysteries. In the recent forms of this sort the 

 adductor muscle, which in all gastropods is attached 

 to the columella at a certain distance within the aper- 

 ture, is attached deeper within the shell than in non- 

 plicate forms. The point of attachment may be an 

 entire turn, or even more, behind the aperture, while 

 in short globose few-whorled shells and in the non- 

 plicate forms it is, as a general rule, little more than 

 half a turn behind the aperture. 



' ' Now let us consider the dynamics of the case. We 

 have, reduced to its ultimate terms, a twisted, shelly, 

 hollow cone, subangulate or even channelled at two ex- 



1 Transactions of the Wagner Free Institute of Science, Philadelphia, Aug., 

 1890, p. 58. 



