THE DEVKLOPiMENT OF PALUDINA VlVIl'ARA. 123 



(b) Theoretical Considerations. 



I do not mean to attempt to give here anything like a 

 complete historical summary of the many views which have 

 been held on the subject of Grasteropod torsion and asym- 

 metry. This has already been done more or less fully many 

 times (see especially Simroth [16] and Boutau [2] ), and I 

 shall therefore confine myself merely to a very brief conside- 

 ration of those views which lend themselves to criticism from 

 an embryolog'ical standpoint, and upon which a study of the 

 development even of a single form may throw some light. 

 Such theories, therefore, as that put forward by Lang, which 

 claims only to be phylogenetic, and for which confirmation is 

 not sought from the facts of ontogeny, will be passed over 

 altogether ; while theories of authors who, like Biitschli, seek 

 to base their conclusions to a greater or less extent upon 

 embryology may be of some interest in this connection, and 

 will therefore be considered. At the same time it must be 

 remembered that the remarks upon these theories are pro- 

 fessedly based only on embxyology, and need not necessarily 

 invalidate their phylogenetic value, though they may weaken 

 the author's argument. 



For the sake of convenience the theories under considera- 

 tion may be placed in two classes. In the first of these are 

 placed those theories which maintain that the present condi- 

 tion of the Prosobrauchia has been brought about by a simple 

 process of unequal growth, resulting in the forward movement 

 of the palleal complex in a horizontal plane ; while the second 

 comprises those more recent theories of Pelseneer, Amaudrut, 

 and Boutan, which regard asymmetry primarily as the 

 concomitant of a twist which causes the palleal complex to 

 move in a vertical plane. 



Biitschli (4) was the first to put forward in an exact and 

 careful way the point of view which is now common to all 

 theories of the former class, lie puts aside the older view 

 of Spengel, which obviously runs counter to known embryo- 



