450 Transformations of Planorbis. [June, 



ology that, during an early larval period, " in most Gasteropods, 

 the shell and mantle extend much more to the left than towards 

 the right side and that the commencement of the spiral shell is 

 thus produced." This same explanation applies to the falling 

 over on one side of the young Anomia and oyster, in so far as the 

 side upon which they fall is heavier than the other or upper side. 



These seem to be readily accounted for as the direct results of 

 hereditary peculiarities which have arisen in their ancestors and 

 become embryonic through the action of the law of quicker de- 

 velopment or acceleration explained above, and into the same 

 category comes also the straight anomalous bag-like shell or 

 plate of the embryo, and the split bivalve shell of the young oyster, 

 which according to Professor Brooks is never an embryonic plate 

 as in other Lamellibranchs. 



The problem has not been approached among animals as it has 

 among the plants by Sachs, Darwin and others, and we do not 

 know how to distinguish between the direct effects of gravitation 

 upon the growth of any animal at any one stated period of its 

 life, and the effects of the proximate causes arising from the in- 

 herent tendencies of heredity. 



Notwithstanding these imperfections in the evidence, and the 

 absence of experimental proof, it has appeared to me that the 

 discussion of this question would not be without usefulness in 

 calling attention to what seems to me one of the most fruitful 

 lines for experimental research. This, though now attracting 

 much interest among botanists, on account of Sachs' experiments 

 and Darwin's last book, is neglected by zoologists. 



In conclusion, Hyatt summarizes what he has endeavored to 

 condense in his brief communication. 



I have tried to show the results of the study of the history of a 

 single species, Planorbis levis, and its evolution into many distin- 

 guishable forms, of which 14-19 may with justice be called by dif- 

 ferent names and considered as distinct species. I have also 

 striven to bring into comprehensible shape the following concep- 



First, that the unsymmetrical spiral forms of the shells of these, 

 and of all the mollusca, probably resulted from the modification 

 of the action of the laws of heredity, produced by gravit; 



Second, that there are many characteristics in these ; 

 groups of shells which are due solely to the uniform ; 



e modification 

 gravitation. 



