PHYSAD^. 19 



to a great extent, in contending that external agents have 

 produced the different characters presented by the order 

 Bimana. 



The experiment of Cross will be an experimentum 

 crucis for the question of the mutability of species, should 

 it be ascertained that the resulting Acarus be not produced 

 from the egg of a known species; for it will doubtless be 

 proven that it is not developed from an animalcule, a 

 zoophyte, or a planarian worm. 



The great family of the Helicidise requires a careful study; 

 because, from the diversity of form which it presents, it 

 appears to offer better means than any other, towards the 

 solution of several great physiological questions. The 

 principle of the revolution of the spiral, in Planorbis, which 

 results in an apex upon the same side with the obliquity 

 of the aperture, in some species, and upon the opposite one 

 in others, is to be discovered; and the causes of the pecu- 

 liarities presented by the Helices, must be accounted for; 

 such as the reversal of the aperture in Anostoma and 

 Strophostoma, the possession or the want of teeth, and the 

 laws which govern their production. We are at present 

 quite ignorant of the extent to which these characters may 

 be considered generic; and whilst one author separates 

 Monoceros from Purpura, another unites them, because he 

 cannot detect the peculiarity in the organization of the 

 soft parts, which would enable him to account for the for- 

 mation of the teeth, although the presence of the latter 

 demonstrates the existence of the former. 



Ckicquesalunga, October, 1842. 



