ORGANIC EVOLUTION 353 
pond-shell similar to the planorbis, which is so common at 
the present time. 
Fig. 104 shows some of these transformations, the finer 
Fic. to4.—Planorbis Shells from Steinheim. (After Hyatt.) 
gradations being omitted. The shells from these two sources 
bear directly upon the question of whether or not species have 
held rigidly to their original form. 
33 
