ORGANIC EVOLUTION 



355 



pond-shell similar to the planorbis, which is so common at 

 the present time. 



Fig. 104 shows some of these transformations, the finer 



FIG. 104. 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. 



