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 



JV 



■si 



23^ 281 



20 



cz ^ 





C3D 



! 



C3Q 



131 



/ .1 S3 



171 



■QS)-c^c 



in 



11 



Fig. 104. — Planorbis Shells from Steinheim. (After Hyatt.) 



gradations being omitted. The shells from these two sour - 

 bear directly upon the question of whether or not species have 

 held rigidly to their original form. 



