2OO Darwin, and after Darwin. 



Near the village of Steinheim, in Wiirtemberg, 

 there is an ancient lake-basin, dating from Tertiary 



FIG. 89. Transmutations of Planorbis. (After Hyatt) 



times. The lake has long ago dried up ; but its 

 aqueous deposits are extraordinarily rich in fossil 

 shells, especially of different species of the genus 



