200 Darwin, and after Darwin. 
Near the village of Steinheim, in Wiirtemberg, 
there is an ancient lake-basin, dating from Tertiary 
Fic. 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 
