34 EVOLUTION 



oldest form has a more or less smooth shell, 

 the youngest has a conspicuously ridged shell, 

 and there are fifteen gradations between the 

 two. Before the complete series was known 

 it was usual to distinguish half a dozen or 

 more species; but with the beautifully gradu- 

 ated, really continuous series before us, we 

 feel — fossils as they are — that we see a species 

 varying before our eyes. If conditions had 

 arisen that assured survival and success only 

 to the markedly ridged forms, the inter- 

 mediate gradations would soon have fallen 

 into the minority and disappeared as living 

 creatures from the scene, and a ridged species, 

 apparently discontinuous, would have been 

 established. 



Similarly in the neighbourhood of Stein- 

 heim in Wlirtemberg, in calcareous deposits 

 that mark the floor of an old Tertiary lake, 

 there are enormous quantities of a small 

 snail, Planorbis multiformis, which has 

 been carefully studied by Hyatt and others. 

 And again, since the whole history has been 

 unearthed, we see evolution before our eyes. 

 The particularly interesting feature is that 

 there are four or so primitive forms which 

 are very like one another, and that each of 

 these is the starting-point of a series the 

 termini of which are very different. The 

 contrast between the beginning and the end 



