EVIDENCES OF EVOLUTION 35 



reptilian and avian characters, the latter, of 

 course, fully predominating. 



FOSSIL SERIES. One of the finest examples 

 of a well-preserved series of kindred forms 

 is afforded by an extinct freshwater snail, 

 Paludina neumayri, which is very abundant 

 in some Tertiary deposits in Slavonia. The 

 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- 

 heirn in Wurtemberg, 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. 



B2 



