Introductory. 19 



disconnected forms were described as distinct species ; 

 but as soon as the connecting forms were found 

 showing a progressive modification from the older to 

 the newer beds, the whole were included as varieties 

 of one species. 



FIG. i. Successive forms of Paludina, from the Tertiary deposits 

 of Slavonia (after Neumayr). 



Of course, other cases of the same kind might be 

 adduced, and therefore, as just remarked, in their 

 work of naming species naturalists may only have 

 been marking out the cases where intermediate forms 



c 2 



