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. 
Fic. 1.—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 
C2 
