OF EVOLUTION. 97 



our being able to perceive the workings of such 

 phenomena. 



I cannot conclude this chapter on molluscan 

 variation without referring to the very remarkable 

 discoveries which have been made during the 

 last few years in some of the later Tertiary lake 

 basins of Germany and Austria, bearing upon the 

 modification, through time, of the characters of 

 certain well-known freshwater genera of mol- 

 lusks. The so-called " Paludina beds" of Slavo- 

 nia, which date from about the middle Tertiary 

 period, will best illustrate my purpose. From 

 these deposits, which run continuously from what 

 are known as the " lower " to the " upper Paludina 

 beds," and whose physical development appears 

 to have been practically unbroken, Prof. Neumayr, 

 of Vienna, has brought to light a number of 

 forms, eight or more, of Paludina, which differ 

 so materially from one another that to the casual 

 observer they appear like so many distinct 

 species ; and as such have they actually been 



described. But it has been shown that the 



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