40 The Ma\e-Up and the Wordings of the Earth 



The mere succession of forms in a given region may tell us 

 nothing at all about the descent of the later inhabitants 

 from the earlier ones. In some cases we can be certain that 

 the successors were not the descendants of the previous 

 occupants. 



Examples of Continuity 



In Slavonia there are some lakes that have been left 

 behind by receding waters and that have been drying up 

 slowly for millions of years. Certain fresh- water snails have 

 been for countless generations living and dying in these lakes; 



Fig. 6. Varieties of Paludina 



The fresh-water snails represented by the series of shells show enough 

 differences between any two of them to be considered as distinct species. 

 Certainly the two extremes, a and k, would not be considered of the 

 same species if the intermediate forms were unknown. After Neumayr. 



and millions of the shells have dropped to the bottom, sunk 

 into the mud, and become part of the underlying rock forma- 

 tion. By cutting down into these underlying rocks we can 

 obtain shells dating back through the ages in a continuous 

 series. If we compare the oldest shells with those of living 

 snails we can see very marked differences (Fig. 6) . If, how- 

 ever, we follow the shells down through the rocks — that 



