272 THE TRANSFORMATIONS OF THE ANIMAL WORLD 



physiological changes, such as the loss of pulmonary 

 respiration, undergone by certain forms of deep- 

 water Limnseas dwelling in the vast depths of Lake 

 Leman. Isolation is here accompanied, as in 

 almost all similar cases, by concomitant modifica- 

 tions in the conditions of the environment. The 

 example, often quoted since the researches of 

 Neumayr and Paul, of the ornamented Paludines 

 of the great Levantine lakes is particularly in- 

 structive on this point. These great sheets of 

 fresh water, which covered the basin of the Danube, 

 the Balkan peninsula, and a part of the present 

 ^Egean Sea, during the Pliocene epoch, must doubt- 

 less, owing to the intensifying phenomena of 

 evaporation, have presented a very complete 

 saturation with salts of lime, which has aided the 

 thickening of the shell in the form of keels and 

 tubercules which become more marked as we rise 

 higher in the series of strata. This chemical or 

 other similar explanation is all the more probable 

 that the phenomenon not only affects the Paludines, 

 but nearly all the other genera of Molluscs in these 

 Levantine formations. The divergence which sepa- 

 rates the ornamented from the smooth Paludines 

 of our existing fresh waters is marked enough not 

 only to justify the creation of a great number of 

 species, but even of those real genera, which have re- 

 ceived the names of Tulotoma, Tylopoma, and Bosco- 

 vicia. In this case, the influence of isolation is 

 associated with changes in the nature of the waters, 

 and leads to the formation of manifold small 

 branches with longer or shorter parallel evolution. 

 The strangely varied fauna of the Molluscs of 



