258 THE MAMMALIA. 



while, at the same time, the afflux of fresh water 

 exercised a greater influence upon the lessened 

 amount of sea water. These circumstances, which 

 reduced the food of marine animals nay, which 

 was obviously detrimental to their existence were 

 accompanied, moreover, by the gradual separation 

 of the great ocean into numerous basins, occasioned 

 by the rising of the land already alluded to ; this 

 prevented the animals from migrating, and the 

 altered condition of the water increased even 

 further owing to the separation. As a proof of 

 this we have the Black Sea, the Caspian, and the 

 Aral Sea, which remained longest in connection, 

 and several other seas in Central Asia. Those 

 species of invertebrates and fishes which, owing 

 to their peculiar organisation, could exist only in 

 large, open seas, and not in an inland sea with a 

 lesser amount of salty substances, and were unable 

 to accommodate themselves to the change in the 

 physical, thermal, and biological conditions, died 

 out together with the Cetacea. Those that sur- 

 vived and were able to adapt themselves to circum- 

 stances, like some of the molluscs, &c., decreased 

 in size.' 



