DISTRIBUTION AND MIGRATIONS 



253 



between the seas on the one hand and the fresh-waters on the 

 other, the conditions in the two regions being, for the most 

 part, of a very different nature. Certain fishes, Hke some of 

 the Sharks (Carcharinus) , Saw-fishes (Pristidae), and Sting Rays 

 (Trygonidae) , ascend rivers for considerable distances, and 

 others Hke the Flounder (Flesus) and Stickleback (Gasterosteus) 

 are equally at home in either salt or fresh water. They are 

 unable to survive a sudden change from fresh to saline water, 

 but can pass quite rapidly from the sea to the brackish estuary, 

 and from thence to the fresh water proper, and vice versa, 

 without seeming to notice the difference. Others, such as the 

 Salmon (Salmo) and Shad {Alosa) migrate annually from the sea 





Fig. 97. 

 Land distribution in Eocene times. (After Gregory.) 



to the rivers for spawning purposes, and others, again, like the 

 Common Eel (Anguilla) leave the fresh water for the sea as the 

 breeding time approaches. 



Two main categories of marine fishes may be conveniently 

 distinguished: oceanic and coastal or littoral. In the open 

 oceans, from the surface down to about one hundred and fifty 

 metres, large, swift, predaceous fishes such as the Tunny ( Thunnus) 

 and Sword-fish (Xiphias) are found, together with swarms of 

 smaller forms such as the Lantern-fishes (Myctophidae) ; the 

 region from one hundred and fifty to five hundred metres is 

 mostly occupied by small silvery fishes of various kinds, the 

 majority with large eyes. Below this the bathypelagic fishes 

 occur— Wide-mouths {Stomiatidae), Ceratioid Angler-fishes, and 

 so on— mostly blackish in colour with comparatively small eyes ; 



