290 ANIMAL LIFE 



forma ever reach Honolulu, nor are Hawaiian shore-fishes 

 ever seen on the coast of California. For these reasons 

 natural boundaries of the great realms of distribution are 

 found in the sea. 



The other great check to distribution is found in heat 

 and cold. Most of the tropical animals can not endure 

 frost. The arctic animals, however fierce or active, are 

 enfeebled by heat. The timber line, north of which and 

 above which frost occurs the year round, therefore serves 



FIG. 176. Alligators ; animals found only in the warm waters of tropical and sub- 

 tropical regions. 



as a boundary of limitation. Another equally marked is 

 the frost line. Even the fishes of the tropics are extreme- 

 ly sensitive to slight cold. Off Florida Keys the cutlass- 

 fish is sometimes seen stiff and benumbed on the water, 

 where the temperature is scarcely below 60 Fahr. A 

 " norther " on the Gulf of Mexico will sometimes bring 

 fishes which live in considerable depths to the surface, 

 through chilling the water. These barriers are rarely 

 crossed by localized species, but many forms, especially 

 birds, keep within a relatively uniform temperature through 

 migration. The summers are spent in the north or in the 

 mountains, the winters in districts that are warmer. 



