290 ANIMAL LIFE 
fornia ever reach Honolulu, nor are Hawaiian shore-fishes 
ever scen 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. 
