ADAPTATIONS 



343 



drop back down to the sea and survive to repeat the acts of 

 reproduction. 



Catadromous fishes, as the true eel (Anguilla), reverse this 

 order, feeding in the rivers and brackish estuaries, apparently 

 finding their usual spawning ground in the sea. 



A large part of the life of the animal is a struggle with the 

 environment itself: in this struggle only those that are adapted, 

 live and leave descendants fitted like themselves. The fur of 

 mammals fits them to their surroundings] As the fur differs 

 so may the habits change. Some animals are active in winter: 

 others, as the bear, and in Northern' Japan, the red-faced mon- 

 key, hibernate, sleeping in caves or hollow trees or in burrows, 

 until conditions are favorable for their activity. Most snakes 

 and lizards hibernate in cold weather. In the swamps of 

 Louisiana, in winter, the bottom may often be seen covered 

 with water snakes lying as inert as dead twigs. Usually, 

 however, hibernation is accompanied by concealment. Some 



FIG. 204. Head of rainbow trout, Salmo irideus, with gill cover bent back to show 

 gills, the breathing organs. 



animals in hibernation may be frozen alive without apparent 

 injury. The blackfish of the Alaska swamps, fed to dogs when 

 frozen solid, has been known to revive in the heat of the dog's 

 stomach and to wriggle out and escape. As animals resist 

 heat and cold by adaptations of structure and habits, so may 

 they resist dryness. Certain fishes hold reservoirs of water 

 above their gills, by means of which they can breathe during 



