ADAPTATIONS 143 



80. Adaptations concerned with surroundings in life. 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, hibernate, sleep- 

 ing 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, 



FIG. 80. The giant gall of the white oak (California), made by the gall insect Andri- 

 cus calif ornicus. The gall at the right cut open to show tunnels made by the 

 insects in escaping from the gall. From photograph. 



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 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 or habits, 

 so may they resist dryness. Certain fishes hold reservoirs 



