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, 
Fie. 80.—The giant gall of the white oak (California), made by the gall insect Andri- 
cus californicus. 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 
