ADAPTATIONS 



343 



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

 reproduction. 



Catadronious fishes, as the true eel (An(iuill(i), reverse this 

 order, fce(Hng in the rivers and l)rackish estuaries, apparently 

 finding their usual spawning ground in tlie sea. 



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

 environment itself: in this str\iggle only tliose that are adapted, 

 live and leave descendants fitted like themselves, 'i'lie 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 swami)S of 

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

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

 however, hibernation is accompanied by concealment. Some 



u 



Fig... 204. — Head of rainbow trmit, ^(i.'mo irnU-us, witu yiii cover beut back u> show 



gills, the breathing organs. 



animals in hibernation may be frozen alive without aj^parent 

 injury. The blackfish of the Alaska swam]-)s, fed to tlogs when 

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

 stomach and to wriggle out and escai)e. As animais 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 



