MAMMALIA 461 



forms however that do migrate. These migrations may be re- 

 lated to the breeding habits or to food supply rather than to tem- 

 perature primarily. Rats, the rat-like lemmings, fur-seal, and 

 reindeer migrate. Wonderful stories are told of the lemmings 

 of Sweden and their fierce instinctive migration, from which noth- 

 ing can turn them, until they plunge into the sea and swim until 

 they perish. 



The hibernating mammals must find a place where they will 

 not quite freeze. All their vital activities are lowered and their 

 temperature falls gradually until in some instances it closely 

 approaches the freezing point. Usually this state must be ap- 

 proached somewhat gradually. The larger hibernating mam- 

 mals like the bear must seek caves or be satisfied with a* hollow 

 log or a shallow den. Burrowing forms fare best, as they merely 

 dig themselves in below the frost line. The smaller ones must 

 find the safer places, since they lose their heat more rapidly. 



Different species vary greatly in the duration of their winter 

 sleep. A few hibernate through the whole of the cold weather 

 without interruption, as the woodchuck. Some may hibernate 

 only in the coldest weather and for a brief time. Others wake 

 up from time to time and seek food. Although the vital proc- 

 esses are low they keep going. This uses up material. Since 

 no food is taken, the fatty reserves of the body are drawn upon. 

 Animals go into the sleep fat and come out thin. 



It is in the higher mammals that one finds the greatest dis- 

 play of intelligence to be seen in the animal kingdom, and it 

 is in man that intelligence and reason whose first beginnings in 

 animals no one can mark find their culmination. That these 

 high qualities are closely correlated with the great development 

 of the brain there can be no doubt. The great progress of man 

 in getting mastery of the earth is one of the most interesting 

 aspects of the same general problem of evolution and adapta- 

 tion which gives unity to the subject matter of zoology. Thus 

 the sciences which pertain to man in all his various interests 

 have in some measure their foundation in the science of zoology. 

 (See Chapter XXV.) 



465. Mammals in Relation to Man. Mammals are not merely 

 closest to man in matter of kinship, but they are his most 



