KEGULATION OF TEMPERATURE. 231 



condition necessary to their very maintenance, they seem to be the 

 principal source of warmth produced. Since the loss of heat is 

 greater when the temperature of the external medium is lowered, the 

 activity of the vegetative organs must considerably increase in the 

 colder season of the year, and in the northern climates. 



In addition to the continual supply of new quantities of heat, a 

 second cause contributes to the maintenance of the constant tempera- 

 ture of the warm-blooded animals. This is the protection afforded 

 by the special nature of the covering of the body. While the Ver- 

 tebrates with a variable temperature have a naked or armoured skin, 

 Birds and Mammals have a more or less close covering of hairs or 

 leathers, which limits to a great extent the loss of heat by radiation. 

 The large aquatic animals, on the other hand, have a scanty covering 

 of hair, but they develop thick layers of fat beneath the cutis, which 

 serve for the retention of heat, and at the same time for hydrostatic 

 purposes. 



There is in all cases a mutual relation of a complicated kind be- 

 tween the factors which favour the withdrawal of heat and the 

 conditions of the retention and the formation of heat, a relation which 

 in spite of many variations in its individual factors results in the 

 equalization of the heat generated and the heat lost. Some Mam- 

 mals are able to maintain their proper temperature only within certain 

 limits of the external temperature ; these animals are to a certain 

 extent incompletely homothermic, and when the temperature sinks 

 below a certain point they fall into the so-called winter-sleep 

 (hibernation], i.e., a state of rest characterised by an almost com- 

 plete absence of movement, and by a diminution in the energy of all the 

 vital processes. In the class of Birds, whose higher temperature 

 permits of no interruption or limitation of the vital functions, there 

 is no example of hibernation. But these animals have numerous 

 means of heat adjustment at their disposal ; in particular, the swift- 

 ness of their flight enables them to leave their homes at the approach 

 of the cold season, and to betake themselves to warmer climates, 

 where food is abundant. The common migrations of the migratory 

 birds, migrations which sometimes extend over great distances, to a 

 certain extent take the place of the winter-sleep of the hibernating 

 animals; in the Mammalia, whose organisation permits of hiberna- 

 tion, migrations like those of Birds are very rare. 



The most essential peculiarity of Birds, and one with which many 

 characteristics both of external appearance and of internal organi- 

 sation are correlated, is their power of flight. This peculiarity in 



