vi FORM AND FUNCTION 105 



cold diminishes its activity ; when the thermometer 

 goes down a few degrees below freezing, it torpifies. 



The warm-blooded animal generates heat within him- 

 self, and, in a certain measure, is superior to external 

 conditions. The cold-blooded animal is their slave. 

 It might be thought that fish live through great 

 cold in hard winters. But since water is densest and 

 heaviest when it is at a temperature of 39 F., ponds 

 and pools in rivers are not so cold some way below 

 the ice as might be thought. It is true that when 

 fish die during a frost, it is usually from want of 

 oxygen, the ice not having been broken to allow 

 oxidation of the water at the surface. There is no 

 reason, however, to suppose that fish can stand very 

 great cold any more than other cold-blooded animals 

 It is true of them as a class that they are at the mercy 

 of their surroundings. 



It is impossible here to spend more space on so ab- 

 struse a subject. I would refer the reader to Dr. 

 Michael Foster's Textbook of Physiology, where the 

 subject is admirably handled. He is not there speak- 

 ing of birds ; but, in this respect, what is true of one 

 warm-blooded animal is probably true, roughly speak- 

 ing, of all. 



Problems connected with the Holloiu Bones of Birds. 



Not long ago the problems connected with this 

 subject were settled in a very offhand way. The 

 heated air in the air-sacks and bones being lighter than 

 the surrounding atmosphere made the bird a balloon, 

 and so flight was easy. This theory has withered 



