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 Hollow 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 
