228 EVOLUTION AND GENETICS 



supply of undigested food in the crop, and for rapid digestion. 

 The respiratory system is so modified that air enters the alveoU 

 of the lungs through one system of tubules and passes out through 

 another, so that fresh air passes constantly over the respiratory 

 epithelium. These things result in rapid metabolism, which is 

 correlated with high body temperature. Many birds maintain a 



Fig. 135. — Flying fish, Dadijloplerus volilans. (From Lull.) 



temperature of 105° F., and the best fliers reach 110° to 112°. High 

 temperature is of value not only in promoting rapid metabolism 

 but also as a protection against the low temperature of the upper 

 air. 



The extent to which animals are adapted for flight varies greatly 

 however. Some forms are able to move through the air to a 

 limited degree only, by gliding, while others are capable of true 

 flight. 



Gliding is a self-explanatory term. If an animal is provided 

 with sufficiently extensive membranes, it is buoyed up by the air 

 pressure induced by gravitation, and is able to coast from higher 

 to lower levels on the air. The same principle enables birds to soar 

 on air currents without moving the wings, but this demands a 

 nice adjustment of equilibrium of which less highly adapted organ- 



