2 THE BIOLOGY OF BIRDS 



of the careful from the adventurous. Not that it was 

 necessarily a dichotomy, for at many a level in the genealo- 

 gical tree, we seem to see a trifurcation, rather than a 

 bifurcation. Between the extremes of great activity and 

 great passivity, others found a via media. 



** Living more nearly up to income " may sound meta- 

 phorical, but what is meant is that the ratio of disruptive, 

 down-breaking processes (katabolism) to constructive, up- 

 building processes (anabolism) is high in the bird's consti- 



A 

 tution. In every viable body the fraction -; must always 



K. 



be greater than unity ; in the bird the denominator is large. 

 Continual oxidation or combustion of carbohydrate material 

 is needed to keep the muscles, we shall not say " contract- 

 ing," but " able to continue contracting." The bird's breath- 

 ing is very rapid. The heart beats very quickly. The blood 

 is astonishingly rich in red blood-corpuscles. The bird is 

 selective in its diet, rarely eating much that is unprofitable, 

 and it makes the most of its food, for the digestion is 

 singularly perfect, as may be seen in the relatively small 

 amount of faecal matter. It takes millennia to make a 

 guano deposit, and the guano is mostly from the kidneys, 

 not from the food-canal of birds. In general, then, the 

 bird is a high-geared engine. That is evident from the 

 high temperature, which is from 2° to 14° F. higher than 

 that of mammals. 



§ 2. Running and Parachuting before Flying 



Given a spare muscular type of body, good breathing, 

 a strong heart, perfect digestion, and rich blood, it is natural 

 to expect active habits — running, jumping, climbing, and 

 parachuting from tree to tree. An examination of a bird's 

 wing shows, in varied development, a web or skin (patagium) 

 extending from the shoulder-joint along the upper or pre- 

 axial margin of the arm (see Fig. 17). It is possible that 

 this was the incipient wing with which primitive birds started, 

 and that they raced along the ground as bipeds, taking occa- 



