vii FLIGHT 261 



there arc facts at our disposal which arc full of mean- 

 ing-, and which in a rough and ready way settle the 

 question. (1) Birds have large appetites and rapid 

 digestions, a great proof of vigour. (2) In proportion 

 to their size they give off from their lungs more car- 

 bonic acid than other animals, and this means greater 

 destruction of tissue, which implies a greater expendi- 

 ture of energy. (3) Their temperature is exceptionally 

 high, irrefragable evidence of the rate at which they 

 live. (4) They are capable of as great rapidity of 

 movement as mammals, and they tire less soon. 

 This points to superior quality of muscles. 



Proportion of Whig Area to Weight in Large and 

 Small Birds. 



This is a more difficult question than it might on 

 first thoughts appear to be. One elementary cause 

 of error may be disposed of at once : the doubling of 

 the size of the bird will not mean the doubling of the 

 length and breadth of the wings, for in that case the 

 wing area would be quadrupled. 1 This will explain 

 the following figures : the comparative wing areas of 

 a Herring Gull and a Great Tit are represented by 

 541 : 31, i.e., the Gull's expanse of wing is rather more 

 than eighteen times that of the Tit, whereas the length 

 is less than six times as much. 2 It is the areas, then, 

 and not the lengths or breadths, that we must compare, 



1 See fig. 29. 



2 I take the areas from Pieter Harting, quoted by Marey 

 (Animal Mechanism, p. 224), and the lengths from Howard 

 Saunders's Manual of British Birds. The results are too broad 

 to be affected by differences in different specimens. 



