20 THE FLIGHT OF BIRDS 



put big and small birds in this respect more or 

 less on a level. They appealed to the elementary 

 principles of geometry. If you take two cubes, 

 a side of one of which is twice the length of the 

 other, the larger one is in bulk eight times as great 

 as the smaller one, but its surface area is only four 

 times as great. This holds true of other figures 

 of three dimensions that are not cubes. Magnify 

 a bird till it is eight times its former bulk and you 

 will only have multiplied its surface area by four. 

 In order, then, to compare bird with bird correctly, 

 you should take (so say these theorists), the cube 

 root of its weight (for the weight is practically the 

 bulk, i.e. three dimensions multiplied together) 

 and the square root of its surface area (since that 

 is two dimensions multiplied together). When we 

 adopt this method we find the preponderance of 

 the small bird in point of wing-area per pound 

 weight not so very great. And we may, if we are so 

 constituted, derive a certain comfort from feeling 

 that we are following out geometrical principles. 

 But these principles have, it must be owned, in the 

 case of some species been widely departed from. 

 What are we to make of the legs and neck of the 

 Flamingo ? If any small bird of ordinary build 

 were symmetrically enlarged, should we ever arrive 

 at elongations so enormous ? It is, of course, true 

 that if the bulk of a small bird were multiplied 

 many times, and the area (not the bulk) of his wings 

 increased in proportion, he would have a greater 

 expanse of wing than his muscles could possibly 

 work. If the Stork had a wing-area as great in 

 proportion to his weight as the Swallow, then 



