62 ANIMAL LIFE 



in width (fig. 8). The form, then, not only of the wing, 

 but of every flight feather — hollow and sustaining below, 

 rounded and unresistant above, short and rounded 

 for more laborious flight, long and pointed for easy, 

 sustained flight — has a definite significance for all 

 the varied needs of birds and their more elaborate 

 evolutions, such as sailing and soaring. And in those 

 cases in which flight is abandoned and the wing, 

 as in penguins, is converted into a fin, the feathers take 

 on the form most suitable to the needs of an aquatic 

 life and form scale-like coverings. 



Flight, however, is not the only movement for 

 which birds are fitted ; and if the fore part of the 

 body is exquisitely adapted to meet the require- 

 ments of aerial life, the hinder part is mainly a 

 mechanism for terrestrial and aquatic existence, for 

 running, perching, or swimming. Such a double 

 adaptation is a rare thing in animals, and is only 

 shared by insects. All others that specialise in 

 movement — fish, whales, bats — have become incap- 

 able of rapid and sustained travel in an alternative 

 manner. Birds alone are capable of both flight and 

 bipedal movement. 



The legs of a bird have a double office. They 

 sustain and propel the body's weight, which is disposed 

 so as to fall about the knee-joint. 



For sustenance the limbs are pillar-like, and 

 distribute the pressure over four divergent toes. 

 For propulsion the joints are elongated, and the hip- 

 girdle, or ' back ' of the bird, affords a large area, 



