STRUCTURAL ADAPTATIONS 367 



ing the spindle and giving the same general form as in relatively 

 short-necked types, such as Gulls and Eagles. So far, it may 

 be reniarked, there is no explanation of the reason why birds, 

 closely related, both structurally and genetically, such as the 

 Herons and Storks, should assume such different forms during 

 flight. The passage through the air is further aided by the 

 smooth, closely fitting feathers, thereby reducing friction to a 

 minimum. 



Resuming the question of flight later, we may pass on to 

 remark that the wings rarely serve any other purpose, during 

 adult life, than as organs of serial locomotion. In the case of 

 the Penguins they serve as paddles (p. 385) ; and in some other 

 instances they are armed with sharp spurs or bony bosses, and 

 constitute formidable weapons of offence (p. 161). When not 

 in use the fore-limbs are 'held closely pressed to the sides of the 

 body, so that, in short-winged forms, when the flank feathers 

 overlap the free edge of the wing, no trace of the limb is 

 visible. 



Birds present a really remarkable uniformity in the matter 

 of shape. The differences whereby one group may be dis- 

 tinguished from another are differences of degree only — length 

 of beak, neck, leg or tail, peculiarities of toes and ornament, 

 a horizontal or perpendicular carriage of the body, sum up the 

 main variations in this matter. This uniformity of likeness is 

 to be accounted for, as in the case of the extinct Pterodactyles 

 and the Bats among the Mammalia, by the fact that once 

 launched on an aerial career they become too highly specialised 

 to adapt themselves to any new line of development. The 

 more generalised lizards, for example, or the carnivora and 

 ungulata among the Mammalia, offer far more variations and 

 profound modifications of structure than can be met with 

 among the birds. They have, in short, proved capable of 

 adapting themselves to many different environments. And 

 this because during the earlier stages of evolution they retained 

 the use of the fore-limbs for general purposes, while in the case 

 of flying animals these organs are devoted solely to the require- 

 ments of flight. The exigencies of independence for a living 

 on this mode of locomotion curtails development in all other 

 directions. 



The fore-limbs of the bird being no longer used for the 



