22 LECTURES ON BIOLOGY 



A further stage of wing-degeneration is reached in the 

 Ostriches (Struthionidae). The skeleton-parts of the arm are 

 still existent but far too weak to exercise their functions and 

 raise the heavy body aloft. Ostriches have entirely lost the power 

 of flight and have become typical runners. But the effect of 

 this change in the mode of life has not been confined to the 

 extremities, for the bones have lost their pneumatic structure 

 and become massive. The breastbone of the normal bird carries 

 a strong, median ridge, or keel, the so-called ' crista,' to which 

 are joined the powerful muscles of flying. But as the ostrich 

 has no longer any use for the crista it has entirely disappeared. 



But of all birds the Kiwis (Apterygidse) of New Zealand supply 

 the most remarkable instance of transformations brought about 

 by the relinquishing of the power of flight. In these birds the 

 feathers have come to resemble hair and the wings become 

 mere stumps which fulfil no functions and are barely noticeable 

 externally. 



Still more remarkable than the degeneration of wings due to 

 disuse is their tranformation into fins, a phenomenon which 

 may be observed in the Penguins (see coloured plate). A more 

 radical change in a bird can hardly be conceived. If it were 

 not for the plumage it would be difficult to say whether these 

 creatures really are birds. The legs have been shifted far back, 

 thus giving the penguins on land an erect appearance ; in the 

 water they are extended backwards and employed as a rudder. 

 The tail is furnished with short, stiff feathers which have to 

 support the body in the sitting attitude. In the place of the 

 large jointed wings we find in the penguins broad uniform 

 ' rowing-plates ' or paddles covered with scale-like feathers. 



Those who see a penguin for the first time in its watery element 

 swimming and diving like a fish and keeping for minutes under 

 water would feel convinced that these birds had in the begin- 

 ning been specially endowed for a ' life on the ocean wave.' 

 But anatomy proves the exact contrary, for it shows that they 

 have gradually developed from typical flying birds : the fins of the 

 penguin still contain the same skeleton-parts as the well-known 

 fliers (one upper-arm, two fore-arm, and two carpal bones, and 

 three hand and two finger bones which have become fused 

 together). But while in these the tendency is all towards the 



