44 DOCTRINE OF EVOLUTION 



more obvious is the change of structure in the case of 

 masts of vessels, which originally bore the sails for 

 propelling the ship. When steam engines were em- 

 ployed to give motive power, masts did not disappear. 

 They now provide the derrick supports of trading 

 steamers; in battleships their function is changed to 

 that of fighting tops and signal yards. Even the poles 

 carried by canal boats to bear windmills must be re- 

 garded as the reduced vestiges of masts originally con- 

 structed to carry sails; and their adaptive evolution, 

 like that of countless structures in animals, has been 

 accomplished by degeneration. 



The birds are another class of backboned animals 

 which exhibit identical principles of relationship. A 

 heron has long legs and wide-spreading toes, which 

 keep its body out of the water as it stalks about the 

 marshes where it seeks its food ; its bill is a long slender 

 pincers. Compare it with an eagle; the latter has a 

 short and heavily hooked beak to tear flesh, while its 

 stout legs bear strongly curved talons to hold its 

 struggling prey. Swimming birds like the swan and 

 duck and loon possess feet which are constructed in 

 general like those of the former examples, but they are 

 webbed and shortened to serve as paddles. In the 

 penguin we find a counterpart of the seal among 

 mammals ; its feathers are much reduced and its fore 

 limbs are no longer wings enabling the animal to fly, 

 but they are paddles which it uses when it swims in 

 pursuit of fish. Finally the ostrich and wingless bird 

 of New Zealand — the Apteryx — have wings that are 



