3 ANIMAL LOCOMOTION-. 



surfaces are specially modified), it is more troublesome to 

 swim than to walk, and to fly than to swim. This arises from 

 the displacement produced, and the consequent want of sup- 

 port. The land supplies the fulcrum for the levers formed 

 by the extremities or travelling surfaces of animals with 

 terrestrial habits; the water furnishes the fulcrum for the 

 levers formed by the tail and fins of fishes, sea mammals, 

 etc.; and the air the fulcrum for the levers formed by the 

 wings of insects, bats, and birds. The fulcrum supplied by 

 the land is immovable ; that supplied by the water and air 

 movable. The mobility and immobility of the fulcrum con- 

 stitute the principal diff'erence between walking, swimming, 

 and flying ; the travelling surfaces of animals increasing in 

 size as the medium to be traversed becomes less dense and 

 the fulcrum more movable. Thus terrestrial animals have 

 smaller travelling surfaces than amphibia, amphibia than fishes, 

 and fishes than insects, bats, and birds. Another point to be 

 studied in connexion with unyielding and yielding fulcra, is 

 the resistance offered to forward motion. A land animal is 

 supported by the earth, and experiences little resistance from 

 the air through which it moves, unless the speed attained is 

 high. Its principal friction is that occasioned by the contact 

 of its travelling surfaces with the earth. If these are few, the 

 speed is generally great, as in quadrupeds. A fish, or sea mam- 

 mal, is of nearly the same specific gravity as the water it in- 

 habits; in other words, it is supported with as little or less effort 

 than a land animal. As, however, the fluid in which it moves 

 is more dense than air, the resistance it experiences in forward 

 motion is greater than that experienced by land animals, and 

 by insects, bats, and birds. As a consequence fishes are for 

 the most part elliptical in shape ; this being the form calcu- 

 lated to cleave the water with the greatest ease. A flying 

 animal is immensely heavier than the air. The support 

 Avhich it receives, and the resistance experienced by it 

 in forward motion, are reduced to a minimum. Flight, 

 because of the rarity of the air, is infinitely more rapid than 

 either walking, running, or swimming. The flying animal 

 receives support from the air by increasing the size of its 

 travelling surfaces, which act after the manner of twisted 



