58 The Life Story of the Fish 



effortlessness with which a porpoise coasts along in front 

 of a ship. The reason for this is that, while the porpoise 

 swims on much the same principle, his movements are in 

 the up-and-down, instead of in the side-to-side, plane, and 

 therefore cannot be seen as we look down on him from the 

 bow of a steamer. They become apparent only when an extra 

 wide move brings him up out of the water entirely. The 

 porpoise being a mammal which has readapted itself to the 

 water and having different body musculature from the fish, 

 it has found the vertical motion preferable to the horizontal 

 and more useful in giving it access to the air which it has to 

 breathe. 



The amazing part of the whole phenomenon of readapta- 

 tion is that this land animal, which has had to learn to 

 swim, is a better swimmer than most fish. It slips through, 

 and in and out of, the waves like a shadow, and it has been 

 estimated to make over thirty miles an hour. The salmon's 

 speed has been variously estimated at from seven to four- 

 teen miles per hour, and the top for fish at about thirty, in 

 the tunas.^ This "typical" fish with the "ideal" shape has 

 grooves in its body surface into which the fins can be re- 

 tracted while swimming so that they will offer less resistance 

 to the water, and its form is the last word in stream-lining, 

 while its wide, deeply forked tail fin fastened to the body 

 by the narrow "caudal peduncle" is another indication of 

 speed. Man himself, in the person of a champion athlete 

 running one hundred yards, can make only about twenty 

 miles per hour. The horse can double this, the antelope 

 triple it, and some birds have been estimated to do better 

 than a hundred miles an hourj but in view of the heavy 



^ It is difficult to measure the speed of wild animals with any degree of 

 accuracy. Sixty miles an hour has been attributed, although without 

 substantiating evidence, to that distant relative of the tunas, the sailfish. 

 To place the maximum for fish at thirty is therefore conservative. 



