TUNNIES AND THEIR RELATIONS 



pace, and without resting. Their main propeller is 

 single, median, terminal, and the whole construc- 

 tion is arranged in such a way as to ensure the 

 utmost output of power for considerable stretches of 

 time. 



Swimming is often compared to flying, and re- 

 semblances between the two activities are pointed out. 

 The rapid flight of a large bird through the air has 

 much, apparently, in common with that of a fish in 

 the water. Both agree, certainly, in that they move 

 about in an environment which contains and envelops 

 them completely; they both differ from progress on 

 the land when the earth always provides the starting- 

 point. But, except for this likeness, they differ in 

 every possible way. The motors and propellers are 

 different both as regards construction and situation. 

 The preliminary action of the motors may be identical, 

 since it is furnished by muscular energy, but its 

 application is carried out in an entirely different 

 manner. 



In flying, the density of the individual is greater 

 than that of the air and, since the force of gravity is 

 considerable, the principal effort consists in supporting 

 the body and preventing it from falling. The wings 

 beat in order to support the animal as well as to propel 

 it, for, by reason of its inconsiderable density, air 

 opposes to progress only the slightest resistance. The 

 propellers, the wings, are therefore placed close to the 

 body's centre of gravity, and balance one another while 

 still acting as supports. 



Nothing of this sort is true of swimming, especially 

 when the rate of swimming is rapid, as in the case of 

 the tunnies. The main effort is no longer devoted to 

 supporting the body, for the density of the individual 

 and that of the surrounding water are very similar, 

 and the creature has little difficulty in keeping itself 

 up. What is an absolute necessity for the bird here 

 no longer exists or, at least, only in a very slight 



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