SWIMMING 



one side, and antagonistic bending by the body then hitches it to the 

 other side, but all actions and all motions are equalized. i 



As our cutworm cannot swim because there are two antagonistic pro- 

 pulsive strokes of equal force and two antagonistic body divisions of 

 equal mass and shape, it is evident that in order to make a swimming 

 organism out of it we would have to make certain interdependent altera- 

 tions as follows: There would have to be provision for holding the 

 forward part of the body relatively inert to furnish a base from which 

 propulsive movements could be initiated. This could be accomplished 

 by increasing the mass of the anterior part of the animal and altering 

 its shape so that it offered more resistance to torsional stress. This 

 would have the effect of displacing the center of mass of the entire ani- 

 mal forward of the middle. A relative increase in the mass of the an- 

 terior part would cause a relative decrease in that of the posterior por- 

 tion, and the latter should also experience some flattening and broaden- 

 ing, to provide a greater area for operating against the water. The result 

 may be likened to a man that is propelling a skiff by a single sculling- 

 oar from the stern. But this sculling-oar may be very short and light, 

 or very long and heavy. If the former, conditions are comparable to 

 those encountered in fishes of the Ostraciidae type, in which the entire 

 body is incased in an unyielding covering and locomotion is accom- 

 plished by vibration of the tail tip. Or if the sculling-oar be very long 

 and heavy, conditions may better be compared to the mackerel, whale 

 and similar marine types which propel themselves by oscillations of the 

 entire body. Both these groups might conceivably have body forms of 

 essentially the same degree of stream-line perfection, and the center of 

 gravity would therefore tend to be located at the same point in both, 

 although varying somewhat with the speed of movement. But the 

 actual center or pivot of motion would be very different indeed. In 

 the ostraciiform fish it would be located in the tail proper and far from 

 the center of mass, while in the other sort it would be much farther for- 

 ward and nearer the center of mass. In effect, a fish can hold its body 

 rigid and move only the extreme tip of the tail in order to glide forward, 

 or it can move the entire body; and this is what introduces difficult 

 physics. Not only is the amount of force applied in swimming inde- 

 terminate, but the location of the exact area over which it operates is 



^ It must be understood in this connection that there is no assertion made of 

 the inability of a short, worm-like body to swim through the water. Some 

 larval forms can do so very well, but this ability depends upon some particular 

 specialization which they have been able to develop. 



[15] 



