THE EVOLUTION OF THE FUNCTION 

 AND STRUCTURE OF THE FINS IN 

 FISHES 



By H. H. SWINNERTON, D.Sc, F.Z.S., F.G.S. 



Most people have watched a fish in its native element and 

 have been fascinated by the grace and ease of its movements ; 

 very few have stopped to inquire about the relation of these 

 to its form and structure. This is all the more surprising 

 because the subject is not without a practical bearing. Investi- 

 gators interested in naval architecture have made many experi- 

 ments which throw much light upon this relationship. 



The general resemblance between the hull of a boat and 

 the body of a fish is familiar to all. This, of course, is traceable 

 to the similarity of the purpose to be served, viz. easy move- 

 ment through water. The advantages of the form adopted can 

 be most easily realised by considering first the case of an un- 

 suitable form such as a rectangle (fig. i). Experiments on this 

 subject have consisted either in drawing the body through the 

 water, or in causing the water to flow past the object.^ The 

 results are the same in both cases. In front of the rectangle a 

 mass of still water accumulates, which forms a natural bow. The 

 rest of the water moves past this bow and along the sides of 

 the rectangle without offering any more resistance to it than 

 would be off'ered to a boat with a bow. At the stern, however, 

 the water turns suddenly in and produces a strong swirl, which 

 tends to suck the body back. The resistance to easy movement 

 is thus felt mainly not at the bow but at the stern. 



The case is very different when a spindle-shaped body is 

 used (fig. 2). In place of the watery bow there is now a proper 

 one. Towards the stern the water is let back gently into its 

 place, thus preventing a swirl. The same conditions are fulfilled 

 by a fish's body. This increases from the tip of the bluntly 



' See for example Prof. H. S. Shaw, "Experiments on the Nature of the 

 Surface Resistance in Pipes and on Ships," Inst. Nav. Archit. 1897. 



447 



