THE FUNCTION AND STRUCTURE OF FINS 451 



fins. A fish deprived of its pectoral members sinks downward 

 at the head and assumes an oblique position in the water. 

 Removal of both the pectoral and pelvic fins of one side causes 

 the fish to roll over to that side ; and if the fins are removed 

 from both sides the animal turns belly upwards like a dead 

 fish."^ The chief mistake made by the performer of these 

 experiments was in assuming that the cutting off of the fins 

 was a no more serious operation for the fish than having his 

 hair cut is for a man. As a matter of fact the mere shock of 

 the operation is sufficient to upset the delicate control which the 

 fish has over the movements of all its muscles, including those 

 which govern the walls of the air bladder. 



Turn now to observations upon able-bodied fish, e.g. a gold- 

 fish. When it is advancing rapidly through the water the 

 larger part of each paired fin lies flat against the body, whilst 

 the upper border alone projects to form a shallow keel (fig. 5). 

 If now a piece of silk or cotton be tied round the fish so as to 

 hold the fins flat against the side and thus put them completely 

 out of action it will be found that the fish does not " turn belly 

 upwards like a dead fish" but that it swims about as merrily 

 as ever. It suffers one inconvenience however, for it now 

 bumps its snout frequently against the sides of the aquarium or 

 other objects in its path. After about five minutes it learns to 

 obviate this by bending its tail fin sharply to the right or left. 

 This leads to the production of a swirl which slows down 

 and eventually stops the movement of the fish (cp. fig. 3). At 

 other times it turns sharply aside if space permits. 



From this experiment it is evident that the fish's body is in a 

 state of stable— and not unstable — equilibrium. It would indeed 

 have been strange if after all these millions of years nature had 

 not succeeded in producing a creature which was in a state of 

 stable equilibrium in its own medium, especially when such 

 stability would be a marked advantage. Nevertheless the centre 

 of gravity of the fish is not very much below the centre of 

 gravity of the water it displaces. This was demonstrated with 

 a fish that had both pairs of fins tied up. A piece of lead was 

 tied on to its back. The weight of this rolled it over immediately. 

 The size of the lead was reduced bit by bit until it was able to 

 keep upright as long as it was swimming rapidly. Even then if 

 it slowed down it rolled over. The weight of the hapless victim 



' Cainbridi^e Natural History^ vol. Fishes, p. 353. 



