9^ LOCOMOTION OF PISHES, 



chiefly lateral in fislies; and, of course, in those 

 which swim, as the majority of them do, on their 

 belly, it moves in the water from side to side ; 

 whereas in those which, like the osseous flat fishes, 

 swim on their sides, it moves upwards and down- 

 wards ; but its effects are, of course, precisely the 

 same in urging the animal forwards, except that, in 

 the latter case, the animal advances in the diagonal, 

 intermediate, not between a force urging it to the 

 right and another urging it to the left, but between 

 a force urging it downwards and another urging it 

 upwards. On the other hand, in the cetaceous 

 tribes the movements of the tail are only or chiefly 

 perpendicular ; so that, in them, swimming as they 

 do on their bellies, the tail moves upwards and 

 downwards, and they are urged forwards, therefore, 

 on precisely the same principles as the flat osseous 

 fishes while swimming on their sides. 



Nor is the tail of fishes employed merely as an 

 oar to effect their progress in the water, but also as 

 ft rudder to direct it ; the slightest continued incli- 

 nation of this organ to the right side, for example, 

 while the body is still in motion, necessarily deter- 

 mining the direction of the course of the animal in 

 the same degree to this side, the resistance now 

 offered by the water to the course of the animal 

 directly forwards being greater on this side than on 

 the other ; and the same thing results if the fish 

 move the tail through a greater sweep, or with more 

 force, from right to left, than in the opposite direc- 

 tion. And if either this inclination of the tail to 



