PROGRESSIVE MOTION IN FISHES. 413 



the centre of gravity, c, will move onwards in the 

 direction c b, parallel to p r. This impulse is 

 not destroyed by the farther flexion of the tail 

 towards the left side, because the principal force 

 exerted by the muscles has already been ex- 

 pended in the motion from r to m, in bringing it 

 to a straight line with the body ; and the force 

 which carries it on to l is much weaker, and 

 therefore occasions a more feeble reaction. 

 When the tail has arrived at the position l, indi- 

 cated by the dotted outline, a similar action of 

 the muscles on the right side will create a resist- 

 ance and an impulse in the direction of k l, and a 

 motion of the whole body in the same direction, 

 c A. These impulses being repeated in quick 

 succession, the fish moves forwards in the diago- 

 nal c D, intermediate between the directions of 

 the two forces. By bending the whole body 

 almost in a circle, and then suddenly straighten- 

 ing it, fishes are often able to leap to the top of a 

 high cataract, in ascending against the stream of 

 a river. 



Such being the plan upon which progression 

 is to be effected, we find that every part of the 

 mechanism of the fish is calculated to promote 

 its execution. The principal muscular strength 

 is bestowed upon the movements of the tail ; and 

 the largest assemblage of muscles consists of those 

 which give it the lateral flexions that have been 

 just described. For this purpose all the impor- 



