SWIMMING OF FISHES. 



369 



wards in the diagonal c d, intermediate between 

 the directions of the two forces. By bending the 

 whole body almost in a circle, and then suddenly 

 straightening 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 on which progression is to 

 be effected, we find that every part of the me- 

 chanism 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 



i86 



the axis of the fish is, at the same time, continually changing, the 

 direction of the projectile force changes likewise at every instant. 

 The combined effect of all these changes, both of 

 direction and of intensity in the impelHng forces, 

 renders the problem of determining the exact form 

 of the curve described by the centre of gravity 

 of the fish during its progression, exceedingly in- 

 tricate. 



Mr. Lubbock, who has pointed out to me the 

 necessity of taking all these circumstances into 

 account, conceives that this curve must have some 

 resemblance to that delineated in Fig. 185*, com- 

 mencing from G, the position of the centre of 

 gravity, before the fish begins to move ; receding to 

 the point marked 1, by the first flexion of the tail 

 to the right; then advancing to 2, by its return to 

 the straight position ; again receding to 3, by the 

 next flexion to the left : and so on, through a series 

 of points of contrary flexure 1,2, 3, 4, 5, &c. But, 

 in consequence of the velocity acquired, the motion 

 will not be exactly in the direction of the resultant 

 of the pressure ; and the fish will not, in fact, have 

 resumed the straight or uncurved position at these 

 points of contrary flexure, but rather sooner. 

 The theory of Barthez, which has been adopted by subsequent 

 writers, appears to me to be incorrect. 

 VOL. I. B B 



