and th6 Anatomy 6f Fishes. 296 



gravity of water, an increase of bulk could only retard, instead 

 of facilitating their progress through that element ; — indefed, it 

 must be, on the contrary, an increase of specific gravity which 

 should accelerate the motion in an animal provided with 

 powerful external organs for propelling itself through the water. 

 This organ (or gasometer, if it might be compared with an 

 instrument of modern mechanics) appears to be of much less 

 importance to the swimming of fishes than is generally sup- 

 posed ; and it is probable that its paramount utility will be dis- 

 covered in some other function, when the touchstone of natural 

 philosophy, by actual experiment, shall be duly applied to the 

 subject. 



Men are prone to form hypotheses to suit appearances and 

 their own convenience ; they observe that the lampreys, flound- 

 ers, &c., affect the bottom, and their air-bladder, being less 

 obvious, they place the cause for the effect, and assert they 

 reside at the bottom, because they are not provided with a 

 swimming-bladder ; whilst, if they would take the trouble of 

 searching and judging for themselves, they would find this 

 organ, or something analogous, I presume, in all fishes*. 

 That this organ is smaller in those fishes is true, for th^ 

 obvious reason that their habits and economy render it less 

 necessary for them to raise themselves in the water : the bladder, 

 therefore, less frequently called into use, is, for the same reason, 

 smaller than in most other fishes, as we constantly find the 

 development of the muscles and organs of animals to bear a 

 near relation to the frequency with which they are called into 

 action. Of this there are abundant examples amongst men : 

 mountaineers, or those accustomed to much walking, have 

 stout and strong legs ; whilst sailors and blacksmiths excel in 

 the size and strength of the arm, and the pectoral muscles. 



One remarkable fact respecting the float or air-bladder in 

 fishes is, that it is sometimes double, and at other times single, 

 in individuals of the same species. This has been observed 

 by Blumenbach, and I have found it to prevail in the cuirass of 



il ,* I find it exists in all the species of pleuronectes and jietromyzons, which I 

 ^ave had the opportunity to examine, and even in that extraordinary animal the 

 hag-fish {gastrubranckus ccecits, or myocine glutinosa, Lin.), although this fish, 

 being supposed d^atj^te qC ^iwon, ^pvdd seldom veuture to quit tha bottom. 



