INTRODUCTION. 75 



principles as a bird rises in the air ; and in most 

 other fishes, under these circumstances, by means 

 of either these fins or the tail. It is true, the tail 

 of fishes, in general, being placed upright, and not 

 flat, as it is in the cetaceous tribes, and moving 

 from side to side, and not upwards and downwards, 

 is calculated, not so much to raise them in the 

 water — as that of the cetaceous tribes does — as to 

 propel them forwards in a horizontal line ; but it 

 must be remembered, that some of the fishes which 

 are destitute of an air-bladder, for example, most 

 of the flat fishes, swim, not on their bellies, but 

 on their sides, so that their tail, in fact, lies flat 

 upon the water, its motions are perpendicular, and 

 it is, consequently, as well adapted for raising 

 them in this fluid, as that of the cetacea. The 

 circumstance of these tribes swimming on their 

 sides, is a corroboration of the opinion, that one 

 use of the air-bladder is to keep the back of those 

 fishes, which possess it, uppermost. It does by 

 no means follow, however, that fishes, destitute 

 of an air-bladder, may not have other equally 

 effectual means of keeping the back uppermost 

 in their motions through the water. In the 

 eel-shaped lampreys and myxines, the imperfect 

 cartilaginous spinal column is probably as light, 

 or lighter, than the aggregate of the other parts; 

 in the rays, the same motions which raise them 

 in the w^ater, necessarily keep the back upwards; 

 and in the other fishes, above enumerated, as 

 wanting an air-bladder, it is easy to conceive 



