Species in the case of certain Fishes. 453 



its food especially above it or at the surface of the water, will 

 have tlie snout turned up and the mouth very oblique ; while 

 the sharks, which also most frequently hunt at tlie surface, 

 will, on the contrary, in general have the mouth completely 

 inferior. But in these two cases it is in the preponderant 

 intervention of other organs that we must seek the explana- 

 tion of the differences of modifications. The former of these 

 fishes, with the organization of its fins, can with difficulty 

 struggle against the influence of the air-bladder, which tends 

 to retain it in the horizontal position ; the latter, destitute of 

 air-bladder, can, on the contrary, not only easily keep a por- 

 tion of the head out of water, and the mouth open at the 

 surface, but can also turn or twist in various directions, thanks 

 to the arrangement of the organs of locomotion and the 

 unequal development of the lobes of the caudal fin. I might 

 select, nearer home, what may be called parallel examples, 

 among the fishes which, in contrast to the above, live and 

 hunt preferently at the bottom of the water. According as 

 these are required to take their food most frequently from 

 above, in front of, or beneath them, and according as the 

 different developments of the air-bladder or the fins permit 

 one position or another in the act of prehension, we shall 

 usually see in them, with a slightly different situation of tlie 

 eye, a more or less oblique arrangement of the buccal cleft, 

 which is then superior, horizontal, or inferior. Compare, 

 among others, in these respects, our goby, the bullhead, and 

 our barbels. 



It would not be difl[icult to multiply these examples, even 

 in different classes ; but I prefer still to limit myself in order 

 now to compare fishes more similar to one another in form^ 

 and to establish here a parallel between various Cyprinida, 

 leading different modes of life, and the various forms of one 

 and the same species, according as the latter is subjected to 

 one or another condition of existence. For this pm-pose I 

 select a family all the members of whicli are equally provided 

 with an air-bladder in communication with the exterior, and 

 which consequently must be able to pass with more facility 

 from one pressure to another. 



If I compare, among others, our various representatives of the 

 genera Alburnus^ Scardinius^ Leuciscus, Ahramis^ Chondro- 

 stoma, Tinea, Carpio, and Barhiis, I see at once that to an 

 habitual station more or less near the surface or the bottom of 

 the water, there usually corresponds a more or less oblicjue 

 ai-rangement of the buccal cleft, sometimes almost superior, 

 sometimes completely inferior. I then remark that with a 

 slightly different diet, most frequently necessitating the pre- 



Ann. d: Mag. N. Hist. Ser. 4. Vol xix. 31 



