46o NATURE AND MAN. 



gullet. Such fish cannot take into it any air from the outside ; so 

 that the air which is found in the sac in some instances, would 

 seem to have been secreted from the blood. It is commonly sup- 

 posed that the fish uses this bladder for so regulating its specific 

 gravity as to rise or sink in the water ; but there is no adequate 

 basis for this hypothesis. For there is no muscular structure in 

 the bag to cause it to increase or diminish in size ; and there is 

 no outside arrangement of muscles that can be conceived to 

 answer this purpose. Moreover, when deep-sea Fish, having a 

 closed swimming-bladder, are brought to the surface, their swim- 

 ming-bladders burst in consequence of the removal of external 

 pressure, and the fish are killed. The most singular thing is, 

 that there are genera of fish, the Scomber (or Mackerel tribe) for 

 instance, of which some species have a swimming-bladder, and 

 others none ; and it cannot be affirmed that the latter are less 

 able to swim at different depths than the former. This swimming- 

 bladder, in certain other forms of fish, retains its original com- 

 munication with the pharynx ; and air can then pass into it from 

 the outside. Carp in ponds are often seen to swallow air; and 

 you may occasionally see gold-fish, which are a kind of carp, 

 coming to the surface of the water of the globes in which they are 

 kept, discharging air-bubbles and taking in a fresh supply. It 

 seems pretty certain, then, that there are fish which use this rudi- 

 mentary lung really for the purpose of respiration ; certainly the 

 Ganoid fishes do, which are a most important group in the evolu- 

 tionary series, connecting Fishes with Reptiles. 



Now, of the first appearance of this organ, and of its develop- 

 ment into a closed air-bladder, it seems to me that Natural Selec- 

 tion gives no account whatever. Let it be supposed that the 

 pharyngeal pouch "formed itself" in some ancestral fish as an 

 "aimless" variation; how can it be conceived to have been of 

 such service to the animals which possessed it, that they beat 

 others in the struggle for existence, — when we do not find this to 

 be the case even with the fully-developed swimming-bladder? 

 And how can we account for the progressive elongation of the 

 pouch into a closed swimming-bladder, if, in this condition, it is 

 of no use to its possessors ? To me it seems as if the whole 

 evolutionary history of this organ plainly poii ts to its ulterior 



