256 NATURAL THEOLOGY. 



artifice. A diving-machine might be made to as* 

 cend and descend, upon the hke principle ; name- 

 ly, by introducing into the inside of it an air-vessel, 

 which, by its contraction, would diminish, and by 

 its distension enlarge, the bulk of the machine it- 

 self, and thus render it specifically heavier or spe- 

 cifically lighter than the water which surrounds it» 

 Suppose this to be done, and the artist to solicit a 

 patent for his invention : the inspectors of the mo- 

 del, whatever they might think of the use or value 

 of the contrivance, could by no possibility enter- 

 tain a question in their minds, whether it were a 

 contrivance or not. No reason has ever been as- 

 signed, — no reason can be assigned, why the con- 

 clusion is not as certain in the fish as it is in the 

 machine ; why the argument is not as firm in one 

 case as the other. 



It would be very worthy of inquiry if it were 

 possible to discover, by what method an animal 

 which lives constantly in water is able to supply 

 a repository of air. The expedient whatever it 

 be, forms part, and perhaps the most curious part 

 of the provision. Nothing similar to the air-bladder 

 is found in land-animals ; and a life in the water 

 has no natural tendency to produce a bag of air. 

 Nothing can be further from an acquired organi- 

 zation than this is.^^ 



^ The aea varies in temperature and pressure at different 

 depths, and no doubt the texture of the fish^ and especially of its 

 integument must conform to this variety. The swimming-bladder 

 13 the means of adjustment by which the fish lives at its native 



