258 CARTILAGINOUS FISHES. 



supposition that the water enters by the hole on the one 

 side, passes through, and escapes by the other ; because, 

 unless there were a difference in the two, which could 

 shut the one aperture against the escape, and the other 

 against the entrance of water, it is not easy to see how 

 it could be accomplished, any more than a person could 

 at one and the same time draw in breath by the one 

 nostril, and let it out by the other. If it is found that 

 there is no other ingress for the water of respiration, 

 while the animal is sucking, than the gill openings ; the 

 simplest plan would be to suppose that these and the 

 thorax had a power of alternate expansion to receive, 

 and contraction to expel, like lungs; and that is an 

 alternation of motion and position, for which the struc- 

 ture of gills but ill adapts them. If, however, Sir 

 Everard Home has not solved the difficulty, he has 

 found it out, and that, in such cases, is half the labour. 

 There is a general rule in all these cases ; the estab- 

 lished laws of the matter of which animals are com- 

 posed, and in or by which they perform the functions 

 of life, must never be violated ; and can never be 

 suspended, without the existence of some contrivance 

 sufficient to effect that purpose : and the great beauty 

 of the whole is, that that contrivance is always the 

 simplest possible, does its office completely, but does 

 nothing more. 



