TRIFLES LIGHT AS AIR. 41 



We need scarcely say to the student of nature, 

 that the form and functions of fishes are as ad- 

 mirably adapted for easy movement through the 

 water, as are those of birds for that aerial motion 

 called flight. Suspended in a liquid element of 

 almost equal specific gravity with themselves, ex- 

 ternal organs resembling those of birds in size, 

 would have been disproportionate and unnecessary ; 

 but the air-bladder (the functions of which, by no 

 means entirely understood, have never been satis- 

 factorily explained in all their bearings) is known 

 to possess the power of contraction and dilatation, 

 the exercise of which is followed by a corresponding 

 descent or ascent of the animal's body. Thus a 

 small central and inconspicuous organ effects, in 

 the easiest and most simple manner, the same ob- 

 ject which even the soaring eagle or giant condor 

 can only accomplish by great exertion of the wings, 

 and after laborious and frequently repeated gyra- 

 tions. We shall ere long, however, have occasion 

 to remark in more detail, that the air-bladder, al- 

 though essential to the economy of such species as 

 possess it, is by no means indispensable as a gene- 

 ral attribute of the class, as in many tribes it is 

 entirely wanting. It is not even a generic cha- 

 racteristic, as it does not exist in the red mullets 

 of the British seas, though possessed by the cor- 

 responding species of Asia and America, while 

 of our two kinds of mackerel, the so called Spanish 

 species (Scomber colias) is distinguished by a 

 swimming bladder, and the common mackerel (Sc. 

 scomber) does not possess that organ. 



