FISHES 1341 



fins." The "breast-fins" correspond to the forelegs 

 of other animals or to the arms of man; while the 

 ventral fins correspond to the hindlegs or to man's 

 lower limbs; and these latter fins may be placed, as 

 hind-limbs should be, to the rear of the body (as in 

 sturgeons) ; or they may be found (as in the cod) 

 placed beneath the breast-fins on the throat. 



It may be asked, How do we know that these two 

 pairs of fins represent the limbs of other animals? 

 We reply, because when we investigate their struc- 

 ture we find them to be supported by a bony skeleton, 

 the various portions of which correspond to those ex- 

 isting in the skeleton of the limbs of man or other 

 vertebrates. And it is only through this important 

 principle of tracing out what are known as the ho- 

 mologies or resemblances between parts, and by look- 

 ing at and comparing their structure, that we are 

 enabled to find out the real nature of many organs in 

 animals; similar organs frequently existing under 

 very different and varied guises. 



The other fins of fishes do not exist in pairs, but are 

 placed in the middle line of the body. Hence they 

 are named the median or unpaired fins. Thus we 

 find the back or dorsal fins to represent the unpaired 

 fins, as also do the tail and anal fins ; the latter being 

 placed on the lower surface of the body. These un- 

 paired fins, if they correspond to any other structures 

 in the fishes, are simply to be regarded as special 

 developments of the skin, and therefore bear no true 

 relationship to the limbs of other animals. We may 

 find one or more dorsal and one or more anal fins; 

 but the tail-fin, by the action of which, as every one 



