ANIMALS AND THEIR ENVIRONMENTS. 303 



called " back " is darkcoloured, whilst the opposite and 

 presumed under surface is white. Again, the idea that the 

 darker surface is the back, would be strengthened by the 

 observation that it bears the eyes, and further that the fish 

 swims with this surface uppermost. Notwithstanding these 

 apparently well-founded observations, however, the zoologist 

 finds ample reason for a complete denial of their validity 

 and correctness. He would firstly direct attention to the fact 

 that, in the middle of each flat surface of the fish, and in the 

 breast region, a certain short fin (Figs. 59 and 60) is to be dis- 

 cerned. These fins form a pair, possessed by all save the very 

 lowest fishes ; they are named "pectoral" or "breast fins," 

 and correspond, as may be proved by an examination of their 

 skeleton, with the fore limbs of other vertebrate animals. In 

 the flat-fishes, it usually happens that one pectoral fin is of 

 smaller size than the other. Moreover, there are other two 

 fins, also paired, to be discerned in these fishes, placed be- 

 low the breast fins, one on each flat surface of the body, but 

 exhibiting a somewhat rudimentary structure and only a 

 slight development as compared with their representatives in 

 other fishes. These latter are the two " ventral " fins of the 

 zoologist, and an examination of their skeleton and nature 

 shows that in reality they represent the hind limbs of the 

 fish, just as the breast fins correspond to the fore members. 



A very cursory examination of other fishes in which both 

 sets of fins exist, would satisfy us that the paired fins are 

 invariably borne on the sides of these animals. This rule 

 of fish-structure accords with the position of the limbs 

 in all other vertebrate animals. These appendages are 

 always paired, and are invariably lateral in their position 

 and attachments. We are therefore forced to conclude 

 that, unless the flat-fishes present extraordinary ex- 

 ceptions to the laws of limb-development and situation 

 represented in all other vertebrate animals, they must, like 

 other fishes, carry their paired fins or limbs on the sides of 

 their bodies. Otherwise, we must assume that they bear the 

 limbs on their backs and on the lower surfaces of their 



