372 FISHES 



ventral, which is nearer to the mouth, belongs naturally to the 

 upper side. The asymmetrical feature next in importance is 

 the difference between the two sides in colour : the lower side 

 is white, the upper side coloured, the colours are dark brown 

 ranging to black, with spots or markings ranging from yellow 

 through orange to red ; the plaice, for instance, is characterised 

 by conspicuous red spots. 



There are then three chief peculiarities in which a flat-fish 

 resting on the ground with its original plane of symmetry in a 

 horizontal position, differs from an ordinary fish which swims 

 with its plane of symmetry vertical: (i) the position of both 

 eyes on the upper side ; (2) the absence of pigment from the 

 lower side ; (3) the extension of the dorsal fin along the line 

 outside the dorsal eye, instead of between the eyes as in other 

 fishes. In addition to these there are minor differences between 

 the upper and lower sides : the scales or tubercles are less 

 developed on the lower side, the pectoral fin on the lower side 

 is usually smaller, and as already mentioned the jaws and teeth 

 are usually larger on the lower side, reduced on the upper. 

 Obviously these peculiarities of structure are all adaptations to 

 the peculiarity of position and mode of life, and they serve as 

 excellent examples of what we mean by adaptation. The flat- 

 fishes are evidently descended from ordinary symmetrical 

 ancestors which swam in a vertical position, and in the course 

 of evolution their structure has undergone the modifications 

 which are more suitable for their peculiar position and habits. 

 To have a flattened shape and to lie upon the sand or gravel at 

 the sea-bottom, is a mode of life which offers distinct advan- 

 tages to a fish in the way of food or concealment from enemies, 

 and it has been adopted by other fishes as well as flat-fishes. 

 Skates, for instance, and the angler have adopted this mode of 

 life, but they have always rested on the ground in the vertical 

 position, and either were originally or have become flattened 

 dorso-ventrally without alteration of their original symmetry. 

 The ancestral flat-fishes must have been originally flattened from 

 side to side, and therefore to obtain the advantage of contact 

 with the ground were compelled to lie on one side. In this posi- 

 tion the lower eye would be useless and therefore the head has 

 been modified so as to bring that eye round to the upper side. 

 Anatomy shows that the change is produced by a twisting or 



