III. 



The Evolution of Fiat-Fishes. 



IN starting to deal with any biological question it is well to plant 

 our feet firmly on the solid ground of fact. In the present 

 instance we can ascertain, without much risk of ambiguity, what are 

 the structural peculiarities of the Flat-fishes. All we have to do is 

 to compare them carefully with various symmetrical fishes. The most 

 important and most obvious peculiarities are (i) the position of the 

 eyes, both on the upper side of the body ; (2) the absence of pigment 

 from the lower side ; (3) the anterior extension of the dorsal fin 

 separating the blind side of the head from the eyed side. To these 

 may be added a fourth, which is less important, being more marked 

 in some genera or species than in others, and in some apparently 

 absent, namely, the greater development of the jaws and teeth on the 

 lower side. Mere dissection of adult specimens shows that the 

 anomalous position of the eyes is due to a distortion of the facial 

 region of the skull. The cranial region of the skull is but slightly 

 altered, but the interorbital parts of the two frontal bones are bent 

 away from their original position in the dorsal median line down to 

 the side of the head, and they are also compressed into a thin plate. 

 But the eyes have pretty nearly the same relations to the interorbital 

 septum as in an ordinary fish. There is an eye on each side of the 

 interorbital septum, as usual. 



It is, in fact, the curious condition of the dorsal fin in the flat- 

 fish, even more than the mere distortion of the eyes, which makes it 

 so different from an ordinary fish. If the fin terminated some 

 distance behind the eyes, or if it was prolonged in the direction which 

 it ought to follow, that is along the line which divides the two 

 frontal bones from one another, it would be plain at a glance 

 which was the left side of the head and which the right. 

 It would then be obvious that the left eye was still on the 

 left side of the head, and the right eye on the right. But 

 the dorsal fin does neither of these things. The ectethmoid bone 

 belonging to the blind side is much enlarged, and sends back a 

 process outside the eye belonging to that side to meet another 

 process from the cranial region of the skull. Thus the eye 

 which has migrated, the upper eye when the fish is held in a 

 vertical plane, is enclosed in a complete bony orbit, while the 

 lower eye is merely bounded on its outer side by the jaw-muscles. 



