52 REPORT OF COMMISSIONERS OF INLAND FISHERIES. 



the torsion of the cranium carries the eye over the hne of the profile; 

 in other species the dorsal fin has already extended down to the 

 snout before metamorphosis has taken place, and in this case the 

 eye has to pass under the tissues supporting the forward extension 

 of the dorsal fin and thus seems to pass from side to side through 

 the head. 



The attempts to explain how these unique modifications in the 

 fiat-fishes were brought about, probably originated with Lamarck 

 nearly one hundred years ago. As he was the first to conceive and 

 apply systematically a thoroughgoing theory of descent, no one 

 before him had ever recognized that there was any such problem as 

 the origin of the flat-fishes. In accordance with his theory of "use 

 and disuse," he considered that the shifting of the position of the 

 flat-fishes w'as due to the fact that they acquired the habit of living 

 in shallow water, and then were forced to swim on their sides in order 

 to follow their prey near the shore; the eye on the lower side, as the 

 result of the constant straining upward toward the light, finally, in 

 the course of many generations, migrated over to the other side. A 

 very similar explanation has been recently offered by Cunningham 

 (1897). "The action of the eye-muscles would probably have some 

 effect [in twisting the orbital region], and the weight of the fish, 

 resting on the ground, would force the lower eye-ball toward the 

 upper side, and so distort the face. In all probability both these 

 influences have contributed to the result." *He also suggests an ex- 

 planation of the forward extension of the median fins on the same 

 basis; the constant action of the numerous small muscles attached 

 to the fin-rays in the young fish would tend to differentiate an 

 increasing number of rays from the embryonic tissue "while the 

 direction of the muscular strains, the constant endeavor by muscular 

 contraction to draw forward the anterior ends of the fins, would 

 determine the direction of their extension. Thus the mode in which 

 the fins were used would produce in the course of generations the 

 structure and relations which they now possess." There are various 



* Cunningham, Science Progress, 6. 1897, 502-3. 



