53 



have been worked in its present position just as well if the oblique muscles had been 

 attached to the side of the interorbital septum, as the right muscles are. But, on the 

 theory for which I contend, there must have been some fixed point or fulcrum which 

 remained fixed while the action of the muscles altered the position of neighbouring 

 parts in relation to this fixed structure. The left ectetmoid of the sole fulfils the 

 precise conditions required of such a fulcrum. A man cannot lift himself up in a 

 basket, and the eye muscles could not b} T physiological effects have removed themselves 

 bodily by their own action to a new position. But two muscles by straining on their 

 own attachment, when unresisted by other muscles could twist neighbouring structures 

 round that attachment, must in fact do so, the bone to which they were attached 

 remaining in the same position and becoming enlarged. This is exactly what we find 

 to have taken place in the sole. Thus the change which has taken place in evolution 

 in the eyes and orbits of the sole is exactly of the same kind and in the same direction 

 as, but much greater in degree than, the change which must have taken place in the 

 individual fish which lay on its side and looked out witli its lower eye beyond the 

 e:lge of its head. Why, then, should we hesitate to conclude that the evolution has 

 been due to the accumulation, by inheritance, of the modifications due to known 

 physiological effects of functional activity? The only reason for hesitation is that 

 some zoologists say that acquired characters are not inherited, an assertion which 

 seems to me to be contrary to the evidence on the subject. However, this is not the 

 place to enter upon a discussion of the question of the heredity of acquired characters. 

 My purpose has been merely to describe the relations of the eye muscles to the orbits 

 in the sole, relations which I believe are not accurately described in any existing 

 anatomical treatise, and to point out that these relations are such as would necessarily 

 have resulted if the distortion of the orbits and the migration of the left eye hid been 

 due in the course of evolution to the constant action of the oblique muscles of the 

 left eye. It must be remembered that the other peculiarities in the structure of the 

 sole, namely, the extension of the dorsal fin to the snout and the asymmetrical 

 development of the jaws, are not in any sense consequences of the distortion of the 

 orbits. In many flat fishes both eyes are on one side, as in the sole, while the jaws of 

 the two sides are almost perfect!) 7 symmetrical, and the dorsal fin terminates behind 

 the eyes. I believe that both these peculiarities in the sole are modifications due to 

 physiological causes connected with the habit of lying on the left side ; but it is certain, 

 I think, that these physiological causes have nothing to do with the action of the 

 oblique muscles of the left eye. In endeavouring to trace the evolution of the sole 

 from a symmetrical ancestor each modification must be explained separately ; the 

 distortion of the eyes and orbits may be explained by the action of the oblique 

 muscles of the eyes, but this cause does not in the least explain the absence of colour 

 on the under side, nor the greater size of the jaws on the lower side, nor the anterior 

 extension of the dorsal fin. 



