1894.] TEIiBOSTEAJS" MOBPHOIOGT. 445 



of the modifications due to known physiological efiiects of functional 

 activity." 



It is evident, in the specimen before us, that whether the fish 

 made any efforts to look upwards or not, it could certainly never 

 have succeeded in doing so, so that the supposed original stimulus 

 of the rotation may in this instance be considered as wanting. 

 Yet the metamorphosis of the skull has proceeded on the normal 

 lines, and so far from the efforts of the eye having apparently 

 affected the rotation of the ectethmoid, the condition we have 

 observed suggests rather that the rotation of the ectethmoid has 

 slightly displaced the eye. It may be said that the eye, intercepted 

 too soon by the pseudomesial, may nevertheless, by the continuance 

 of its efforts, be responsible for the rotation of the ectethmoid : 

 but, as the dii-ection of the strain would be diverted, by the oblique 

 muscles having to pull round the obstacle instead of directly, one 

 would expect that any resulting rotation would be less normal 

 than in the present instance. At the same time the reduction in 

 the anterior part of the ectethmoid seems very possibly connected 

 with functional inactivity. 



In the present state of our knowledge, it may be argued that an 

 acquired character may become so fixed that the defection of what 

 was originally the cause becomes of no moment in development; 

 so that the condition of pur specimen is no bar to the acceptance 

 of this theory in the case of Flat-fish evolution. While unwilling 

 to dispute such a proposition, I am inclined to think that the 

 almost entire absence of modifications from the normal type, which 

 might be attributed to the physiological effects of the arrest of the 

 migration of the eye, point rather to the theory of Natural Selection 

 as the true interpretation of the evolution in question. 



EXPLANATION OP THE PLATES. 



Plate XXVIIL 



Fig. 1. Molva ahyssorum, 47 inches long. X ^^ ca, 

 2. vulgaris, 35| inches long. X A ca. 



Plate XXIX. 



Fig. 3. Abdominal viscera of M. ahyssorum, % , from the left side ; slightly 

 displaced. 

 3 a. The intestine of another specimen of M. ahyssorum. 

 4. Abdominal viscera of M. vulgaris, J, from the left side; slightly 

 displaced. 



Plate XXX. 



Fig. 5. Ocular side of head of Plaice. Skin and part of superficial jaw- 

 muscles (»z.) removed so as to expose the membranous wall of the 

 right orbital cavity {m..w.) and the right recessus orbitalis (r.). 

 X ica. 



e.s. Cut edge of stin. 

 i.s. Bony interorbital septum. 

 6. Right recessus orhitalis of Plaice ; the outer wall partly removed. 

 Enlarged and slightly diagrammatic. 

 0. Opening into orbital cavity, 

 s. Muscular septum. 



