220 MISCELLANEOUS OBJECTIOIiS TO THE 



Cetaceans, with the functions of the parts slowly clmnging 

 during the progress of development, as are the gradations 

 in the beaks of the different existing members of the duck- 

 family. We should bear in mind that each species of duck 

 is subjected to a severe struggle for existence, and that the 

 structure of every part of its frame must be well adapted 

 to its conditions of life. 



The Pleuronectid^e, or Flat-fish, are remarkable for 

 their asymmetrical bodies. They rest on one side — in the 

 greater number of species on the left, but in some on the 

 right side; and occasionally reversed adult specimens 

 occur. The lower, or resting- surface, resembles at first 

 sight the ventral surface of an ordinary fish; it is of a white 

 color, less developed in many ways than the upper side, 

 with the lateral fins often of smaller size. But the eyes 

 offer the most remarkable peculiarity; for they are both 

 placed on the upper side of the head. During early youth, 

 however, they stand opposite to each other, and the whole 

 body is then symmetrical, with both sides equally colored. 

 Soon the eye proper to the lower side begins to glide 

 elovvly round the head to the upper side; but does not pass 

 right through the skull, as was formerly thought to be 

 the case. It is obvious that unless the lower eye did thus 

 travel round, it could not be used by the fish while lying 

 in its habitual position on one side. The lower eye would, 

 also, have been liable to be abraded by the sandy bottom. 

 That the Pleuronectidae are admirably adapted by their 

 flattened and asymmetrical structure for their habits of 

 life, is manifest from several species, such as soles, flound- 

 ers, etc., being extremely common. The chief advantages 

 thus gained seem to be protection from their enemies, and 

 facility for feeding on the ground. The different mem- 

 bers, however, of the family present, as Schiodte remarks, 

 *^a long series of forms exhibiting a gradual transition 

 from Hippoglossus pinguis, which does not in any consid^ 

 erable degree alter the shape in which it leaves the ovum, 

 to the soles, which are entirely thrown to one side.""* 



Mr. Mivart has taken up this case, and remarks that a 

 sudden spontaneous transformation in the position of the 

 eyes is hardly conceivable, in which I quite agree with him. 

 He then adds: '^ If the transit was gradual, then how such 

 transit of one eye a minute fraction of the journey toward 



