290 MISCELLANEOUS OBJECTIONS TO THE [Chap. VIL 



tions in the beaks of the different existing members of 

 the duck-family. We should bear in mind that each 

 siDecies of duck is subjected to a severe struggle for ex- 

 istence, and that the structure of every part of its frame 

 must be well adapted to its conditions of life. 



The Pleuronectidffi, or Plat-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 colour, 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 coloured. Soon the eye propei 

 to the lower side begins to glide slowly 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 whilst lying in its habit- 

 ual position on one side. The lower eye would, also, 

 have been liable to be abraded by the sandy bottom. 

 That the Pleuronectidse are admirably adapted by their 

 fiattened and asymmetrical structure for their habits of 

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

 flounders, &c., being, extremely common. The chief 

 advantages thus gained seem to be protection from their 

 enemies, and facility for feeding on the ground. The 

 different members, however, of the family present, as 

 Schiodte remarks, " a long series of forms exhibiting a 



