240 ORIGIN OF SPECIES 



as those of the shoveller, in which case they would have 

 served exclusively as a sifting apparatus. From this stage, 

 in which the lamellae would be two-thirds of the length of 

 the plates of baleen in the Balaenoptera rostrata, gradations, 

 which may be observed in still-existing Cetaceans, lead us 

 onwards to the enormous plates of baleen in the Greenland 

 whale. Nor is there the least reason to doubt that each step 

 in this scale might have been as serviceable to certain an- 

 cient Cetaceans, with the functions of the parts slowly chang- 

 ing during the progress of development, as are the grada- 

 tions ill 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 Pleuronectidse, 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 sur- 

 face 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 pecu- 

 liarity; 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 proper 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 for- 

 merly 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 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, floun- 

 ders, &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 



