316 THE ORIGIN OF SPECIES 



natural selection into lamellae as well-developed as those 

 of the Egyptian goose, in which case they would have 

 been used both for seizing objects and for sifting the 

 water; then into lamellae like those of the domestic duck; 

 and so onward, until they became as well constructed 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 ros- 

 trata, gradations, which may be observed in still-existing 

 Cetaceans, lead us onward 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 ancient Cetaceans, with 

 the functions of the parts slowly changing 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 Pleuronectidae, 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 speci- 

 mens 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 



