CHAP. VII.] THEORY OF NATURAL SELECTION. 173 



prominent, recurved teeth, like those of the Merganser (a member 

 of the same family), serving for the widely different purpose ot 

 securing live fish. 



Returning to the whales. The Hyperoodon bidens is destitute 

 of true teeth in an efficient condition, but its palate is roughened, 

 according to Lacepede, with small, unequal, hard points of horn. 

 There is, therefore, nothing improbable in supposing that some 

 early Cetacean form was provided with similar points of horn on 

 the palate, but rather more regularly placed, and which, like the 

 knobs on the beak of the goose, aided it in seizing or tearing its 

 food. If so, it will hardly be denied that the points might have 

 been converted through variation and 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 onwards, 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 Balae- 

 noptera 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 

 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 Pleuronectidas, or Flat-fish, are remarkable for their asym- 

 metrical bodies. They rest on one side, in the greater number 

 of species on the left, but in some on the right side; and occa- 

 sionally 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, how- 

 ever, 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 

 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 habitual position on one side. The lower eye 

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



