Chap. TIL] THEORY OF NATURAL SELECTION, 293 



to the species, of endeavouring to look upwards with 

 both eyes, whilst resting on one side at the bottom. We 

 may also attribute to the inherited effects of use the 

 fact of the mouth in several kinds of flat-fish being bent 

 towards the lower surface, with the jaw bones stronger 

 and more effective on this, the eyeless side of the head, 

 than on the other, for the sake, as Dr. Traquair supposes, 

 of feeding with ease on the ground. Disuse, on the other 

 hand, will account for the less developed condition of the 

 whole inferior half of the body, including the lateral fins ; 

 though Yarrell thinks that the reduced size of these fins 

 is advantageous to the fish, as " there is so much less 

 room for their action, than with the larger fins above." 

 Perhaps the lesser number of teeth in the proportion of 

 four to seven in the upper halves of the two jaws of the 

 plaice, to twenty-five to thirty in the lower halves, may 

 likewise be accounted for by disuse. From the colour- 

 less state of the ventral surface of most fishes and of 

 many other animals, we may reasonably suppose that the 

 absence of colour in flat-fish on the side, whether it be 

 the right or left, which is undermost, is clue to the exclu- 

 sion of light. But it cannot be supposed that the peculiar 

 speckled appearance of the upper side of the sole, so like 

 the sandy bed of the sea, or the power in some species, 

 as recently shown by Pouchet, of changing their colour 

 in accordance with the surrounding surface, or the 

 presence of bony tubercles on the upper side of the 

 turbot, are due to the action of the light. Here natural 

 selection has probably come into play, as well as in 

 adapting the general shape of the body of these fishes, 

 and many other peculiarities, to their habits of life. "We 

 should keep in mind, as I have before insisted, that the 

 inherited effects of the increased use of parts, and perhaps 



