Chap. VII.] THEORY OF NATURAL SELECTION. 293 



nation of the abnormal condition of the Pleuronec- 

 toids." 



We thus see that the first stages of the transit of the 

 eye from one side of the head to the other, which Mr. 

 Mivart considers would be injurious, may be attributed 

 to the habit, no doubt beneficial to the individual and 

 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 colourless 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 

 due to the exclusion of light. But it cannot be sup- 

 posed 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 surround- 

 ing surface, or the presence of bony tubercles on the 



