CHAP. VII.] THEORY OF NATURAL SELECTION. 175 



thus produced. With the Pleuronectidae, on the other hand, the 

 older they grow the more habitually they rest on one side, owing 

 to the increasing flatness of their bodies, and a permanent effect is> 

 thus produced on the form of the head, and on the position of the 

 eyes. Judging from analogy, the tendency to distortion would no 

 doubt be increased through the principle of inheritance. Schiodte 

 believes, in opposition to some other naturalists, that the Pleuro- 

 nectidae are not quite symmetrical even in the embryo ; and if this 

 be so, we could understand how it is that certain species, whilst 

 young, habitually fall over and rest on the left side, and other 

 species on the right side. Malm adds, in confirmation of the above 

 view, that the adult Trachypterus arcticus, which is not a member 

 of the Pleuronectidse, rests on its left side at the bottom, and swims 

 diagonally through the water ; and in this fish, the two sides of 

 the head are said to be somewhat dissimilar. Our great authority 

 on Fishes, Dr. Giinther, concludes his abstract of Malm's paper, by 

 remarking that " the author gives a very simple explanation of the 

 abnormal condition of the Pleuronectoids." 



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 con- 

 dition 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 can- 

 not 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 



