242 ORIGIN OF SPECIES 



dered rather crooked. These fishes, however, are soon able 

 to hold themselves in a vertical position, and no permanent 

 effect is thus produced. With the Pleuronectidse, 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 in- 

 creased through the principle of inheritance. Schiodte be- 

 lieves, in opposition to some other naturalists, that the Pleu- 

 ronectidae are not quite symmetrical even in the embryo; and 

 if this be so, we could understand how it is that certain spe- 

 cies, whilst young, habitually fall over and rest on the left 

 side, and other species on the right side. Malm adds, in con- 

 firmation of the above view, that the adult Trachypterus arc- 

 ticus, 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 remark- 

 ing 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 in- 

 herited 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. Tra- 

 quair 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 



