Page - 242 - in The Origin of Species

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