190 THE ORIGIN OF SPECIES 



that such an incipient transformation must rather have been in- 

 jurious." But he might have found an answer to this objection in 

 the excellent observations published in 1867 by Malm. The Pleu- 

 ronectidae, while very young and still symmetrical, with their eyes 

 standing on opposite sides of the head, cannot long retain a vertical 

 position, owing to the excessive depth of their bodies, the small 

 size of their lateral fins, and to their being destitute of a swim- 

 bladder. Hence, soon growing tired, they fall to the bottom on one 

 side. While thus at rest they often twist, as Malm observed, the 

 lower eye upward, to see above them; and they do this so vigor- 

 ously that the eye is pressed hard against the upper part of the 

 orbit. The forehead between the eyes consequently becomes, as 

 could be plainly seen, temporarily contracted in breadth. On one 

 occasion Malm saw a young fish raise and depress the lower eye 

 through an angular distance of about seventy degrees. 



We should remember that the skull at this early age is carti- 

 laginous and flexible, so that it readily yields to muscular action. 

 It is also known with the higher animals, even after early youth, 

 that the skull yields and is altered in shape, if the skin or muscles 

 be permanently contracted through disease or some accident. 

 With long-eared rabbits, if one ear flops forward and downward, 

 its weight drags forward all the bones of the skull on the same 

 side, of which I have given a figure. Malm states that the newly- 

 hatched young of perches, salmon, and several other symmetrical 

 fishes, have the habit of occasionally resting on one side at the 

 bottom; and he has observed that they often then strain their 

 lower eyes so as to look upward; and their skulls are thus ren- 

 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 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 

 Pleuronectidae are not quite symmetrical even in the embryo ; and 

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

 while 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 Pleuronectidae, rests on its left side at the bottom, 

 and swims diagonally through the water; and in this fish, the two 



