818 THE ORIGIN OF SPECIES 



site sides of the head, cannot long retain a vertical 

 position, owing to tiie excessive depth of their bodies, 

 the small size of their lateral fins, and to their being 

 destitute of a swimbladder. 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 up- 

 ward, to sec above them; and they do this so vigorously 

 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 dis- 

 tance of about seventy degrees. 



"We should remember that the skull at this early age 

 is cartilaginous 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 per- 

 manently contracted through disease or some accident. 

 With long-eared rabbits, if one ear lops 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 1 

 rendered rather crooked. These fishes, however, are soon 

 able to hold themselves in a vertical position, and no 

 permanent effect is thus produced. With the Pleuronec- 

 tidae, on the other hand, the older they grow the more 

 habitually they rest on one side, owing to the increasing 



