TEEOR Y OF NA TUBAL SELECTION. ^'i I 



the other side of the head could benefit the iiidividudl i,-, 

 indeed, far from clear. It seems, even, that sucli an in- 

 cipient transformation must rather have been iiijurious." 

 But he might have found an answer to this objection in 

 the excellent observations published in 1867 by Malm. 

 The Pleuronectidffi, while very young and still symmetri- 

 cal, with their eyes standing on opposite sides of the head, 

 cannot long retain a vertical position, owing to the exces- 

 sive 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 ob- 

 served, the lower eye upward, to see 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, tem- 

 porarily 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 

 cartilaginous and flexible, so that it readily yields to mus- 

 cular action. It is also known with the higher animals, 

 even after early vouth, that the skull vields and is altered 

 in shape, if the skin or muscles be permanently contracted 

 through disease or some accident. With long-eared rab- 

 bits, 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 tlie 

 newly-hatched young of perches, salmon, and several other 

 symmetrical fishes, have the hal3it 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 rendered rather crooked. These 

 fishes, however, are soon able to hold themselves in a ver- 

 tical 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, tlie tendency 

 to distortion would no doubt be increased through the 

 principle of inheritance. Schiodte believes, in opposition 



