Chap. VII. J THEORY OF NATURAL SELECTION. 291 



a sudden spontaneous transformation in the position of 

 the eyes is hardly conceivable, in which I quite agree 

 with him. He then adds : " if the transit was gradual, 

 then how such transit of one eye a minute fraction of 

 the journey towards the other side of the head could 

 benefit the individual is, indeed, far from clear. It seems, 

 even, that such an incipient transformation must rather 

 have been injurious." But he might have found an 

 answer to this objection in the excellent observations 

 published in 1867 by Malm. The Pleuronectidse, whilst 

 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 swimbladder. Hence soon growing tired, 

 thev fall to the bottom on one side. Whilst thus at 

 rest they often twist, as Malm observed, the lower eye 

 upwards, 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, 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 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 forwards and 

 downwards, its weight drags forward all the bones of 

 the skull on the same side, of which I have given a 



