THEORY OF NATURAL SELECTION 241 



series of forms exhibiting a gradual transition from Hippo- 

 glossus pinguis, which does not in any considerable degree 

 alter the shape in which it leaves the ovum, to the soles, 

 which are entirely thrown to one side." 



Mr. Mivart has taken up this case, and remarks that 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, in- 

 deed, 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 excel- 

 lent observations published in 1867 by Malm. The Pleuro- 

 nectidas, whilst very young and still symmetrical, with their 

 eyes standing on opposite sides of the head, cannot long re- 

 tain 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, they 

 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 be- 

 tween 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 car- 

 tilaginous 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 dis- 

 ease 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 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 upwards; and their skulls arc thus rcn- 



