THEORY OF NATURAL SELECTION. 207 



ventral surface of an ordinary fish ; it is of a white color, 

 less developed in many ways than the upper side, with the 

 lateral fins often of smaller size. But the eyes offer the 

 most remarkable peculiarity ; for they are both placed on 

 the upper side of the head. During early youth, however, 

 they stand opposite to each other, and the whole body 

 is then symmetrical, with both sides equally colored. Soon 

 the eye proper to the lower side begins to glide slowly round 

 the head to the upper side ; but does not pass right through 

 the skull, as was formerly thought to be the case. It is 

 obvious that unless the lower eye did thus travel round, it 

 could not be used by the fish while lying in its habitual 

 position on one side. The lower eye would, also, have been 

 liable to be abraded by the sandy bottom. That the Pleu- 

 ronectidaB are admirably adapted by their flattened and 

 asymmetrical structure for their habits of life, is manifest 

 from several species, such as soles, flounders, etc., being 

 extremely common. The chief advantages thus gained seem 

 to be protection from their enemies, and facility for freeding 

 on the ground. The different members, however, of the 

 family present, as Schiodte remarks, "a long series of forms 

 exhibiting a gradual transition from Hippoglossus 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 toward 

 the other side of the head could benefit the individual is, 

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

 cipient 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, 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 grow- 

 ing 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 see above them ; and they do this so vigorously 

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



