186 MISCELLANEOUS OBJECTIONS TO THE [Chap. VII. 



struggle for existence, and that the structure of every part of its 

 frame must be well adapted to its conditions of life. 



The Pleuronectida?, or Flat-fish, are remarkable for their asym- 

 metrical bodies. They rest on one side, — in the greater number of 

 species on the left, but in some on the right side ; and occasionally 

 reversed adult specimens occur. The lower, or resting-surface, re- 

 sembles at first sight the ventral surface of an ordinary fish : it is of 

 a white colour, 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 oppo- 

 site to each other, and the whole body is then symmetrical, with 

 both sides equally coloured. 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 whilst lying in its habitual 

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

 to be abraded by the sandy bottom. That the Pleuronectidse are 

 admirably adapted by their flattened and asymmetrical structure 

 for their habits of life, is manifest from several species, such as 

 soles, flounders, &c, being extremely common. The chief ad- 

 vantages thus gained seem to be protection from their enemies, 

 and facility for feeding 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 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 inju- 

 rious." But he might have found an answer to this objection in 

 the excellent observations published in 1867 by Malm. The 

 Pleuronectida, 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 



