Seite - 240 - in The Origin of Species

240 ORIGIN OF SPECIES as those of the shoveller, in which case they would have served exclusively as a sifting apparatus. From this stage, in which the lamellae would be two-thirds of the length of the plates of baleen in the Balaenoptera rostrata, gradations, which may be observed in still-existing Cetaceans, lead us onwards to the enormous plates of baleen in the Greenland whale. Nor is there the least reason to doubt that each step in this scale might have been as serviceable to certain an- cient Cetaceans, with the functions of the parts slowly chang- ing during the progress of development, as are the grada- tions ill the beaks of the different existing members of the duck family. We should bear in mind that each species of duck is subjected to a severe struggle for existence, and that the structure of every part of its frame must be well adapted to its conditions of life. The Pleuronectidse, or Flat-fish, are remarkable for their asymmetrical 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, resembles at first sight the ventral sur- face 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 pecu- liarity; 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 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 for- merly 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 one side. The lower eye would, also, have been liable to be abraded by the sandy bottom. That the Pleuronectidae are admirably adapted by their flattened and asymmetrical structure for their habits of life, is manifest from several species, such as soles, floun- ders, &c., being extremely common. The chief advantages 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