One other significant piece of evidence must be named. Among the _Annulosa_ we found unsymmetrical bilateralness in creatures having habits exposing them to unlike conditions on their two sides; and among the _Vertebrata_ we find parallel cases. They are presented by the _Pleuronectidæ_--the order of distorted flat fishes to which the Sole and the Flounder belong. On the hypothesis of evolution, we must conclude that fishes of this order have arisen from an ordinary bilaterally-symmetrical type of fish, which, feeding at the bottom of the sea, gained some advantage by placing itself with one of its sides downwards, instead of maintaining the vertical attitude. Besides the general reason there are special reasons for concluding this. In the first place, the young Sole or Flounder is bilaterally symmetrical--has its eyes on opposite sides of its head and swims in the usual way. In the second place, the metamorphosis which produces the unsymmetrical structure sometimes does not take place--there are abnormal Flounders that swim vertically, like other fishes. In the third place, the transition from the symmetrical structure to the unsymmetrical structure may be traced. Almost incredible though it seems, one of the eyes is transferred from the underside of the head to the upper side: the transfer being effected by a distorted development of the cranial bones--atrophy of some and hypertrophy of others, along with a general twist. This metamorphosis furnishes several remarkable illustrations of the way in which forms become moulded into harmony with incident forces. For besides the divergence from bilateral symmetry involved by presence of both eyes upon the upper side, there is a further divergence from bilateral symmetry involved by differentiation of the two sides in respect to the contours of their surfaces and the sizes of their fins. And then, what is still more significant, there is a near approach to likeness between the halves that were originally unlike, but are, under the new circumstances, exposed to like conditions. The body is divisible into similarly-shaped parts by a plane cutting it along the side from head to tail: “the dorsal and ventral instead of the lateral halves become symmetrical in outline and are equipoised.”