188 MORPHOLOGICAL DEVELOPMENT. 



its supporting parts more fully developed, therebv lias the 

 under half of its body made more different from the upper 

 half, and that the bird is subject by its mode of life to yet 

 another set of actions and reactions ; we shall see that these 

 facts are quite congruous with the general doctrine, and fur- 

 nish further support to it. 



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 Pleuronectidce 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 bila- 

 terally-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 speci- 

 fic reasons for concluding this. In the first place, the young 

 Role 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 unsym- 

 metrical 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 under-side of the head to the upper- 

 side. Until lately it was supposed that the change by 

 which the two eyes, originally placed on opposite sides, come 

 to be placed on the same side, is effected by a distortion 

 of the cranium ; but it is now asserted that actual migration 

 of an eye occurs. According to Prof. Steenstrup, the eye 

 passes between the ununited bones of the skull ; but according 

 to Prof. Thomson, it passes under the skin. Be the course of 

 the metamorphosis what it may, however, it furnishes several 



