THE GENERAL SHAPES OF ANIMALS. 205 



the mechanical converse between organisms of these several 

 types and their environments — on remembering that the 

 fish habitually moves through a homogeneous medium of 

 nearly the same specific gravity as itself, that the terrestrial 

 reptile either crawls on the surface or raises itself very in- 

 completely above it, that the more active mammal, having 

 its supporting parts more fully developed, thereby has 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 

 furnish 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-S3 r mmetrical 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 un- 

 symmetrical 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 struc- 

 ture 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: the 

 transfer being effected by a distorted development of the 

 cranial bones — atrophy of some and hypertrophy of others, 





