ACQUIRED DIFFERENCES. 5 



Passing on to Vertebrates we find a striking example 

 among fishes in the case of the Pleuronectidae, where in 

 some species the right half of the body, in other the left, 

 has assumed a singular superiority over the other. The 

 fish which originally had its dorsal surface upwards and 

 its ventral downwards has turned on its side, developed 

 pigment on what becomes its upper surface and lost it 

 on the lower, and by a peculiar twist of the skull the 

 mouth is brought chiefly to the under side and the visual 

 terminals to the upper. In these fishes the modification 

 may have been due to some geological change in the sea- 

 bed, rendering a bottom habit more advantageous, or to 

 some change either in the food sought or in the enemies 

 avoided, which rendered the bottom habit beneficial. 

 They are found fully developed as far back as the Miocene 

 period. 



I, personally, am ignorant of any conspicuous example 

 of asymmetry among Amphibians or Reptiles, and among 

 the Birds they appear to be few. Many birds sleep 

 habitually standing on one leg, but so far as I know (and 

 in this I am supported by the more extensive observations 

 of Dr. William Ogle*), these birds use the right and the 

 left leg indifferently. In the Cockatoo and Parrot tribe, 

 however, there is a preference for the one side ; these 

 birds have developed marked prehensile power in the leg, 

 and, for example, in opening a nut, the bird usually 

 stands on the right leg and holds the nut in its left. 



The great majority prefer to stand on the right leg, 

 but a few use the left, while holding an object in the 

 other claw. 



In Mammalia structure and function are usually 

 symmetrical, but there are exceptions. Where the 

 extremities are used for the simple purposes of support 



-Med. Chir. Trans., Vol. liv. 



