RECAPITULATION. 503 



viscera and abdominal cavity. The dorsal fin extends on 

 to the head, its base being attached, not to the median line 

 of the head, which is bent by the distortion of the eyes, 

 but along the outer edge of the orbit belonging to the 

 lower side, this edge being now in line with the dorsal edge 

 of the body. This extension of the dorsal fin separates both 

 eyes and orbits entirely from the lower side of the head, 

 and adds greatly to the disguise beneath which the original 

 symmetry of the fish is concealed. The increase in the 

 extent of the median fins corresponds to the increased use 

 which is made of them in the peculiar habits of the flat-fish. 

 The lateral muscles of the body and tail are no longer 

 constantly employed in locomotion, but the fish glides over 

 the bottom by means of an undulating motion of its 

 marofinal fins. This motion involves the constant action of 

 the numerous small muscles attached to the fin-rays. Such 

 an action commencing in the individual at a period when 

 the differentiation of the fin-rays from the embryonic tissue 

 belonging to the fins was not completed, would tend it is 

 believed to increase the development of such embryonic 

 tissue and differentiate an increasing number of rays from 

 it, while the direction of the musular strains, the constant 

 endeavour by muscular contraction to draw forward the 

 anterior ends of the fins, would determine the direction of 

 their extension. Thus the mode in which the fins were 

 used would produce in the course of generations the 

 structure and relations which they now possess. 



The third chief general peculiarity of the flat-fishes is 

 the absence of pigment from the skin of the lower side, an 

 absence which has been showm by experiment to depend on 

 the absence of light. It is a fact that pigment is developed 

 on the lower side of these fishes when that side is exposed 

 to light, and it is also a fact that when such exposure is 

 maintained the lower side does not develop pigment so 

 •quickly or so abundantly as the upper side. It is main- 

 tained that the former fact proves that the character was 

 originally acquired by the individual, the latter that it is 

 now strongly inherited. 



There are certain conditions of life which have produced 



