3 H LEISURE-TIME STUDIES. 



their popular name from their exceeding thinness of body, 

 and which are allied to the familar tape or ribbon-fishes. 

 The deal-fish rests on its left side, and, like the flat-fishes, is- 

 a bottom-living species. Moreover, it swims diagonally 

 through the water from its want of symmetry, and evinces a 

 disparity in development between the two sides of the head. 

 The occurrence of allied conditions in the heads of higher 

 animals and in other and distinct groups of fishes, would 

 seem to argue clearly and forcibly in favour of like conditions 

 producing like results to those seen in the flat-fishes. Nor 

 must we lose sight of the fact that disuse of the fins of the 

 lower side in the flat-fishes will account for their lesser size, 

 as compared with those of the upper surface ; and that the 

 jawbones are stronger and teeth more numerous on the 

 lower side of the head. This latter result accrues naturally 

 from the more constant use of the jaws on the lower side of 

 the head than on the eyed side in the act of feeding on the 

 ground, a fact pointed out by Dr. Traquair, and which serves 

 to illustrate the influence of "use" in developing structures, 

 as opposed to the effects of " disuse " in rendering organs 

 useless and abortive. From every consideration, we are 

 forced to conclude that the flat-fishes present us with typical 

 examples of animals which owe their peculiar form and 

 habits to the circumstances of their life, associated with the 

 action of environments upon their frame. We learn from 

 the consideration of such features of living beings, not only 

 how perfectly adaptation to circumstances is correlated with 

 structure and life at large, but also how plastic and mobile 

 under the sway of outward forces the living organism may 

 prove. Whilst no less powerfully does the consideration of 

 the flat-fishes and their modifications support the ideas that 

 the existing order of nature is largely due to secondary 

 causes and to mechanical forces which acquire dominance 

 and power over living beings through the effects of perpetu- 

 ated habit, and of use or disuse continued through long 

 periods of time. 



Within the confines of the group of vertebrate animals 



