58 REPORT OF COMMISSIONERS OF INLAND FISHERIES. 



than the old forms and thus to appropriate for itself a hitherto 

 unclaimed field for its home. Thus natural selection could act and 

 allow the new forms to become permanent. These early modifica- 

 tions having been established, further changes then may be assumed 

 to have arisen by continued mutation and become fixed by natural 

 selection and ultimately to have produced the great variety of 

 existing species and their adaptation to the var3ang conditions of 

 bottom existence. 



According to this theory, the ultimate source of the modifications 

 of the flat-fishes is to be looked for in the germ cell and in the unknown 

 factors which caused essential changes in the germinal structure of the 

 egg of the symmetrical ancestor of the flat-fishes. Just what this 

 change was, to what cause it was originally referable, how the 

 structure of the germ-cell of the asymmetrical fishes differs from that 

 of the symmetrical fishes, such questions must be left until the theory 

 of evolution by mutation has so far advanced that it can show how- 

 mutations result from the conditions which influence germinal 

 structure. 



From the systematic point of view, the flat-fishes are grouped 

 together as a suborder called the Heteroso7nata; they are further 

 divided into two families, the Flounder family (Pleuronectidce) and 

 the Sole family (Solidce). The so-called American Sole is the only 

 common representative of the Sole family in American North Atlantic 

 w^aters; all the other common flat-fishes belong to the Flounder 

 family. The species belonging to the Flounder family may be 

 grouped into three sub-groups or tribes, the Halibut tribe {Hippo- 

 glossince), the Flounder tribe {Plcuronectince) , and the Turbot tribe 

 (Psettince). Their classification may be put into S3'noptic form as 

 follows: 



