234 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



independence of the type of chicasma and the kind of migration of the 

 eye, in some species at least, has been pointed out in this paper. It 

 thus appears that the asymmetry of a flatfish is made up of numerous 

 more or less independent elements, which in the typical individual are 

 brought together by a combination of events, but which may from time 

 to time show evidence of their independence by appearing in unusual 

 ways. What the factors are that control these elements in the asym- 

 metry of the fish is unknown, but how they may be discovered has been 

 indicated by Agassiz ('79, p. 12), who initiated experiments on the 

 unmetamorphosed fishes to ascertain the influence of light from below, 

 experiments which when carried out still further by Cunningham and 

 MacMunn ('94, p. 791) showed that this factor is of importance in 

 determining pigmentation. 



Although it must be admitted that in the halibut, bastard halibut, 

 and staiTy flounder the evidence of the independence of the factor or 

 factoi'S determining the crossing of the optic nerves and those controll- 

 ing the migrations of the eyes is as complete as it well can be under 

 the cii'cumstances, it does not follow that in other species these factors 

 are so unrelated, nor that they have always been independent in tlie 

 three species named. The fact that in every species of Pleuronectidae that 

 turns in only one direction (Table III.) the nerve of the migrating eye is 

 always dorsal shows that there has been at least in the past a very 

 intimate relation between the process of chiasma formation and that of 

 eye migration. It seems beyond a doubt that in the ancestral Pleuronec- 

 tidae the process of forming a chiasma was narrowed down to the produc- 

 tion of that type which was mechanically most advantageous for the 

 migrating eye, and thus a stock arose in which a particular type of chiasma 

 was associated with a particular type of asymmetry. From this stand- 

 point the occurrence of reversed specimens, as in the three species already 

 mentioned (Table IV.), cannot be regarded a primitive trait, as implied 

 by Thilo (:02, p. 30G),but must be looked upon as a new departure, for 

 all these species show in their optic chiasmata the stamp of an ances- 

 tral condition uniform for each one. 



Although phylogenetic questions, like taxonomic, are seldom well 

 answered on the basis of single characters, single characters are often 

 very important in the investigation of these questions. From this 

 standpoint the crossing of the optic nerves has a significant bearing 

 on the general questions of the origin and the present classification 

 of the flatfishes. The flatfishes have undoubtedly descended from sym- 

 metrical fishes, and, as Johannes MUller ('46) long ago pointed out. 



