234 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
independence of the type of chiasma 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 starry flounder the evidence of the independence of the factor or 
factors 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 circumstances, it does not follow that in other species these factors 
are so unrelated, nor that they have always been independent in the 
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. 306), 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 undvubtedly descended from sym- 
metrical fishes, and, as Johannes Miiller (46) long ago pointed out, 
