Suborder Heterosomata 693 
longer obtains. In 49 individuals of four species of dextral 
soles, the left nerve is uppermost in 24, the right nerve in 25. 
Among sinistral soles, or tonguefishes, in 18 individuals of two 
species, the left nerve is uppermost in 13, the right nerve in s. 
Professor Parker concludes from this evidence that soles 
are not degenerate flounders, but rather descended from primi- 
tive flounders which still retain the dimorphic condition as 
to the position of the optic nerves, a condition prevalent in 
all bony fishes except the flounders. 
The lack of symmetry among the flounders lies, therefore, 
deeper than the matter of the migration of the eye. The asym- 
metry of the mouth is an independent trait, but, like the migra- 
tion of the eye, is an adaptation to swimming on the side. Each 
of the various traits of asymmetry may appear independently of 
the others. 
The development of the monomorphic arrangement in 
flounders Professor Parker thinks can be accounted for by the 
Fig. 596.—Larval Flounder, Pseudopleuronectes americanus. 
(After S. R. Williams.) 
principle of natural selection. In a side-swimming fish the 
fixity of this trait has a mechanical advantage. The unmeta- 
morphosed young of the flounder are not strictly symmetrical, 
for they possess the monomorphic position of the optic nerve. 
The reversed examples of various species of flounders (these, 
by the way, chiefly confined to the California fauna) afford 
“striking examples of discontinuous variation.” 
A very curious feature among the flounders is the possession 
in nine of the California-Alaskan species of an accessory half- 
lateral line. This is found in two different groups, while near 
relatives in other waters lack the character. One species in 
Japan has this trait, which is not found in any Atlantic species, 
