236 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
perfectly sound, for there are not a few species, like the starry flounder, 
the bastard halibut, etc., in which the ventral position of the nerve of 
the migrating eye occurs in many adults. The death rate of these indi- 
viduals, as compared with that of individuals having the nerve of the mi- 
grating eye dorsal, would, however, be significant. Duncker (: 00, p. 339) 
has determined this for Pleuronectes flesus. Ina large collection of material 
from Plymouth, England, including the dextral and the sinistral indi- 
viduals in natural proportion, it was found that among the smaller, and 
presumably younger, individuals the sinistral specimens were relatively 
more abundant than among the larger ones, the proportion being about 
one hundred to eighty-five. As Duncker correctly concludes, the death 
rate of the sinistral individuals must therefore be higher than that of the 
dextral ones. As this is a dextral species, it follows that individuals in 
which the nerve of the migrating eye is ventral are more open to early 
death than those in which this nerve is dorsal, and that therefore there 
is good reason to suppose that the dorsal position of the nerve of the 
migrating eye is a real advantage in the Pleuronectidae. 
Numerous attempts have been made to explain the phylogenetic pro- 
cess by which the asymmetry of the flatfish has been established. 
Most of these deal with the migration of the eye, and Cunningham 
(90, p. 51; ’92, p. 193) has set forth in a clear way the two chief lines 
of argument. One of these is based upon Darwinian principles, and 
the other, which is on the whole favored by Cunningham, involves La- 
marckian methods. This second explanation is somewhat elaborated by 
Cunningham, in that he has ascribed the migration of the eye chiefly to 
the action of the oblique eye muscles. In any fish that was flattened 
sidewise and had taken up with side swimming, the oblique muscles 
of the eye that faces downward would be continually brought into play 
to lift the eye to a position of greater service, and if the effect of this 
action could be inherited, the migration of the eye might thus be 
accounted for. It would be hazardous in the present state of our knowl- 
edge to assert that such changes cannot be inherited, though this does 
not prove that they are. Granting that they are handed on from genera- 
tion to generation, it is, in my opinion, conceivable that operations such 
as those described by Cunningham may have brought about the migra- 
tion of the eye. But with the monomorphic chiasma the question seems 
to me wholly different. The Pleuronectidae have descended from a stock 
with two types of optic chiasmata essentially like those of the present 
symmetrical teleosts, and of these two types, that one has been retained 
which in each group is mechanically advantageous for the migration 
