236 bulletin: museum of COMrARATIVE ZOOLOGY. 



perfectly sound, for there are not a few species, like tlic 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. In a 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 advantnge 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 Cuimingham 

 ('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 diff"erent. The Pleuronectidae have descended from a stock 

 with two types of optic chiasmata essentially like those of the ])resent 

 symmetrical teleosts, and of these two types, that one has been retained 

 whicli in each group is mechanically advantageous for the migration 



