PAEKER: OPTIC CHIASMA IN TELEOSTS. 237 



of the eye. The selection and preservation of this type seems to me 

 entirely inexplicable from the standpoint of Lamarckian factors, for the 

 optic nerves are in no way open to muscle influence as the eye is; the 

 whole change is, in my opinion, at once suggestive of a process of elimi- 

 nation. Hence I regard the origin of the monomorphic chiasmata of the 

 Pleuronectidae as an operation in which the Lamarckian factors have 

 played no part, but which may be entirely explained through natural 

 selection. Although natural selection seems to be the only way of 

 accounting for the origin of the monomorphic chiasmata of the Pleu- 

 ronectidae, I do not wish to be understood to imply that the whole 

 asymmetry of the flatfishes has been thus produced. I can see no 

 reason why continued muscle action may not in the end modify the 

 position of an eye or why some direct influence of the environment, such 

 as light, may not have much to do with pigmentation; nor am I con- 

 vinced that such changes may not be inlierited. 



It seems to me entirely possible from our present knowledge that the 

 asymmetry of a flatfish may be in part the result of tlie action of La- 

 marckian factors and in part the outcome of natural selection, for these 

 two operations are not at all incompatible and may perfectly well work 

 together. But what I wish particularly to point out in this connection 

 is that in the origin of the monomorphic chiasmata of the Pleuronectidae 

 natural selection seems to be the only available means. 



From another standpoint the flatfishes are biologically interesting. 

 Their asymmetry is of a very pronounced type, and its particular phase 

 sometimes characterizes a whole tribe, as the dextral Pleuronectinae 

 and the sinistral Psettinae. Notwithstanding this evidence of general 

 stability, species may occur almost anywhere among modern forms in 

 which a complete reversal of symmetry of external characters at least 

 may exist. This is well shown in P-leuronectes flesus, Platichthys stel- 

 latus, etc., and indicates that this group of animals is open to discon^ 

 tinuous variation of a profound and fundamental kind. Flatfishes are 

 not peculiar in this respect, for discontinuous variation, as Bateson ('94) 

 has pointed out, has long been recognized in other groups. Thus in 

 the gasteropods reversed (sinistral) shells of the common Buccinum 

 and of the European garden snail have long been known. Reversed 

 specimens of this kind may establish themselves as a special race, as in 

 the case of Fusus antiquus of Vigo Bay, Spain. Sometimes whole 

 species are characterized by reversal, as among the Pupas, or even whole 

 genera, as in Clausilia and Physa. Not only do the gasteropods show 

 these differences, but some lamellibranchs, like Chama, are also reversed. 



