PARKER: OPTIC CHIASMA IN TELEOSTS. ZO 
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 inherited. 
It seems to me entirely possible from our present knowledge that the 
asymmetry of a flatfish may be in part the result of the 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 Pleuronectes 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. 
