52 POPULAR SCIENCE MONTHLY. 



Panmixia can not account for the reduction of the color, since 

 it returns in some species when they are exposed to the light, and 

 disappears to a certain extent in others when kept in the dark. 

 Panmixia, Romanes thinks, may have helped to discharge the color.* 

 In many instances the coloration is a protective adaptation, and 

 therefore maintained by selection. Panmixia might in such in- 

 stances lower the general average to what has been termed the 

 " birth mean." Proteus is perhaps such an instance. But in this 

 species the bleached condition has not yet been hereditarily estab- 

 lished, and since each individual is independently affected " the 

 main cause of change must have been of that -direct order which 

 we understand by the term climatic." 



Since, however, the bleached condition, which in the first in- 

 stance is an individual reaction to the absence of light, has become 

 hereditarily established in Amblyopsis so that the bleaching goes 

 on even when the young are reared in the light, it is evident that 

 in Amblyopsis we have the direct effect of the environment on 

 the individual hereditarily established. 



The Eyes of the Amblyopsim:. — The structure of the eyes has 

 formed the basis of a separate, fully illustrated paper.* The promi- 

 nent features in the eyes of the various species must, however, be 

 known before the question of the origin of these forms and the 

 causes of degeneration can be seriously considered. The eyes of 

 the species of Chologaster are normally formed, possessing a lens, 

 pupil, vitreous body, retina, and optic nerve, and all the eye mus- 

 cles normal to the fishes. The eyes are functional. The retina 

 is, however, very much simplified. The eye of papilliferus is, in 

 this respect, more perfect than the eye of cornutus. In papilliferus 

 the outer nuclear layer consists of two series of nuclei, the inner 

 layer of about five series of nuclei, and the ganglionic layer of a 

 complete single layer of nuclei except where the optic fibers pass 

 between them, for an optic-fiber layer is not present. In Cholo- 

 gaster cornutus the outer nuclear layer has been reduced to one 

 or two series, and the ganglionic layer to cells widely separated 

 from each other or in rows and little groups, but no longer form- 

 ing a complete layer. In Amblyopsis and Typhlichthys the largest 

 eyes are not more than one twentieth the diameter of those of 

 Chologaster, or one thousandth of their bulk; the lens is nearly, 

 if not quite, obliterated; the same is true of the vitreous body and 

 the optic nerve in the adult. Beyond this the eyes differ much. 

 In Amblyopsis scleral cartilages are present and prominent, the 

 pigmented layer is prominent, the outer and inner nuclear layers 

 form one layer only, two or three cells deep. In T. subterraneus 



* Archiv f. Entwiekelungsraechanik, viii, pp. 545-617, Plates XI-XV. 



