To illustrate the distribution of eyes in this group, we may take as an example the genus Serolis. All the species of this genus are provided with eyes except Serolis antarctica—a species that extends from 600 to 1,600 fathoms.
The eyes of all the deep-sea species are relatively larger than those of the shallow-water ones, except Serolis gracilis, whose eyes seem to be disappearing.
But these large eyes of the deep-sea species of Serolis are not capable of any greater perceptive power. In fact, the evidence of degeneration they show, both in minute structure and in the diminution of pigment, proves that they can be of very little use to these animals for perception (see Figs. 4 and 5).
This increase in size, accompanied by degeneration of structure, is just what we should expect to find in the eyes of deep-sea animals, and it is difficult to explain why it is that we do not find more examples of it.
If the animals that now live in the depths of the sea are descended from the shallow-water forms of bygone epochs, they must have passed through many different habitats with diminished light until they reached their present dark abode in the abyss.
Fig. 4.—Semi-diagrammatic section through the eye of Serolis schythei, a shallow-water species (4–70 fathoms). C, lens; V, crystalline cone; R, rhabdom; N, nerve. (After Beddard.)
Fig. 5.—Diagrammatic section of the eye of Serolis bromleyana, a deep-sea species (400–1,975 fathoms), showing the degenerate character of the eye. The corneal facets C, and the crystalline cones V, are the only structures that can be recognised. (After Beddard.)
In every new region they came to, the forms with larger and better eyes would be at an advantage in the fainter light, and would be more likely to survive and transmit their favourable variation in this respect to their offspring than their less fortunate neighbours. Thus down to the depth of the limit of sunlight we should expect to find, as we do find in fishes, large-eyed species.