722 THE EYE IN EVOLUTION 



An alternative explanation is to suppose that there is an innate tendency for the 

 eye to disappear which is normally opposed by natural selection becavise of its biological 

 utility. It is doubtless true that a loss-mutation may become effective and the organ 

 may disappear if its utility has ceased. It is to be remembered, however, that individuals 

 may show a cajaacity for the eye to retrogress or develop according to its usefulness. 

 Thus on the one hand, the eyes of larval cave-salamanders {Proteus, Typhlotriton) 

 usually regress at metamorphosis, but will develop if the larvto are grown artificially 

 in the light (Kammerer, 1912) ; these sightless Amphibians thus appear to become 

 blind in each successive generation. On the other hand, Ogneff (1911) found that if 

 goldfish were kept in the dark for 3 years their eyes became degenerate and functionless 

 while the eyes of many sj^ecies of ojaen-water fish become redviced if their biological 

 value is lessened by increasing their food and eliminating predators from their 

 environment. 



We have already seen that ocular regression of this type may occur in 

 most Invertebrates, particularly worms, Molluscs and Arthropods ; the 

 phenomenon is also encountered in all classes of Vertebrates with the 

 exception of Birds. It is interesting that in most cases there is a correspond- 

 ing increase in the development of other senses, such as the chemical, 

 olfactory or tactile sense, which are of greater use than vision in dark 

 surroundings. 



THE SEDENTARY HABIT 



A SEDENTARY HABIT may lead to the eyelessness in sessile forms. Thus 

 among actively swimming Lamellibranchs such as the common scallop, 

 Pecten, eyes of an extremely elaborate type are found, but in sluggish and 

 quiescent forms they may be primitive, as in the bivalve, Litna, or absent as 

 in the mussel, Aiiodonta. Among Crustaceans, those species of the Amphi- 

 pod, Gammarus, which live in pools, or the Isopod, Asellus, which lives in 

 holes is completely blind. In other species eyes may be present in the 

 actively swimming nauplius stage, but when the adult becomes sessile these 

 may become vestigial (the acorn-shell, Balanus, which encrusts rocks ; the 

 ship-barnacle, Lepas). We have already seen ^ that in insects the degree of 

 ocular development is generally correlated with that of the wings (Kalmus, 

 1945). 



THE ABYSSAL HABIT 



An ABYSSAL HABITAT renders eyes useless ; for in the deep seas there is 

 perpetual night. The transparency of the different seas varies greatly, a 

 factor which depends largely on the concentration of plankton organisms, 

 but at 370 metres in the Mediterranean and at 1.500 metres in mid-Atlantic 

 there is not sufficient light to affect a photographic plate unless it is exposed for 

 2 hours ; while the pelagic zone (down to 200 metres) is illuminated, the bathy- 

 pelagic zone (200 to 2,000 metres) is thus very dark, and on the deep-sea floor 

 (the benthonic zone), which may be several miles in depth, darkness is complete. 



1 p. 224. 



