616 



THE EYE IN EVOLUTION 



Fundulus 



H. Miiller, 1954 ; and others) ; the excursions are often considerable, the rods, 

 for example, of the killifish, Fundulus, and the catfish, Ameiurus, changing from 

 a length of 90-lOOji. in light adaptation to 30-35ij. in the dark-adapted eye 

 (Detwiler, 1943). Among Amphibians, the Anurans show a more marked degree 

 of migration than Urodeles and in each case the cones move more rapidly and 

 extensively than the rods. In Reptiles the phenomenon is very slight and slow, 

 if it occurs at all ; in Birds cone migration is sometimes rapid and extensive ; 

 and in Mammals no movements of this type have been reported except by Garten 

 (1908) in monkeys (Angelucci, 1892 ; Garten, 1908 ; Hess, 1912 ; Detwiler, 

 1916 ; Laurens and Williams, 1917 ; Kohlrausch, 1918 ; Laurens and Detwiler, 

 1921 ; Loevenich, 1948). 



The pupillary response and these retinal photo -mechanical changes 

 are supplementary in time in so far as the former is immediate and 

 rapid in its action while the latter is slow. It is also interesting that 

 their efficiency shows a mutual relationship. The photo-mechanical 

 migrations in the retina are more marked in the lower Vertebrates 

 (apart from Selachians) ; in Fishes (apart from Selachians) pupillary 

 movements are slow and restricted and the muscles respond auto- 

 nomously to light. At the other end of the scale the nervously con- 

 trolled pupil of the higher Vertebrates is so active and effective that 

 retinal migrations have become superfluous and have disappeared. 



The STATIC ORGANIZATION OF THE RETINA is of importance in 

 arhythmic animals. Obviously such a retina must be duplex, but to 

 attain sensitivity the rod population must be considerably higher than 

 that of the cones. The most effective distribution is seen in Primates 

 wherein the cones are massed in an area centralis suitably situated to 

 subserve form vision ^ while the rods are particularly numerous in the 

 peripheral retina ; in such a case the central area is relatively blind 

 in dim illumination, a circumstance of little inconvenience, however, 

 since acuity is impossible in these circumstances in any case and 

 light-perception is as easily served by the peripheral as by the central 

 retina. 



By the use of one or more of these expedients a very high degree 

 of ABSOLUTE SENSITIVITY TO LIGHT Can be attained. Theoretically it 

 would be expected that the most sensitive eyes in the vertebrate 

 phylum are to be found in bathj^pelagic fishes which make use of the 

 minute available traces of light by means of a large eye with a huge 

 pupil, a brilliant tapetum and an enormous and heavily summated 

 rod-population. Among Selachians a typical example of such a com- 

 bination is seen in the deep-sea luminous shark, EtmojJterus ; it is also 

 seen in the Chimaeras and bathypelagic Teleosts. In some species of 

 the latter the state of dark adaptation has become permanent, the 

 retinal epithelium losing its pigment and photo-mechanical changes 



1 p. 6r)6. 



