7. THE DISTRIBUTION AND EVOLUTION OF VISUAL SYSTEMS
323
In summary, marine fishes characteristically possess rhodopsins;
the only present exceptions having been found among the Labridae and
closely related Coridae, some of which possess predominantly porphyropsins. All the mammals and birds yet investigated use vitamin Ai
alone in vision, and ordinarily make from it rhodopsins with A max at
500 ± 2-3 m/x (chicken, pigeon, owl, cow, rabbit, pig). Curiously, man
is exceptional with A max 493 m/x (24); and it is reported that Crescitelli
has found the A max of opossum rhodopsin at the same wavelength (40).
Recently Dowling in our laboratory has found rat rhodopsin to have
A max 494m/x (41).
The position of the reptiles requires further clarification. Some time
ago we found the retinas of the freshwater turtles, Pseudemys scripta
and P. mohilensis, to contain vitamin A 2 , apparently with no admixture
of vitamin A x (12). These retinas contain a great predominance of cones,
and we were unable to obtain any measurable photosensitive pigment
from them. We supposed from finding vitamin A 2 that the cone pigment
is cyanopsin; and Granit's measurements (13) of the spectral sensitivity
of cone vision in the Greek tortoise, Testudo graeca, showing a close correspondence with the absorption spectrum of cyanopsin, agree well with
this supposition. In that case the few rods present in these retinas should
be expected to contain porphyropsin.
On the other hand, alligator (18) and rattlesnake (25) retinas contain typical rhodopsins, based on vitamin Ai, and with A max about 500
m/x. Brown and Brown (42) have also found exclusively vitamin Ai, and
as visual pigments rhodopsin (A max about 500 m/x) and iodopsin
(A max about 560 m/x) in the retinas of the marine green turtle, Chelonia
mydas.
Crescitelli (25) has found that in seven species of nocturnal geckos
the visual pigments are rhodopsins, based upon vitamin A x , yet with A max
ranging between 518 and 528 m/x, overlapping therefore with A max characteristic of porphyropsins. Crescitelli suggests that this observation may
support Gordon Walls's transmutation theory, according to which the
geckos are derived from diurnal lizards which possessed cones; and their
rods are to be regarded as secondarily transmuted cones. In that case
their possession of a rhodopsin lying toward the red from the usual
position might be a vestige from this ancestral condition. It may be
added that a red-sensitive retinenex (cone?) pigment with A max 525-530
m/x has been found in the marine scup (29). It is possible that further
exploration will reveal a more widely distributed class of rhodopsins in
which the displacement of A max to wavelengths considerably longer than
500 m/x is evidence of "conishness."
The foregoing discussion can be summarized as follows: All known
323
In summary, marine fishes characteristically possess rhodopsins;
the only present exceptions having been found among the Labridae and
closely related Coridae, some of which possess predominantly porphyropsins. All the mammals and birds yet investigated use vitamin Ai
alone in vision, and ordinarily make from it rhodopsins with A max at
500 ± 2-3 m/x (chicken, pigeon, owl, cow, rabbit, pig). Curiously, man
is exceptional with A max 493 m/x (24); and it is reported that Crescitelli
has found the A max of opossum rhodopsin at the same wavelength (40).
Recently Dowling in our laboratory has found rat rhodopsin to have
A max 494m/x (41).
The position of the reptiles requires further clarification. Some time
ago we found the retinas of the freshwater turtles, Pseudemys scripta
and P. mohilensis, to contain vitamin A 2 , apparently with no admixture
of vitamin A x (12). These retinas contain a great predominance of cones,
and we were unable to obtain any measurable photosensitive pigment
from them. We supposed from finding vitamin A 2 that the cone pigment
is cyanopsin; and Granit's measurements (13) of the spectral sensitivity
of cone vision in the Greek tortoise, Testudo graeca, showing a close correspondence with the absorption spectrum of cyanopsin, agree well with
this supposition. In that case the few rods present in these retinas should
be expected to contain porphyropsin.
On the other hand, alligator (18) and rattlesnake (25) retinas contain typical rhodopsins, based on vitamin Ai, and with A max about 500
m/x. Brown and Brown (42) have also found exclusively vitamin Ai, and
as visual pigments rhodopsin (A max about 500 m/x) and iodopsin
(A max about 560 m/x) in the retinas of the marine green turtle, Chelonia
mydas.
Crescitelli (25) has found that in seven species of nocturnal geckos
the visual pigments are rhodopsins, based upon vitamin A x , yet with A max
ranging between 518 and 528 m/x, overlapping therefore with A max characteristic of porphyropsins. Crescitelli suggests that this observation may
support Gordon Walls's transmutation theory, according to which the
geckos are derived from diurnal lizards which possessed cones; and their
rods are to be regarded as secondarily transmuted cones. In that case
their possession of a rhodopsin lying toward the red from the usual
position might be a vestige from this ancestral condition. It may be
added that a red-sensitive retinenex (cone?) pigment with A max 525-530
m/x has been found in the marine scup (29). It is possible that further
exploration will reveal a more widely distributed class of rhodopsins in
which the displacement of A max to wavelengths considerably longer than
500 m/x is evidence of "conishness."
The foregoing discussion can be summarized as follows: All known