224 HENRY LAURENS AND S. R. DETWILER 
jn pure-cone retinae, where the pigment, as assumed, is necessary 
for the absorption of the hght scattered by the highly refractive 
cones, there is practically no pigment migration. But Detwiler 
('16) found a demonstrable pigment migration and cone con- 
traction in the eyes of both turtles and lizards. Garten further 
argues (p. 109) that photomechanical changes should not take 
place in the crocodilian eye, because of the exclusive presence 
of rods or cones in the various portions of the retina. But, as 
we have demonstrated above, the structural conditions, at least 
in Alligator mississippiensis, differ from his description, and 
changes in the position of pigment and of visual cells do take place 
in light and darkness. 
The duplicity theory. The duplicity theory of von Kries, or 
the theory of the double retina of Parinaud, based on the findings 
of Max Schultze ('66), is of the greatest importance in compara- 
tive work on vision. The hypothesis is generally regarded as 
well substantiated, particularly by the facts of twihght and 
day vision. For brief accounts and references to the literature 
of the theory and its development the reader is referred to Nagel 
('05), Helmholtz ('11), and Parsons ('15). Briefly stated, the 
theory holds that the rods are sensitive only to light and darkness, 
and by virtue of their power of adaptation m the dark through 
the regeneration of visual purple they form the apparatus for 
vision in dim light. The cones, on the other hand, are the 
apparatus subserving bright vision as well as the perception of 
color. But in another way, the rods are the apparatus for 
achromatic scotopic vision (twilight vision), the cones the 
apparatus for photopic vision (day vision). The cones are not 
necessarily assumed to be utterly useless at night, but only rela- 
tively so, being quickly fatigued, on account of their high 
threshold. 
The presence and relative distribution of rods and cones is 
therefore a matter of the first importance. But without prej- 
udice it can be said that this is a very unsatisfactory matter as 
far as the comparative literature is concerned. Early contri- 
butions to the histology of the visual neuro-epithelium either 
have not been reinvestigated, but assumed to be correct, or 
