RETINA OF ALLIGATOR MISSISSIPPIENSIS 227 
('13) also state that the rods of nocturnal bnds (owls) in bright 
Ught approach, or are similar, in function to cones. But this 
is without anatomical foundation for the simple reason that 
the retinae of such birds contain numerous cones (Garten, '07; 
Hess, '13, p. 581) which show, with the pigment, photomechanical 
changes. In this connection the view of Parsons ('15, p. 204) 
may be quoted : 
If we regard the rods as the more primitive type of visual neuro- 
epithelium, as we are probably justified in doing, the persistence of 
recognizable rod attributes in the cones, even if modified, differentiated, 
and rendered more complex, might well be expected. Apart therefore 
from the difficulties of isolating the physiological results of excitation of 
the rods from those of excitation of the cones it may be anticipated 
that the latter cells will retain some measure of the functions which are 
in the highest degree characteristic of their prototypes. Hence, if it 
should ever be conclusively proved that the rod-like foveal cones of the 
human eye possess some trace of visual purple and are endowed with 
some slight degree of light-adaptation it would not be surprising; 
neither, on the other hand, would it militate seriously against the view 
that the rods and cones have become essentially diverse in function. 
Troland ('16) has demonstrated by careful experiments, cor- 
roborating the earlier work of v. Kries and Nagel, that the phe- 
nomenon of Purkinje does not take place in the rod-free portion 
of the human retina. And Watson ('15) and Lashley ('16) 
show that Hess' contention that the spectrum is shortened for 
the bird's eye as compared with the range of wave lengths seen 
by the human eye, is not correct. 
It is not out of place to add that in the condition known as 
night-blindness, in which the rods are insensitive, or practically 
so, dark adaptation is almost abolished or is much slower than 
normal, and that Purkinje's phenomenon is much less marked 
than in the normal eye or absent altogether. 
One further remark concerning Hess' work. He ('10) claims 
that many turtles are nocturnal and cites authorities supporting 
his contention. Ramon y Cajal ('11, p. 361) says that reptiles 
in general (in which of course he is incorrect, witness the alli- 
gator and the gecko) do not see in darkness. Rochon-Duvig- 
neaud ('17) does not believe that turtles can be called nocturnal 
because they are incapable of flight from an enemy or of pursuing 
