PHOTOMECHANICAL CHANGES IN THE RETINA 549 
make it probable that evolution has worked in different ways 
with these cells, some remaining much more strongly developed 
than have others, but through it all there has been a retention, 
to a greater or less degree, of the primtive phototropic response. 
Assuming that pigment migration, when present, may have a 
significant relation to the mechanism causing light perception, 
it is evident that animals, in which positional changes of the 
pigment are absent, must have produced a compensatory adjust- 
ment in another way. 
The movements of the visual cells likewise show certain in- 
consistencies. In all vertebrates whose cones are capable of 
movement, a shortening occurs in the light. The rods, on the 
contrary, are not as uniform in their responses, since among 
amphibians light presumably causes a slight shortening, whereas 
in fishes and birds a striking elongation takes place. If the 
responses of the visual cells to light are adaptive, there must 
be a wide difference in the uses to which the elongation of 
rods in some animals and the shortening in others is put. The 
mere statement that the cone myoids shorten in the light in 
order to make better use of the available light energy leaves 
unanswered the other half of the question involving the sig- 
nificance of elongation in the dark, for, as was previously stated, 
the cones remain shortened in twilight and hence elongation of 
these elements is not obviously related to the phenomenon of 
rod vision. 
In a forthcoming paper by the writer, which will include a 
discussion of certain aspects of the discrepant responses of the 
retinal elements, it will also be shown that the temperature reac- 
tions of the visual cells and of the retinal pigment throughout the 
various vertebrate classes likewise are inconsistent and there- 
fore presumably have no common adaptive significance. 
In reviewing the present status of our information concerning 
the movements of the visual cells and retinal pigment, therefore, 
the general conclusion seems justified, that although the move- 
ments of these elements, when present, may have a certain 
unknown significance in connection with the mechanism of light 
perception, we can at present interpret such movements only in 
terms of protoplasmic responses to definite stimulating agents. 
