548 LESLIE B. AREY 
experimentation—a course that has hitherto been followed in 
only a few instances. Inclusive theories, devised at the writing 
desk, can hardly be expected to offer rational explanations of 
such phenomena as are involved in the diversified responses of 
the retinal elements. 
The retinal pigment cell is very comparable to a melanophore 
and the positive phototropism of its granules is quite likely a 
mere retention of a primitive response, for as Parker (06, p. 413) 
asserted: ‘‘It is probable that in all melanophores (sensu lato) 
in which there is a migration of pigment, light . . . . will 
induce a migration toward the source of illumination and the 
absence of light . . . . amigration in the reverse direction.’’® 
Reviewing the conditions in the several vertebrate classes, a 
high degree of variability appears. In most fishes the mi- 
gration of pigment is extensive, although in certain common 
elasmobranchs pigment is entirely absent. Among amphibians 
there are wide differences. In the frog, the movements are 
extensive but are highly specialized, since it is probable that in 
many cases, e.g., reactions to temperature (Herzog ’05), a 
nervous control has been superimposed over the primitive direct 
responses. Of the urodeles, Necturus shows a very limited 
capacity, and Triton a comparatively well developed capacity 
for executing positional changes of the pigment. In reptiles 
and mammals movements are demonstrated only with difficulty, 
yet among birds extensive migration again appears. 
The discontinuity of the pigment response throughout these 
classes, and the variability in the degree of pigmentation as well 
) 
6 This statement as it stands is not true for a few special cases. Steinach 
(91) on the eel and Hertel (’07) on Triton found that a blanching of the skin 
(pigment contraction) resulted from an exposure to the light, and also, as Harless 
(54) first showed, the skin of the frog becomes dark in darkness but brightens 
in the light. Moreover, it is doubtful whether the movement of the pigment 
granules in all melanophores that do expand in the light and contract in darkness 
is respectively toward and away from the source of illumination. In melano- 
phores of fishes the direction of movement appears to be nearly lateral, whereas 
in the frog there is, to be sure, a vertical component of movement, yet in the light 
this migration is away from the source of illumination and in darkness in the reverse 
direction. The influence of the nervous system, which plays an important réle 
in the behavior of melanophores, undoubtedly complicates the primitive responses 
in many animals. 
