544 LESLIE B. AREY 
The responses of all vertebrate rods to light, therefore, are not 
identical. The outer members of amphibian rods appear to 
shorten and thicken through the action of light, but there is a 
diversity of opinion as to whether or not the myoids do the same. 
In the dark, the nuclei of the rod cells of urodeles move toward 
the choroid and may in this way change the position of the rod 
proper; the myoid, however, is said not to participate in causing 
this movement. In all fishes that possess cones there occurs an 
extreme lengthening of the rod myoid in the light, and a shorten- 
ing in the dark; similar responses have been found in certain 
birds having diurnal habits. No experimentation has been 
performed upon the rod cells of mammals or of reptiles, although 
in the latter group, the retinas usually lack these elements. 
Whenever a system of relations between an organism and its 
environment, and especially those involving constant responses 
to definite stimuli, is discovered, the theorist immediately busies 
himself in devising an explanation which will demonstrate the 
so-called adaptiveness of these relations. Although it is doubt- 
-less true that many such explanations reveal the resourcefulness 
of the human mind rather than the ingenuity of nature, yet a 
suggestive and stimulating hypothesis serves its purpose if only 
the possible dynamic interrelations of parts are thereby brought 
to light. Having completed, therefore, a summarization of the 
changes in the retinal elements which are produced by light, 
the question, What is the adaptive significance of these move- 
ments?, may now be fairly raised. 
The significance of the movements of the visual cells and 
retinal pigment through the action of light has been sought 
by approaching the facts from various standpoints; inclusive ex- 
planations, however, are not easily devised, due to the lack of 
uniformity in the responses throughout the various vertebrate 
classes.* 
3 It is interesting to note that from the first discovery of a striking pigment 
migration in the frog’s retina a similar condition was assumed for man. Mis- 
conceptions as to the amount of retinal pigment present in mammals and in 
man, and especially concerning the extent of its movement (e.g., Bailey, 713; 
Cunningham, ’13), appear repeatedly in the literature of retinal physiology. One 
