PROBLEM OF COLOR VISION—DANE. 617 
the food-containing glass, a different color was selected. Thus the 
monkey learned to proceed at once to the receptacle with food, 
whether it was blue, yellow, red, or green. It was tested also with 
a black and lght gray glass. Having learned that the food was in 
the former, successively darker grays were substituted for the empty 
one. The percentage of wrong choices increased and it was found 
that grays were confused which the human eye can distinguish with 
perfect ease and certainty. Kinnaman concludes that “there can be 
no doubt that monkeys perceive colors.” Two colors of equal bright- 
ness are distinguished better than two grays of equal brightness; 
and though the brightnesses are the same, colors may be distinguished 
from grays.” 
In the dancing mouse, however, the cones of which are at least 
very rod-like, Yerkes has recently found that color vision is extremely 
poor. There is some evidence of discrimination of red and green, 
and of red and blue, but none whatever of blue and green. Ap- 
parently such visual guidance as is received results from differences 
in brightness. The mouse discriminates blacks, grays, and whites.” 
Because of the inherent difficulties in the investigation of color 
vision in the lower animals, comprehensive results have not yet been 
obtained, but the newer methods promise notable discoveries, 
DEVELOPMENT. 
Since color vision is a complex differentiation, it might be expected 
that in the course of development an individual should successively 
pass through the simpler stages by which it was acquired. Anatomic- 
ally it has been shown that the retinal layers first become distinct at 
the center of the retinal cup, and that the differentiation of the retinal 
cells decreases from the center toward the periphery. In the chick 
it is said that the cone nuclei may be identified at an earlier stage than 
the rod nuclei, but it is not generally recognized that one form of 
visual cell precedes the other. 
The development of color vision has been theoretically considered 
by Mrs. Ladd Franklin.? Her theory assumes that the colorless 
sensations, white, gray, and. black, are caused by a primitive photo- 
chemical substance called the gray substance, which is composed of 
numerous gray molecules. 
“Kinnaman, A. J. Mental life of two Macacus rhesus monkeys in captivity. 
Amer. Journ. of Psych., 1902, vol. 13, pp. 98-148. 
b Yerkes, R. M. The sense of vision in the dancing mouse. Journ. of Comp. 
Neur., 1907, vol. 17, p. 194. 
¢ Weysse, A. W., and Burgess, W. 8S. Histogenesis of the retina. Amer. Nat., 
1906, vol. 40, pp. 611-634. 
@Wranklin, C. L. On theories of light sensation. Mind, 1893, n. s., vol. 2, 
pp. 4738-489. 
