850 THE AMERICAN NATURALIST. : [VoL XXXII. 
and white alone remains. But this is not surprising, because 
towards the periphery the cones gradually disappear and the 
rods become the predominant elements; but since they are 
cylindrical bodies of uniform diameter, they must contain fibrils 
of one length only, — somewhat shorter than the longest cone 
fibrils, — consequently they cannot give rise to varied color 
sensations. Moreover, the periphery of the retina, judging 
from the manner in which the periphery grows in the inverte- 
brates, is the youngest and least differentiated part, conse- 
quently we should not expect to find fibrils in that region 
which had attained just the length, position, and connections 
necessary in color vision. 
These examples, I believe, are sufficient to show that it is a 
comparatively simple matter to account, from our point of view, 
for the more characteristic phenomena of color blindness. 
(8) The Development of Color Vision may be explained in a 
similar manner. If color vision depends on the nice gradations 
in length or position or connection of these retinidial fibrils, 
the absence of these conditions should produce color blindness, 
but not necessarily inability to distinguish differences of light 
and shade. Many eyeless invertebrates react to very delicate 
gradations of light intensity, probably by means of the irregular 
networks of nerve fibrils between the epithelial cells of the 
naked skin. As these fibrils become phylogenetically more 
regularly arranged within the specialized sensory cells which 
serve to support them, the power to discriminate different 
colors should become more and more highly developed. But 
these conditions do not call for any particular sequence in the 
evolution of color sensations, for there is no reason to suppose 
that one set of fibrils of a given length and position would 
appear before another, except perhaps that a set of medium 
length would probably appear before the extremely short or 
extremely long ones, hence the sensation of yellowish-green 
should be the first one to emerge from that of whiteness, and 
subsequently it should be the most dominant and acute color 
sensation. 
But if our views are correct, the only way in which the 
evolution of color vision can be worked out is by extensive 
