cism of the previous two, and
seems to be harmonious with all the facts. She supposes that the color
sense is now in the third stage of its evolution. In the first stage
the only elements were white and black; the second stage added yellow
and blue; and the third stage red and green. The outer zone of the
retina is still in the first stage, and the intermediate zone in the
second, only the central area having reached the third. In red-green
blind individuals, the central area remains in the second stage, and
in the totally color-blind the whole retina is still in the first
stage.

In the first stage, one response, white, was made to light of whatever
wave-length. In the second stage, this single response divided into
two, one aroused by the long waves and the other by the short. The
response to the long waves was the sensation of yellow, and that to
the short waves the sensation of blue. In the third stage, the yellow
response divided into one for the longest waves, corresponding to the
red, and one for somewhat shorter waves, corresponding to the green.
Now, when we try to get a blend of red and green {222} by combining
red and green lights, we fail because the two responses simply unite
and revert to the more primitive yellow response; and similarly when
we try to get the yellow and blue responses together, they revert to
the more primitive white response out of which they developed.

But, since no one can pretend to _see_ yellow as a reddish green, nor
white as a bluish yellow, it is clear that the just-spoken-of union of
the red and green responses, and of the yellow and blue responses,
must take place _below the level of conscious sensation_. These unions
probably take place within the retina itself. Probably they are purely
chemical unions.

[Illustration: Fig. 37--The Ladd-Franklin theory of the evolution of
the color sense. (Figure text: Stage 1--white, Stage 2--yellow blue,
Stage 3--red green blue)]

The _very first_ response of a rod or cone to light is probably a
purely chemical reaction. Dr. Ladd-Franklin, carrying out her theory,
supposes that a light-sensitive "mother substance" in the rods and
cones is decomposed by the action of light, and gives off cleavage
products which arouse the vital activity of the rods and cones, and
thus start nerve currents coursing towards the brain.

In the "first stage", she supposes, a _single_ big cleavage product,
which we may call W, is split off by the action