SIGHT.
157
The distribution of colours in the environment which is
thus partial has produced a like partial distribution of
colour-perceiving elements in the retina. Many facts concur
to prove that there are fewer fibres for the perception of red
than for the perception of any other colour ; and that these
fibres require a stronger stimulant to produce sensation than
is the case with any others. The peripheral portions of the
retina are incapable of perceiving red ; and white, when seen
on these portions assumes a complementary greenish tint.*
If we try to perceive very small or imperfectly illuminated
figures, those coloured red are the least easily perceived of
any. And in a common abnormality, known as colour-blind¬
ness (or more correctly, dichroismi), the power of perceiving
the red rays, alone or in composition, is wholly wanting.
All these facts point alike to a comparative weakness and
scarcity of the red-perceiving elements. And a moment’s
consideration will show us that this is just the effect which
we might expect to see produced by natural selection. For
it is clearly desirable that the eyes of the frugivorous
animals should be pleasurably stimulated by reds, oranges,
and purples ; and the simplest contrivance for effecting this
end would be to give the greatest possible rest to such ele¬
ments as answer to stimulations of these orders. Accord-
they ought to be only excited by comparatively
powerful stimulations of their proper kind. How greatlv
habits of life may alter the conformation of the eye can be
follow out this subject in a chapter on “The Genesis of Æsthetics,” but
have decided not to do so for want of space.
* See the next section.