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.