Considering the bright color and great splendor of some eyes, particularly in birds, it seems probable that in these cases the organ has a twofold use: first and chiefly, to see; secondly, to intimidate an adversary with those luminous mirrors, in which all the dangerous fury of a creature brought to bay is best depicted. Throughout nature the dark eye predominates; and there is certainly a great depth of fierceness in the dark eye of a bird of prey; but its effect is less than that produced by the vividly-colored eye, or even of the white eye of some raptorial species, as, for instance, of the Asturina pucherani. Violent emotions are associated in our minds—possibly, also, in the minds of other species—with certain colors. Bright red seems the appropriate hue of anger: the poet Herbert even calls the rose “angrie and brave” on account of its hue: and the red or orange certainly expresses resentment better than the dark eye. Even a very slight spontaneous variation in the coloring of the irides might give an advantage to an individual for natural selection to act on; for we can see in almost any living creature that not only in its perpetual metaphorical struggle for existence is its life safeguarded in many ways; but when protective resemblances, flight, and instincts of concealment all fail, and it is compelled to engage in a real struggle with a living adversary, it is provided for such occasions with another set of defences. Language and attitudes of defiance come into play; feathers or hairs are erected; beaks snap and strike, or teeth are gnashed, and the mouth foams or spits; the body puffs out; wings are waved or feet stamped on the ground, and many other gestures of rage are practised. It is not possible to believe that the coloring of the crystal globes, towards which an opponent’s sight is first directed, and which most vividly exhibit the raging emotions within, can have been entirely neglected as a means of defence by the principle of selection in nature. For all these reasons I believe the bright-colored eye is an improvement on the dark eye.