DARWINISM AND EVOLUTION. 3 1 



that in any organism, light tends to develope pigment grains. 

 Pigment grains developed in any part of a transparent organism 

 would, as absorbing light and heat, render that part more irritable 

 by light or heat than the rest. 



In some of the lower Mollusca, small coloured points, some- 

 times singly in the neighbourhood of the brain, and sometimes in 

 considerable numbers on the edges of the mantle, are called ocelli, 

 and are regarded as rudimentary eyes. These are not organs of 

 sight, but parts of the body sensitive to light ; and we can readily 

 understand what this is, by closing our lids and moving the hand 

 before the eyes. 



While only a general sensitiveness to light exists, the inter- 

 cepting of the sun's rays by something which throws a large fiortio?i 

 of the creature into shade, is required to produce an internal 

 change. But when there comes to be a specially sensitive spot, 

 anything that casts a shadow on that spot alone, produces an 

 internal change. The contrast between light and darkness being 

 all that a rudimentary vision recognises, it is clear that nascent 

 vision extends to those objects alone which are just about to touch 

 the organism, either from their motion or its own. This amount 

 of sight, therefore, as Mr. Spencer observes,* is little more than 

 anticipatory touch. Hence in the earliest forms of sight, visual 

 impressions are habitually followed by tactual ones. Now tactual 

 impressions are habitually followed by contractions. From the 

 Zoophytes upwards, touch and contraction form an habitual 

 sequence. So that in a creature whose incipient vision amounts 

 to little more than an anticipation of touch, there constantly 

 occurs this succession : — i, a visual impression ; 2, a tactual 

 impression ; 3, a contraction. 



As we ascend to creatures having more developed eyes, we find 

 an increase in the sphere of surrounding space throughout which 

 external relations can establish corresponding internal relations. 

 A slight convexity of the epidermic layer lying over the sensitive 

 tract, first serves by concentrating the rays, to render appreciable 

 small quantities of light ; and thus brings into view the same 

 bodies at a greater distance, and smaller or less opaque bodies at 

 the same distance. 



In certain starfishes, small depressions in the layer of pigment 

 which surrounds the optic nerve are filled with transparent 

 gelatinous matter, projecting with a convex surface, like the 

 cornea in higher animals. The luminous rays thus concentrated 

 would form an image. 



*j* See Spencer's " Psychology," from which is taken most of what 

 immediately follows. 



