CHAP. VI.] ORGANS OF EXTREME PERFECTION. 135 



classes may incidentally throw light on the steps by which it has 

 been perfected. 



The simplest organ which can be called an eye consists of an 

 optic nerve, surrounded by pigment-cells and covered by translu- 

 cent skin, but without any lens or other refractive body. We may, 

 however, according to M. Jourdain, descend even a step lower and 

 find aggregates of pigment-cells, apparently serving as organs of 

 vision, without any nerves, and resting merely on sarcodic tissue. 

 Eyes of the above simple nature are not capable of distinct vision, 

 and serve only to distinguish light from darkness. In certain star- 

 fishes, small depressions in the layer of pigment which surrounds 

 the nerve are filled, as described by the author just quoted, with 

 transparent gelatinous matter, projecting with a convex surface, 

 like the cornea in the higher animals. He suggests that this serves 

 not to form an image, but only to cencentrate the luminous rays 

 and render their perception more easy. In this concentration of 

 the rays we gain the first and by far the most important step 

 towards the formation of a true, picture-forming eye ; for we have 

 only to place the naked extremity of the optic nerve, which in 

 pome of the lower animals lies deeply buried in the body, and in 

 some near the surface, at the right distance from the concentrating 

 apparatus, and an image will be formed on it. 



In the great class of the Articulata, we may start from an optic 

 nerve simply coated with pigment, the latter sometimes forming 

 a sort of pupil, but destitute of a lens or other optical contrivance. 

 With insects it is now known that the numerous facets on the 

 cornea of their great compound eyes form true lenses, and that 

 the cones include curiously modified nervous filaments. But these 

 organs in the Articulata are so much diversified that Miiller 

 formerly made three main classes with seven subdivisions, besides 

 a fourth main class of aggregated simple eyes. 



When we reflect on these facts, here given much too briefly, 

 with respect to the wide, diversified, and graduated range of 

 structure in the eyes of the lower animals ; and when we bear in 

 mind how small the number of all living forms must be in com- 

 ] larison with those which have become extinct, the difficulty ceases 

 to be very great in believing that natural selection may have 

 converted the simple apparatus of an optic nerve, coated with 

 pigment and invested by transparent membrane, into an optical 

 instrument as perfect as is possessed by any member of the 

 Articulate Class. 



He who will go thus far, ought not to hesitate to go one step 

 further, if he finds on finishing this volume that large bodies of 

 facts, otherwise inexplicable, can be explained by the theory of 

 modification through natural selection ; he ought to admit that a 

 structure even as perfect as an eagle's eye might thus be formed. 



