DIFFICULTIES OF THE THEORY 251 



to look exclusively to its lineal progenitors; but this 

 is scarcely ever possible, and we are forced to look to 

 other species and genera of the same group, that is to 

 the collateral descendants from the same parent-form, 

 in order to see what gradations are possible, and for 

 the chance of some gradations having been transmitted 

 in an unaltered or little altered condition. But the state 

 of the same organ in distinct classes may incidentally 

 throw light on the steps by which it has been per- 

 fected. 



The simplest organ which can be called an eye con- 

 sists of an optic nerve, surrounded by pigment-cells and 

 covered by translucent skin, but without any lens or 

 other refractive body. We may, however, according to 

 M. Jourdain, descend even a step lower and find aggre- 

 gates of pigment-cells, apparently serving as organs of vis- 

 ion, 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 starfishes, small depressions 

 in the layer of pigment which surrounds the nerve are 

 filled, as described by the author Just quoted, with trans- 

 parent gelatinous matter, projecting with a convex sur- 

 face, like the cornea in the higher animals. He suggests 

 that this serves not to form an image, but only to con- 

 centrate 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 toward the 

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

 only to place the naked extremity of the optic nerve, 

 which in some of the lower animals lies deeply buried 

 in the body, and in some near the surface, at the right 



