DIFFICULTIES OF THE THEORY 147 



of Vox populi, vox Dei, as every philosopher knows, cannot be 

 trusted in science. Reason tells me, that if numerous gradations 

 from a simple and imperfect eye to one complex and perfect can 

 be shown to exist, each grade being useful to its possessor, as is 

 certainly the case; if further, the eye ever varies and the varia- 

 tions be inherited, as is likewise certainly the case; and if such 

 variations should be useful to any animal under changing condi- 

 tions of life, then the difficulty of believing that a perfect and 

 complex eye could be formed by natural selection, though in- 

 superable by our imagination, should not be considered as sub- 

 versive of the theory. How a nerve comes to be sensitive to light, 

 hardly concerns us more than how life itself originated ; but I may 

 remark that, as some of the lowest organisms in which nerves can- 

 not be detected, are capable of perceiving light, it does not seem 

 impossible that certain sensitive elements in their sarcode should 

 become aggregated and developed into nerves, endowed with this 

 special sensibility. 



In searching for the gradations through which an organ in any 

 species has been perfected, we ought 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 grada- 

 tions having been transmitted in an unaltered or little altered con- 

 dition. 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 consists of an 

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

 lucent 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 concentrate 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 



