ORGANS OF EXTREME PERFECTION. 161 



dations 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 evei 

 varies and the variations be inherited, as is likewise cer- 

 tainly the case ; and if such variations should be useful to 

 any animal under changing conditions of life, then the diffi- 

 culty of believing that a perfect and complex eye could be 

 formed by natural selection, though insuperable by our 

 imagination, should not be considered as subversive 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 cannot be detected, are capable cf 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 possi- 

 ble, 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 incident- 

 ally throw light on the steps by which it has bean perfected. 



The simplest organ which can be called an eye consists 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 aggregates of pigment-cells, appar- 

 ently 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 sur- 

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

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

 trate 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 - 9 lor we have only to place the 



