OHQANS of extreme perfection. 171 



eye to one complex and perfect can be shown to exist, each 

 grade being useful to its possessor, as is certaitily the case; 

 if further, the eye ever varies and the variations be in- 

 herited, as is likewise certainly the case; and if such varia- 

 tions should be useful to any animal under changing con- 

 ditions of life, then the difficulty of believing that a per- 

 fect and complex eye could be formed by natural selection, 

 though insuperable by our imagination, should not be con- 

 sidered as subversive of the theorv. 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 low- 

 est 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 gradations having 

 been transmitted in an unaltered or little altered condition. 

 But the state of the same organ in distinct classes may in- 

 cidentally 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 translucent skin, but without any lens or other refract- 

 ive 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 pig- 

 ment which surrounds the nerve are filled, as described by 

 the author just quoted, with transparent gelatinous matter, 

 projecting with a convex surface, like tiie cornea in the 

 higher animals. He suggests that this serves not to form 

 an image, but only to concentrate the luminous rays and 



