ORGANS OF EXTREME PERFECTION 191 



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

 such variations should be useful to any animal under chang- 

 ing conditions of life, then the difficulty of believing that a 

 perfect and complex eye could be formed by natural selection, 

 though insuperable by our imagination, should not be consid- 

 ered as subversive of the theory. How a nerve comes to be 

 sensitive to light, hardly concerns us more than how life it- 

 self originated ; but I may remark that, as some of the lowest 

 organisms, in which nerves cannot be detected, are capable of 

 perceiving light, it docs not seem impossible that certain sen- 

 sitive elements in their sarcode should become aggregated 

 and developed into nerves, endowed with this special sensi- 

 bility. 



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 pos- 

 sible, 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 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 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 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 towards the fomiation of a true, picture- 



