TRANSMUTABLE. 143 



phere, and the relations of both these media* to 

 light, at the remote period when the earliest 

 marine animals were furnished with instruments 

 of vision, in which the minute optical adaptations 

 were the same that impart the perception of light 

 to crustaceans, now living at the bottom of the 

 sea. 



With respect to the waters wherein the Trilobites 

 maintained their existence throughout the entire 

 period of the transition formation, we conclude 

 that they could not have been that imaginary 

 turbid and chaotic fluid, from the precipitates of 

 which some geologists have supposed the materials 

 of the surface of the earth to be derived ; because 

 the structure of the eyes of these animals is such, 

 that any kind of fluid, in which they could have 

 been settled at the bottom, must have been 

 pure and transparent enough to allow the passage 

 of light to organs of vision, the nature of which 

 is so fully disclosed by the state of perfection in 

 which they are preserved. With regard to the 

 atmosphere, also, we infer that had it differed 

 materially from its actual condition, it might have 

 so far affected the rays of light, that a corres- 

 ponding difference from the eyes of existing 

 crustaceans, would " have been found in the organs 

 on which the impressions of such rays were then 

 received. 



Regarding light itself, also, we learn from the 

 resemblance of these most ancient organizations 

 to existing eyes, that the mutual relations of 

 light to the eye, and of the eye to light, were 



