402 INFERENCES RESPECTING THE ANCIENT 
have supposed the materials of the sui'ftice 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 
efficient 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 corresponding 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 oroani- 
zations to existing eyes, that the mutual relations 
of Light to the Eye, and of the Eye to Light, 
were the same at the time when Crustaceans 
endowed with the faculty of vision were first 
placed at the bottom of the primeval seas, as at 
the present moment. 
Thus we find among the earliest organic 
remains, an Optical instrument of most curious 
construction, adapted to produce vision of a pe- 
culiar kind, in the then existing representatives 
of one great Class in the Articulated division of 
the Animal Kingdom. We do not find this 
