The results arising from these facts are not confined to animal Physiology; they give information also regarding the condition of the ancient Sea and ancient Atmosphere. and the relations of both these media to Light, at that 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 compound 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 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 organizations 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 peculiar kind, in the then existing representatives
