Remarks on the Trilobite. 31 
inhabited by limuli, in the regions of Europe, which now form 
the elevated plains of central Germany. . 
“The results arising from these facts, are not confined to ani- 
mal physiology ; they give information also regarding the condi- 
tion of the ancient sea and ancient atmosphere, and the relations 
of both these media to light, at that remote period when the ear- 
liest marine animals were furnished with instruments of vision, 
in which the minute optical adaptations were the same that im- 
part the perception of light to crustaceans now living at the bot- 
tom of the sea. 
“With respect to the waters wherein the trilobites maintained 
their existence throughout the entire period of the transition for- 
mation, we conclude that they could not have been that imagin- 
ary turbid and compound chaotic fluid, from the precipitates of 
which some geologists have supposed that 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 dif- 
fered 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 re- 
ceived 
“ 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 vis- 
ion of a peculiar kind, in the then existing representatives of one 
Sreat class in the articulated division of the animal kingdom. 
We do not find this instrument passing onwards, as it were, 
through a series of experimental changes, from more simple into 
More complex forms ; it was created at the very first, in the full- 
