LIFE ON THE EARTH. 9 



In the elements of land, air, and water, both 

 plants and animals are fitted to live by means of 

 contrivances varied in almost every individual case, 

 but always to be conceived of as elegant adapta- 

 tions to some conditions of matter adaptations to 

 gravity, to force of wind, to depth of water, to 

 degrees of light, to periodicity of seasons, and even 

 to local and limited occurrences. Regard the eye 

 which in its perfect state, as in man, is destined 

 to feel the presence or absence of light, to dis- 

 tinguish the colours of the several rays, and to per- 

 ceive the forms of the luminous surfaces. What is 

 it but a triple photographic lens with six curved 

 surfaces calculated for three different media ; calcu- 

 lated for achromaticity and spherical aberration ; 

 provided with a variable self-adjusting aperture, and 

 a variable self-adjusting focal length, adapted to 

 something better and more sensitive than a collodion- 

 plate the beautifully expanded and guarded retina, 

 which after the fraction of a second of time is ready 

 to receive a new impression with undiminished 

 energy. Regard the two eyes, the natural stereo- 

 scope, by whose beautiful joint action solids take 

 their proper aspect, distance is estimated, and the 

 landscape acquires that instructive composition which 

 our artists delight to imitate. 



Again, regard this wonderful organ, modified to 



