VISION. 477 



§ 5. Comparative Physiology of Vision. 



In the formation of every part of the animal 

 machinery we may generally discern the predo- 

 minance of the law of gradation ; but this law 

 is more especially observed in those organs 

 which exhibit, in their most perfect state, the 

 greatest complication and refinement of struc- 

 ture ; for on following all their varieties in the 

 ascending series, we always find them advancing 

 by slow gradations of improvement, before they 

 attain their highest degree of excellence. Thus 

 the organ of vision presents, amidst an infinite 

 variety of constructions, successive degrees of 

 refinement, accompanied by corresponding ex- 

 tensions of power. So gradual is the progress of 

 this developement, that it is not easy to determine 

 the point where the faculty of vision, properly so 

 called, begins to be exercised, or where the first 

 rudiment of its organ begins to appear. 



Indications of a certain degree of sensibility to 

 light are afforded by many of the lower tribes of 

 Zoophytes, while no visible organ appropriated 

 to receive its impressions can be traced. Tliis is 

 the case with many microscopic animalcules ; 

 and still more remarkably with the Hydra, and 

 the Actinia, which show by their movements that 



