VISION. 337 



i 



5. Comparative Physiology of Vision. 



IN the formation of every part of the animal machinery 

 we may generally discern the predominance of the law of 

 gradation; but this law is more especially observed in those 

 organs which exhibit, in their most perfect state, the great- 

 est complication and refinement of structure: for on follow- 

 ing 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 con- 

 structions, successive degrees of refinement, accompanied by 

 corresponding extensions of power. So gradual is the pro- 

 gress of this development, that it is not easy to determine 

 the point where the faculty of vision, properly so called, be- 

 gins to be exercised, or where the first rudiment of its or- 

 gan 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. This is the case with many microscopic animalcules; 

 and still more remarkably with the Hydra, and the *dctinia, 

 which show by their movements that they feel the influ- 

 ence of this agent; for, when confined in a vessel, they al- 

 ways place themselves, by preference, on the side where 

 there is the strongest light.* The Veretillum cynomo- 

 rium, on the other hand, seeks the darkest places, and con- 

 tracts itself the moment it is exposed to light.t In a per- 

 fectly calm sea, the Medusae which are rising towards the 

 surface, are seen to change their course, and to descend again, 

 as soon as they reach those parts of the water which receive 

 the full influence of the sun's rays, and before any part of 



* Such is the uniform report of Trembley, Baker, Bonnet, Goeze, Ha- 

 now, Roesel, and Schaeffer. 



f Rapp; Nov. Act. Acad. Nat Cur. of Bonn, xiv. 645. 



VOL. II. 43 



