426 THE SENSORIAL FUNCTIONS. 



alike the nearest and most distant objects, and 

 take cognizance at once of the most minute por- 

 tions of matter, and of bodies of the largest mag- 

 nitude ! 



§ o. Comparative Physiology of Visio7i. 



In the formation of every part of the animal ma- 

 chinery we may generally discern the predomi- 

 nance 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 structure ; for on following all 

 their varieties in the ascending series, we always 

 lind them advancing by slow gradations of im- 

 provement, before they attain their highest degree 

 of excellence. Thus the organ of vision presents, 

 amidst an infinite variety of constructions, succes- 

 sive degrees of refinement, accompanied by cor- 

 responding extensions 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 atibrded 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 Actinia, 

 which show by their movements that they feel the 

 influence of this agent ; for, when confined in a 



