No. 3-] ACUTE VISION IN VERTEBRATES. 4S7 



the perception of uncolored and diffuse light. Since all colors 

 appear as gray by diffuse light, even though perceived by the 

 cones, and since the rods are more sensitive to faint impressions 

 than the cones, the presence of rods and almost complete 

 absence of cones in night animals is no more than can be 

 expected. Again, since the perception of color is one of the 

 important functions in day animals, and as this is done only by 

 the cones, the relatively greater number of cones in these 

 animals is readily accounted for. 



Acute vision, however, seems to depend on the presence of 

 a fovea. In man the power to see distinctly grows rapidly less 

 from the fovea to the ora serrata. The macula, it is true, sees 

 objects more distinctly than the peripheral parts of the retina, 

 but even this functions with the peripheral part more as a sen- 

 tinel for moving objects than as a point of acute vision. It is 

 true that all animals are attracted more quickly by moving 

 objects than by stationary ones, and it is especially true in those 

 animals whose retinal developm.ent has not proceeded beyond 

 the differentiation of an area. The power of quiet and close 

 discrimination of objects at rest seems to be present only with 

 those animals which possess a fovea. 



Fishes as a rule depend upon sight for their food, excepting 

 such as the shark, which depends almost wholly on its smell. 

 This class of vertebrates does not, however, usually possess a 

 fovea. How distinctly they see we cannot say, but we know 

 that the trout quickly takes the fly when thrown on the water, 

 or the pickerel the whirling spoon as it is drawn before it. 

 They see the objects while in motion, and are apparently unable 

 to distinguish them from the real article of food. An experi- 

 ence in fishing confirms the fact that a pickerel will not bite at 

 a motionless spoon-hook. The retina of these fish has simply 

 a thickening or area at the axis of vision. 



A somewhat similar experiment can be tried with the frog or 

 toad. If one attaches a bit of red flannel, a green leaf, or any 

 other small object to a thread and dangles it before a hungry 

 frog, he will quickly jump for it. A toad may be fed on meat 

 in a similar way, but in no case will the meat be taken unless it 

 is in motion. Neither do these animals show any marked power 



