COLE. — IMAGE-FORMING POWERS OF VARIOUS TYPES OF EYES, 409 



these responses, is indicated by such observations as that of Latter ( : 04, 

 p. 88), who "once observed a brimstone butterfly visiting flowers of the 

 Dog Violet scattered along a bank, and picking out these flowers to the 

 exclusion of all others with gi-eat precision, not approaching even other 

 blue flowers that were present." The description of the feeding of 

 Ranatra by Holmes (-.05* p. 325) furnishes an excellent illustration 

 of the inhibition of the ordinary phototropic responses by attention 

 to particular objects in the visual field. Holmes's description is as 

 follows: "The phototactic response may also be inhibited by efforts 

 to obtain food. Ranatra which are swimming towards the light can 

 often be caused to discontinue their phototactic efforts if several 

 small insects are placed near them. If the phototactic activities are 

 very lively and vigorous, it is more difficult to divert the attention of 

 the insect to the capture of prey. When attention is once directed 

 to seizing the smaller insects, the light is disregarded. When the 

 prey has once been captured and tfie Ranatra is engaged in sucking 

 out its juices little attention is paid to the light. The repast being 

 finished the insect may resume its positive response." 



Attention to moving objects would appear to be more general than, 

 and probably precedes, the response to stationary objects. This is well 

 illustrated in the feeding habits of frogs, toads, and many lizards, 

 which seldom or never notice an insect so long as it is quiet, but are 

 attracted by it at once if it moves. An object moving in the field of 

 vision may, however, affect the elements of the retina in ([uite another 

 way than does an object which is stationary ; for, unless its color 

 intensity is uniform with that of its background, its movement must 

 produce a change in the intensity of the light reaching certain of the 

 visual elements in the retina, and the response may be to this change in 

 intensity rather than to a definite and clear-cut perception of the object 

 as such. The question of how far animals are able to, and do, distinguish 

 stationary objects is rather a difficult one to solve. As has just been 

 said, it is well known that frogs react to small moving objects, such as 

 insects, which constitute their food. It is also a matter of common ob- 

 servation that if a ft'og sitting on the bank of a stream or pool is disturbed, 

 it ordinarily jumps at once in the direction of the water, even if it is 

 approached from a direction parallel to the shore line. The question 

 arises as to whether the frog recognizes the water by its appearance or 

 whether the response is merely a reaction to a larger area of illumina- 

 tion. For it seems (^uite certain that the open water and sky on the 

 one hand must in general form a larger area of illumination than the 

 bank, with usually tall grass, bushes, or similar dark objects, on the other. 

 We have seen (p. 39 o) that, although a frog may sit for a considerable 



