COLE. — IMAGE-FORMING POWERS OF VARIOUS TYPES OF EYES. 411 



pods, larvae of Lepidoptera, etc., are positive ; these are commonly 

 limited, however, iu their movements toward the light by physical 

 conditions, which either bring them into favorable relations with their 

 surroundings (e. g., to their food supply), or at least prevent their com- 

 ing into unfavorable conditions. 



Type C. Response to size of luminous field. 



The animals which give this t}T^)e of reaction appear to be usually 

 positive to light of ordinary intensity, in which case, other things being 

 equal, they turn toward the larger of two areas of illumination. Some 

 of them (e. g., Ranatra and the frog) under certain conditions change 

 the character of their response, and probably turn oftener to the smaller 

 light than to the larger one, though not enough observations have been 

 made to settle this point conclusively. It is probably only when the 

 animals are positive that the reaction to the size of the illuminated 

 area is adaptive. 



Type D. Response to definite objects in the visual field. 



This form of response includes what we ordinarily mean by the 

 term "vision." As Nuel ( : 04, p. 10) says, " Le mot 'voir' supposant 

 generalement une distinction visuelle et une representation visuelle et 

 psychi(iue des objets." The responses to definite objects are not 

 often in the nature of simple reflexes, but are complicated by psychical 

 processes. In most cases they inhibit direct phototropic response, 

 which may be in evidence at certain times only (as in Ranatra and the 

 frog, and possibly in birds), or may apparently be absent altogether. 



In the lower forms we speak of these responses as phototropic reac- 

 tions, photo-reactions, or simply reactions to light ; in the higher forms, 

 whose reactions give evidence of being governed, or at least influenced, 

 by definite objects in the visual field, they are usually termed vision. 

 These stages, at least in the examples adduced, must not be taken as 

 representing a genetic series. They shade insensibly into one another, 

 and we have seen how some forms, such as the fi-og, for example, may 

 give responses which fall into two of the classes. The ordinary re- 

 sponses of the frog to light which enters its eyes fall under what is 

 termed Tj'pe C, and to an undetermined extent under Type D. But 

 when the optic nerves are cut, so that the eyes no longer function, 

 the frog still reacts to light, its responses falling under Type A . Can 

 we say that the frog's " sight " has been destroyed then by cutting the 

 optic nerves ? It still is responsive to directive light much as it was 

 before. Certainly the commonly accepted usage of the terms " sight " 



