410 PROCEEDINGS OF THE AMERICAN ACADEMY. 



time in a position without apparent orientation to the light, if it is 

 disturbed it usually turns at once and jumps toward the light; and if 

 there are two areas of illumination of different size, it turns toward 

 the larger. Is, then, its ordinary response of jumping toward a pool 

 or stream to be considered as a simple reaction of this kind ? It will 

 be seen that this is similar to what, in Mitsukuri's ( : 01) opinion, de- 

 termined the shoreward migration of Littorinas. A series of observa- 

 tions was started in connection with the present investigation in an 

 attempt to determine the question for the frog, but as yet a sufficient 

 number of experiments has not been made to settle the matter defi- 

 nitely. It need only be said that so far as they have gone, the results 

 appear to indicate that the reaction is not so simple as has been 

 suggested — that apparently the objects in the visual field exert an 

 influence beyond that merely of the amount of light received fi-om any 

 direction, or of the size of the area fi:om which the light is received. 



In those higher animals whose actions correspond still less to simple 

 reflexes, as acute vision (the perception of details in the visual field) 

 becomes more perfectly developed, simple phototropic responses become 

 more and more a secondary matter, until they appear to be entirely 

 absent, or at least are not recognizable as such. Birds give evidence 

 of possessing especially acute vision, and under ordinary circumstances 

 certainly show no evidence of simple phototropic responses ; but the 

 way in which migrating birds often, on stormy nights, gather about 

 lighthouses and dash into the glass only to be killed, recalls strongly 

 the flying of moths into a flame, and it seems possible that this is an 

 expression of phototropism in birds which is ordinarily inhibited by 

 other responses. 



Finally, in recapitulation, we may distinguish roughly the following 

 four stages or types of reactions of animals to stimuli received by the 

 photo-receptive organs. 



Type A. Response of eyeless forms. 



These are in general given by animals which live in dark situations 

 and are negatively phototropic to light of ordinary intensity, though 

 they may (e. g., earthworm) be positive to lower intensities. Some, 

 such as Hydra (Wilson, '91), are positive to light of ordinary intensity. 

 The reactions of a frog with optic nerves cut are essentially those of 

 a positive eyeless form. Animals in this group respond only to light 

 intensities. 



Type B. Response of forms with " direction eyes." 



Animals with eyes of tliis type also react to light intensity only, and 

 are more commonly negative ; but some of them, such as many Cope- 



