RETINAL IMAGE TO ANIMAL REACTIONS. 109 



Since the so-called sense organs of many of the simpler animals are 

 merely devices for exciting muscle to action, and since many of these 

 animals possess no true central nervous organs, the responses of these 

 admitted, and yet it is also equally clear that these higher animals 

 have been derived from stocks that were purely tropic in their day. 

 How have the tropisms disappeared from these lines of descent and 

 what are the forms of response that represent the transition between 

 tropisms and the diversified movements of the more complex animals ? 

 Some insight into the answer to this question can be gained by a 

 comparison of phototropism and vision as elucidated through the 

 retinal image. 



It is now well recognized that many of the simpler animals, uni- 

 cellular as well as multicellular, are extremely responsive to light. 

 The amoeba creeps away from a source of illumination, hydra creeps 

 toward it, sea-anemones are for the most part photonegative, earth- 

 worms are positive to weak light and negative to strong light, and so 

 forth. In all these simpler animals the surface of the given form is 

 apparently open to stimulation by light in the same sense that our 

 whole skin may be stimulated mechanically for touch. When light 

 falls on an amoeba, the formation of pseudopodia ceases on the illumi- 

 nated side and continues on the side in shadow ; hence the animal 

 creatures are of a relatively restricted and circumscribed kind and 

 lack the variety and spontaneity of the reactions of the more complex 

 forms. Such restricted responses are represented by forced move- 

 ments, or more particularly by tropisms. reaction types characteristic 

 in general of the simpler animals and consisting of rather direct 

 responses of the organism as a whole by moving either toward an 

 obvious source of stimulation or away from it. Such responses, as 

 Loeb has abundantly shown, are the usual types of movements for 

 plants and the lower animals, and, though there is much ditTerence of 

 opinion as to the way in which a tropism is accomplished, there can 

 be no doubt as to its predominance among the reaction forms of the 

 simpler animals. 



If, then, tropisms are the common types of response for the 

 simpler animals in which receptors are directly connected with mus- 

 cles, or at most connect with them through a very primitive kind of 



