Yerkes, Animal Behavior. 235 



of the lower rocks, however, the paths may undergo sHght deviations toward the 

 latter. 



"This looks as if the Littorinas were attracted at the same time by the two 

 rocks, and as if they followed the direction of the resultant of the two attractive 

 forces. 



" Replacing the rocks by screens of wood, cloth or paper, one obtains the same 

 results. It is then the shapes of the shadows which are important here, not odors. 

 In the presence of these facts one is almost infallibly led to attribute to the Littori- 

 nas all the elements of higher consciousness, and to recognize perception, will and 

 feeling. Littorinas seem in fact to have visual perception of objects which surround 

 them and to recognize rocks at some distance; they would thus be capable of 

 directing their movements toward the rocks, knowing that they would find there 

 a feeling of well-being. They would conduct themselves as do human beings." 



For his e.xperimental study of the influence of light on Littorina, Bohn used 

 what he calls, the revelateur, because it reveals latent tendencies. This device 

 was a parallel sided glass dish 30 x 15 cm., with two movable screens, one black, 

 the other white, which could be placed in the dish as the experimenter desired. 



The number of facts brought to light by the revelateur, (acts which BoHN has 

 seen fit to set forth in italics, each with a number of its own — is too great for the 

 restatement of all of them in this review. Among the most important are the 

 following: A black screen when placed parallel to the direction of the luminous 

 field (the direction of increasing or decreasing intensity) attracts the animals 

 slightly, I. e., their path toward the light swerves toward the screen; a white 

 screen slightly repels them. This attraction or repulsion depends upon the 

 size, brightness and nearness of the screen. At any given point of a luminous 

 field the direction of the field is only that of the resultant of all the attracting and 

 repelling influences of the illuminated surfaces, especially of the largest and highest 

 surfaces (No. 28, p. 28). From this it follows that objects about the animals, 

 pebbles, rocks, etc., according as they are black or white, exercise an attracting 

 or repelling influence, the strength of which depends upon their size and illumina- 

 tion. 



The importance of these facts for theories of phototropism and for our con- 

 ception of what has long been spoken ot as response to the direction of light is 

 obvious. The fact that the reactions to light in these animals are influenced by 

 the size of the illuminated surfaces about them, as well as by the intensity of 

 their illumination, suggests a promising direction for inquiry, and indicates that 

 possibly the situation may not be completely describable in terms of the inten- 

 sity of the light at a given point, or of the relative intensities of illumination 

 of diff"erent portions of the organism. Bohn beautifully illustrates the equilibrium 

 of forces by showing that when gravity and light act on the animals simultaneously 

 and in partial opposition to one another the movements which occur are 

 resultants of the influences of the two forces. Curiously enough, if the incination 

 of the surface on which a littorina is moving be increased and the illumination 

 diminished, the influence of the light becomes zero when the surface is vertical, 

 and changes to a reaction of the opposite sign as the surface is moved on beyond 

 the vertical (No. 28, p. ^S)- 



In a glass tube covered at intervals with black rings, the molluscs move in 



