406 PROCEEDINGS OF THE AMERICAN ACADEMY. 



" trajectories ") as an indication of the resultant of the light forces ; ^ 

 their paths he takes as indicating "veritable lines of luminous force." 

 This comes very close to reasoning in a circle, and is at best merely 

 qualitative, since he has not attempted to make a physical determi- 

 nation to verify his contention that the direction taken by the ani- 

 mals does really coincide with the resultant of light intensities. If 

 Littorina responds only to the intensity of the light, we should expect 

 that its course, in an illuminated field of this nature, would coincide 

 with the resultant of the light intensities, could such be determined 

 by physical measurements ; but if, like Vanessa, it reacts differently to 

 different areas, not in proportion to the intensity of the light received 

 from them, but according to their extent, the direction taken by the 

 snail would probably deviate accordingly from the resultant of intensi- 

 ties. It seems impossible to decide from the results either of Bohn or 

 of Mitsukuri (^Ol), whether Littorina reacts to differences of inten- 

 sity only, — the size of the screens or other objects in the field being 

 of importance merely in determining the amount of light reflected or 

 absorbed, and thus influencing the direction of the resultant "lines of 

 luminous force," — or whether, like Vanessa, it may react to the areas 

 according to their size, more or less irrespective of the intensity of light 

 received from them. Whichever may be the case, Bohn (:05, p. 30) 

 has pointed out that the shaded surfaces of rocks act in the same way 

 as black screens, and that when a Littorina is in the vicinity of two such 

 shaded areas it does not go directly toward either, but strikes a course 

 which is a mean and eventually leads it into the crevice between the 

 rocks. ^° This is apparently another case where the reactions of the 

 animal to light are well adapted to its needs, since the snails are un- 

 doubtedly much better protected in the crevices than they would be 

 upon the exposed surfaces of the rocks. 



To animals commonly living in the light the possession of eyes 

 capable of forming images must be of distinct advantage. Neverthe- 



9 " En un point donne d'un champ lumineaux la direction du champ n'est que 

 la direction de la resultante de toutes les forces attractives et repulsives excrceos 

 par les surfaces c'clairantes, surtout par les surfaces les plus e'tendues, les plus 

 liautes (fenetres, murs)." 



" " Chaque surface d'ombre exerce une attraction proportionellement a son 

 ctendue, et I'animal suit une direction qui est celle de la re'sultante des forces 

 attractives. En particulier, quand une littorina se trouve dans le voisinage de 

 deux rochers prc'sentant des surfaces d'ombre d'e'tendue par trop ine'gale, elle 

 prend une direction qui est celle de la dingonale du paralli'logramme des forces 

 attractives, et ainsi il lui arrive de se mouvoir vers un espace compris entre les 

 deux rochers et de ne rencontrer ni I'un ni I'autre, bien qu'attiree par I'un et par 

 I'autre." 



