362 LIGHT AND TEE BEHAVIOR OF ORGANISMS 



violet, in Daphnia and Simocephalus in the yellow or green, 

 in Bacterium photometricum probably in the infra-red 

 with a secondary maximum in the orange, while in Oscillaria 

 all rays appear to be equally efficient. In plasmodia, 

 protoplasmic streaming in cells, earthworms, some moUusks, 

 and a number of insects and spiders, the region of maximum 

 stimulation is probably somewhere toward the violet end 

 of the spectrum, although it has not been definitely lo- 

 cated. In nearly all organisms without image-forming 

 eyes^ the relative stimulating efficiency of the different rays 

 is apparently constant under different conditions, but in 

 the forms with eyes there is evidence that it varies. Some 

 of the spider and hermit crabs, a number of insects and 

 spiders and many higher forms may be positive to certain 

 rays under certain conditions and to others under other 

 conditions. Bees and fishes can undoubtedly distinguish dif- 

 ferent regions in the spectrum. They can be trained to select 

 any of the primary colors of the spectrum by associating 

 these colors with food. That is, they are positive to (or 

 select) one color at one time and another at a different time. 

 Just what mechanism is involved in this power of selection 

 is unknown. Whether it is on the basis of brightness or on 

 the basis of color vision or neither is a matter concerning 

 which experimental evidence does not warrant a definite 

 conclusion. Many organisms react to ultra-violet much 

 as they do to visible rays. This is in harmony with the 

 following quotation from Schafer referring to man (1898, 

 p. 1055) : '' The invisibility of the infra-red rays is prob- 

 ably due to insensitiveness of the retina, while the ultra- 

 violet rays fail to be seen, partly, at any rate, owing to 

 absorption by the ocular media." 



(5) The presence of certain rays retards the reaction to 

 others in a number of organisms. According to Wiesner, 

 some plants react to red more strongly than to red mixed 

 with yellow. And according to Lubbock and Wilson 



^ Euglena appears to be an exception to this. According to the researches 

 of Engelmann it becomes positive to red in low oxygen pressure. 



