Some Aspects of Attention 287 



that is on the field first (the contact reaction may prevent re- 

 sponse to temperature, or abnormal temperature may modify 

 the contact reaction), or the one that is the more important 

 (gravity yields always to other stimuli). 



In some higher animals the effects of interference of stimuli 

 have been noted. The earthworm will not respond to light 

 if feeding (91) or mating (179). In the turbellarian Con- 

 voluta roscoffensis light is victorious over heat in determining 

 reaction. The animals in their positively phototropic phase 

 will remain in the heated light end of a vessel until they perish. 

 Light and gravity are more nearly balanced in their effects. 

 Convoluta is negatively geotropic, yet if the brightest region 

 is below the surface, the animals will go there. But if this 

 region is only a little brighter than the surface, they will 

 stay at the surface, gravity dominating (140). The sea 

 urchin shows in its behavior a somewhat similar relation 

 between mechanical and chemical stimulation. If weak 

 acid is dropped into the water containing specimens of Arba- 

 cia, their spines begin to interlace. A slight shaking will 

 restore them to the normal position, but if more acid be 

 added, no mechanical stimulation will overcome the effect 

 of the chemical (409). Various facts concerning the inter- 

 relations- of gravity and light as stimuli have been noted in 

 Chapter IX. A very interesting case of the suppression of 

 one reaction by another is reported by Holmes in his obser- 

 vations on the water insect Ranatra. The positive response 

 of this insect to light, very precise and striking, may be 

 wholly suspended when the animal is feeding, when a num- 

 ber of individuals are collected, when the insect stops to 

 clean itself, or even "by the sudden appearance of a large 

 object in the field of vision," behavior which is strongly 

 suggestive of the "distraction of attention" in a human 

 being (186). Roubaud, in a study of the behavior of some 



