Some Aspects of Attention 315 



The facts suggest that the influential stimulus is either the 

 one that is on the field first (the contact reaction may prevent 

 response 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 hght if feeding ( 1 7 1 ) or mating (327). In the turbellarian 

 Convoluta roscoffensis hght is victorious over heat in deter- 

 mining reaction. The animals in their positively photo- 

 tropic phase will remain in the heated Hght end of a vessel 

 until they perish. Light and gravity are more nearly 

 balanced in their effects. Convoluta is negatively geo- 

 tropic, 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 (253). 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 Arbacia, their spines 

 begin to interlace. A sHght shaking will restore them to 

 the normal position, but if more acid be added, no mechan- 

 ical stimulation will overcome the effect of the chemical 

 (734). Various facts concerning the interrelations of 

 gravity and Hght 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 Hght, 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 



