122 The Animal Mind 



thus "seek" the darker regions, while others give it when 

 undergoing a change in the reverse direction, and hence are 

 " positively phototropic." But if nothing distinguishes the 

 negative reaction to photic stimuli from the negative reaction 

 to any other stimulus, then nothing shows the existence of a 

 sensation quality peculiar to the effect of light unless a 

 special receptive apparatus can be demonstrated. In a flag- 

 ellate Protozoon called Euglena, a pigment spot exists near 

 the anterior end. Now although pigment apparently is not, 

 as Hesse (176) has emphasized, a necessary constituent of 

 visual organs, yet its occurrence always suggests some rela- 

 tion to light, as it is essentially a kind of matter having the 

 property of absorbing light. Euglena gives the negative re- 

 action on entering a shadow. Is its pigment spot really an 

 "eye spot" and concerned in this response? Apparently 

 the reaction occurs before the pigment spot has entered the 

 shadow, and as soon as the transparent tip lying in front of 

 the pigment spot has been pushed into the shaded region 

 (no). It is uncertain, then, what the function of the pigment 

 spot is. But in another organism, which is structurally 

 intermediate between the single-celled and the many-celled 

 forms, pigment spots do play a r61e in light reactions. This 

 organism is called Volvox, and it is really a colony of globular 

 flagellates, each with its flagellum turned outward, and each 

 with an "eye spot." Very weak light has no effect on the 

 movements of Volvox; moderate light causes movement 

 toward the source of light, and very strong light causes move- 

 ment away from the source (183). Accurate observation of 

 these movements indicates that the eye spots are essential to 

 them; each individual responds to a change of illumination 

 of its eye spot (262). This much evidence, then, we have that 

 if Volvox possesses consciousness, changes of light intensity 

 produce in it a specific sensation. 



