136 'JoKTual of Comparative Neurology and Psychology. 



window and a petroleum lamp, in an ordinary room, is illuminated 

 by light rays striking it from every conceivable direction, for light 

 under such conditions is reflected from practically all surfaces in 

 the room as well as from those outside. Under the conditions of 

 the experiments cited above, then, the larva? and moths were not 

 exposed to light from two sources but to light from an infinite 

 number of sources, and the direction of the rays was not known. 

 How then, can it be concluded from the results of these and simi- 

 lar experiments (i) "That if there are several sources of light 

 of unequal intensity, the light with the strongest intensity deter- 

 mines the orientation and direction of movement of the animals;" 

 (2) "that symmetrical points of the photosensitive surface of the 

 animal must be struck by the rays of light at the same angle;" 

 and (3) "that if two sources of light at the same intensity and 

 distance act simultaneously upon a heliotropic animal, the animal 

 puts its median plane at right angles to the line connecting the 

 two sources of light ?" 



Let it be clearly understood that in the criticism of Loeb's con- 

 clusions, I do not wish to intimate, that because the reactions of 

 Volvox or any other organism do not take place in accord with 

 those conclusions, they necessarily cannot hold for the organisms 

 LoEB worked with. I do, however, wish to state and emphasize 

 that in my opinion his experimental results as quoted above, do 

 not warrant his conclusions, even for the animals worked on, much 

 less for all organisms which orient in light. 



The experiments upon which Loeb bases his theory of orienta- 

 tion to a single source of light will be discused later (see p. 142). 



8. ORIENTATION IN LIGHT GRADED IN INTENSITY. 



The reaction of Volvox to light from two sources varying in 

 relative intensity seems to me to prove conclusively that orienta- 

 tion is determined by the relative intensity of the light on opposite 

 sides of the organism, while there is no evidence that the direction 

 of the rays has anything to do with orientation in this organism 

 except in so far as it may aff^ect the relative light intensity on 

 opposite sides. If, however, difference in light intensity on oppo- 

 site sides of a colony can be produced with the rays of light approx- 

 imately parallel, and such intensity difi^erence aff'ects the direc- 

 tion of motion, the verdict miist be considered final. 



