Animal Instincts and Tropisms 275 



from it. Thus the positively heliotropic animal 

 would be compelled to move towards the source of 

 light as a consequence of the fact that the intensity 

 of the light increases the more the nearer the animal 

 approaches the source of light. If the source of light 

 be the reflected 

 sky-light the dif- 

 ference of intensity 

 at both ends of a 

 microscopic organ- 

 ism is so slight 

 that it is beneath 

 the limit capable 

 of influencing the 

 motions. 



A simple experi- 

 ment published by 

 the writer in 1889 

 suffices to dispel 

 the idea that the 



energy gradient determines the direction of the mo- 

 tion of an animal in tropistic reactions. Let direct 

 sunlight (S, Fig. 47) fall through the upper half of a 

 window (w w) upon a table, and diffused daylight (D) 

 through the lower half of the window on the same 

 table. A test-tube a c is placed on the table in such a 

 way that its long axis is at right angles to the plane 

 of the window; and one half a b is in the direct sun- 



FIG. 47 



