216 PROCEEDINGS OF THE AMERICAN ACADEMY. 



Thus the less responsive animals of the intermediate but later series 

 mentioned fall into a less prominent curve, as is indicated by the shorter 

 dotted line in the diagram. The curve of positive response approaches, 

 but never actually reaches, the zero line. Even in darkness there is a 

 slight positive migration. This series (No. 18a) represents the average 

 of two series of experiments, one of 54 and the other of 66 deter- 

 minations, each taken at different times during the investigation. This 

 slight positive response — speaking of it as positive with reference to the 

 position of the source of light in the preceding series (17) — may be inde- 

 pendent of conditions of light and due to several causes. As mentioned 

 before, the thermal conditions of the room were not uniform, conse- 

 quently the positive response may have been a response to heat. The 

 movement was away from the window and hence might be ex- 

 plained as a negative response to the repeated inflowing of daylight, 

 when the window was thrown open to make observations. In the last 

 few experiments an opaque screen was put up between the animal and 

 the window. In these cases the average of the responses was slightly 

 negative, so there is some reason to suppose that it was in part the posi- 

 tion of the window in the previous experiment that determined the slight 

 positive migration. The actual phototactic responses to the caudle light 

 in the positive half of Table X. would then be the observed responses 

 minus this small positive movement in the dark. The actual negative 

 responses to the strong intensities would be the observed responses plus 

 this increment. In series 18b the box was placed at a distance of 

 30 cm. (C. P. 0.676) with the light burning, but the opening was cov- 

 ered with a piece of black paper to shut out the influence of the light 

 while leaving that of heat. The small average response of —3.0 may 

 possibly be regarded as a thermotactic one, and, if so, will have to be 

 deducted from the negatively phototactic response to this intensity of 

 light. For intensities less than the 0.676 C. P., the response to the heat 

 would be correspondingly less. 



We can now answer the second and fourth questions (pp. 207-208) by 

 saving, — that the precision of the phototactic response does, on the 

 whole, vary correlatively with the intensity of the light, and that the kind 

 of phototaxis (positive or negative) is not the same for different intensi- 

 ties of light. The slug gives a negative phototactic response to strong 

 light, a positive one to weak intensities, and is neutral to an intensity 

 somewhere between the extremes. 



A few individuals were tested successively at different light intensities 

 in order to find out with what precision an individual's phototaxis might 

 vary with a change of intensity. 



