118 GRAVITY AND PROTOPLASM [Ch. V 



the centrifugal machine was used in the sunlight, movements 

 towards the centre clearly appeared. It was thus proved 

 that negative geotaxis (which is the same as centrotaxis) may- 

 occur in the sunlight. 



The data of geotaxis are incomplete without a consideration 

 of this phenomenon in the higher animals. Lobb is one of the 

 first investigators in this field. In 1888, he found that flies, 

 deprived on both sides of the free ends of the balancers or the 

 wings, and placed upon a board, move always upwards upon it. 

 If the plane of the board is held oblique to the horizontal, the 

 fly always moves along that line which makes the smallest 

 angle with the vertical. Likewise cockroaches seem to be 

 stimulated by gravity when this acts perpendicularly to their 

 ventral surface, so that they tend to move off from a horizontal 

 surface and do not come to rest until they are on a more or less 

 nearlj'- vertical one. Thus, Lobb put twenty-one cockroaches 

 into a truncated, pyramidal box, one of whose sides made an 

 angle of 80° with the horizontal ; another 60°, the third 45°, 

 and the fourth 25°. After an hour, the number of individuals 

 on each face of the box was counted at intervals of 10 minutes. 

 Adding together the results of 10 such counts he found on the 

 steepest wall 94 cases ; on the wall inclined at 60°, 61 cases ; 

 on that inclined at 45°, 28 cases ; on that at 25°, 25 cases ; and 

 on the horizontal surfaces, 2. After several hours 75 to 80% 

 of the animals were found on the steepest side, although it had 

 the smallest area. Later, Loeb ('90 and '91) showed that the 

 holothurian Cucumaria cucumis, the starfish Asterina gibbosa, 

 and the lady-bird beetles (Coccinellidae) are likewise geotactie. 

 Finally, it may be mentioned that several species of the slug, 

 Limax, are geotactie. 



There is in Protista, as already mentioned, a limit to the 

 intensity of the attractive force below which no response will 

 occur. Is there such a limit in Metazoa likewise ? Experi- 

 ments upon this point have been made by Miss Helen Per- 

 kins and myself in connection with my experimental course at 

 Radcliffe College. We have also beeu able to answer the ques- 

 tion, what difference in effect is produced by different intensi- 

 ties of gravity's action. We experimented with the great slug, 

 Limax maximus, which crawls readily upon a glass plate placed 



