212 S. O. MAST 



but because it tends to move in a direct course in the absence 

 of external stimuli. Thus we find that earthworms, e. g., oriented 

 in light, continue in a given direction for some distance after 

 the light is entirely cut off. The most striking demonstration 

 of this is seen in fire-flies in which the orienting stimulus con- 

 sists of a flash of lght. Orientation and movement toward the 

 place where the flash occurred takes place in darkness. Here 

 then there is no possibility of light acting continuously in directing 

 the animal on its course. Why should it be necessary in other 

 forms as Parker assumes? 



5. " But the chief general defect of the volume," says Parker, 

 :< is one that has been inherited from earlier students in this field 

 of work, and consists in the attempt to apply the trial and error 

 method of orientation to the movements of many of the higher 

 invertebrates, such as the earthworm, fly larvae, etc., to the 

 exclusion of the tropism idea." He then singles out the reac- 

 tion of the earthworm for special criticism with reference to 

 this point. On the basis of his own definition previously quoted 

 have we in reality attempted to apply ' ' the trial and error theory 

 to the exclusion of the tropism idea? " On 

 page 200 we find the following sentence: " If the [earthworm] 

 however is rather sluggish so that there is little lateral move- 

 ment of the anterior end it turns from the source of light with 

 very few exceptions " (very few trials). On the same page 

 there is a table which shows that out of 90 exposures to unilateral 

 illumination the specimens used turned directly from the light 

 80 times and toward it only 10 times (scarcely any indication 

 of trial). And again at the bottom of the page I have made the 

 following statement: "In a few other sluggish specimens the 

 exposure to unilateral illumination was not made until after 

 they had come to rest in light of 15 candle meters. Under such 

 conditions the animals did not react at all until a few moments 

 after the exposure, then they very slowly extended and turned 



the interior end from the source of light every time 



There was no evidence of even the slightest preliminary turning 

 toward the source of light " (no trials whatever). After stating 

 that trial reactions may function in the orientation of the earth- 

 worm I wrote, p. 204: 'This does not indicate that constant 

 light cannot also produce orienting stimulations." That is, it 

 does not indicate that there are no tropisms even in accord 



