218 BRADLEY M. PATTEN 



brought about by the 'motor reflex' produced whenever a stimu- 

 lus is encountered. This usually removes the animal from the 

 source of stimulation, for the 'structurally definite side' toward 

 which the reflex is du-ected is that opposite to the side on which 

 the sensitive area is located. If the distribution of the stimulus 

 in space is such that these reflexes hold the animal on a direct 

 path with reference to the source of stimulation, a response 

 resembling a 'tropism' results, but the method of its accomplish- 

 ment is different from that assumed by Loeb. Jennings did not, 

 however, believe that the orientation of all animals was brought 

 about by the motor reflex ('06, p. 271). "In the symmetrical 

 Metazoa we of course find many cases in which the animal turns 

 directly toward or away from a source of stimulation without 

 anything in the nature of preliminary trial movements." 



About this time Holmes ('05) published a detailed account of 

 the movements involved in the orientation of the individual 

 blowfly larva. He lays great stress on the significance of the side 

 to side swinging of the anterior end which occurs when the larva 

 is suddenly stimulated by lateral illumination. These random 

 movements, he believes, afford a means of selecting a favorable 

 direction of advance. He says ('05, p. 105) : 



There is, so far as I can discover, no forced orientation brought 

 about by the unequal stimulation of the two sides of the body, but an 

 orientation is produced indirectly by follo"s\ing up those chance move- 

 ments which bring respite from the stimulus. I do not deny that there 

 may be an orientating tendency of the usual kind, but if there is, it 

 plays only a subordinate role in directing the movements of the animal. 

 The orientation of these forms is essentially a selection of favorable 

 chance variations of action and following them up (p. 106). It may 

 be said to be a form of the trial and error method minus the element of 

 learning by experience. 



Herms has published two articles on the light reactions of the 

 larvae and adults of the Sarcophagid^ flies. The earlier paper 

 ('07) is based largely on a field study of the, light reactions as they 

 affect food habits and migration. The second paper ('11) con- 

 tains an account of numerous laboratory experiments in which 

 he has brought out a number of interesting points, such as the 

 increased rate of crawling under increasing intensity of illumi- 



' Herms bases his classification on Girchner's system, published in 189G. 



