462 G. H. PARKER 



method of presentation is often so vigorously pursued as to 

 give the reader the impression of special pleading rather than 

 sound judgment. Many of the cases chosen to illustrate the 

 " trial and error " method of orientation must strike the impar- 

 tial reader as equally good examples for the support of the 

 " tropism " theory. Thus, of the two methods of readjustment 

 to a change in the direction of illumination ascribed to Stentor, 

 the avoidance reaction is an excellent example of the trial and 

 error method, but the gradually curved course that the animal 

 also often takes under these circumstances is an equally good 

 instance of tropic response. Yet throughout the whole account, 

 it is clearly implied that the tropism theory is inapplicable to 

 the movements of Stentor. Nor is it anywhere made clear how 

 an organism, after it has once become oriented, can continue to 

 move in a straight line without involving the essential elements 

 of the tropism theory. To ignore the significance of this part 

 of the reaction as well as of those phases of the operation of 

 orientation that are essentially tropic, gives the text a partisan 

 tone that, to say the least, is regrettable. 



But the chief general defect of the volume 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 inverte- 

 brates, such as the earthworm, fly larvae, etc., to the exclusion 

 of the tropism idea. Anyone who has watched carefully the 

 orientation of an earthworm or a fly larva in a field of light 

 will have noticed the many fluctuating movements of the head 

 made by these animals during this process. As to the presence 

 of these so-called trial and error movements, there can be not 

 the least question, but as to their significance for orientation 

 there is, in the mind of the review^er at least, great doubt, for, 

 in any animal that orients more or less directly, these move- 

 ments are apparently always subordinate to a principle essen- 

 tially tropic. An example will make this clear. Suppose that, 

 in the case of an earthworm just about to project its head, five 

 preliminary trial and error movements are made. The worm, 

 after having made these movements, is then believed to follow 

 up with an extended locomotor movement that one of the five 

 which was on the whole most favorable for orientation. In 



