The Unity of the Organism 



principles of variation to which all organic phenomena are 

 subject. Furthermore, under the searching investigation 

 and criticism of numerous workers, notably H. S. Jennings 

 and Ills followers, the tropism theory has been deprived, for 

 most biologists, of its inorganically mechanistic character. 

 The principles of "random movements," "avoiding reac- 

 tions," "trial and error," and others, are thoroughly estab- 

 lished and the recognition of them may be said to have so 

 modified the doctrine of tropisms as to make it one of or- 

 ganic mechanism rather than of inorganic mechanism as it 

 virtually would be according to the thoroughgoing elemen- 

 talistic conception of it. The "mechanistic conception of 

 life," one may remark, has very much to commend it if only 

 the machines conceived are recognized to be alive. My re- 

 marks under this head * may be consulted by the reader 

 who wishes to follow this point. 



What is meant by random movements is made clear by 

 the following: "In the earthworm and the larvae of blow- 

 flies which are negatively phototactic it has been shown by 

 the writer that movements which bring the animal toward 

 the light are checked or reversed and only those which hap- 

 pen to direct the animal away from the light are followed 

 up. Whatever immediate orienting tendency the light may 

 have in these cases is relatively unimportant as compared 

 with the clement of selection of favorable movements in di- 

 recting the animal away from the light." 



Here it will be noticed that the end, beneficial to the ani- 

 mal, is reached through a combination of orienting reac- 

 tions of the rigidly tropistic type, -i.e., the type dependent 

 on the movement of the animal directly toward or away 

 from the source of light by the symmetrical plan of the 

 body, and a sort of reaction in which the particular body- 

 form and the direction of light rays are of only secondary 



* See "Machines, living," in the index of The Probable Infinity of Na- 

 ture and Life. 



