280 GENERAL CONSIDERATIONS [Ch. IX 



fact that whereas the amcsba responding to light is constantly 

 elongated in the direction of the infalling ray, the amoeba 

 which is not stimulated from one direction exhibits, most of 

 the time, a stellate appearance. The wandering of the undi- 

 rected organism is illustrated again in the experiment of 

 Pouchet on the larvae of Musca (Lucilia) csesar, kept in the 

 dark (Fig. 73 ; compare Fig. 74, where the same larvce are 

 migrating under the directive influence of light). 



From these experiments we make the deduction that external 

 agents play a role of the utmost importance for morphology, — 

 of the utmost importance because by them alone is determined 

 the direction of migration of the motile cells or the migrating 



Pig. 74. — Tracing made like that of Fig. 73 by fly larvse, when the light falls upon 

 them in the direction of the arrow. A, tlie iirst direction of the light; B, the 

 second direction. To be compared with the undirected movements of Fig. 73. 

 (From Pouchet, '72.) 



protoplasm of whatever sort in the organism. The sense of 

 that migration depends in part upon the internal condition of 

 the protoplasm. The mechanism by which locomotion is effected 

 — that is wholly internal. The mechanism and the energy 

 necessary to make it go are alone impotent to determine any 

 adaptive movement or any other predictable result. To mech- 

 anism and energy m.ust be added a stimulus external to the 

 responding protoplasm in order that an adaptive or orderly 

 result should occur.* 



* There are several other important matters upon which the results of this 

 First Part throw light, such as the Mechanics of Eesponse and the Origin of 

 Adaptation in Response. Since additional facts for the discussion of these 

 topics will be gained from the succeeding Parts, that discussion will he deferred. 



