Sbptembek 6, 1907] 



SCIENCE 



315 



in itself not beneficial nor injurious, but which 

 serves as a sign of a beneficial or injurious agent, 

 because it regularly precedes such an agent. 



This proposition is illustrated by the ap- 

 proach of an enemy which casts first a faint, 

 later a deeper shadow. The organism comes to 

 react to the faint stimulus. 



3. Progress takes place through increase in the 

 complexity and permanence of physiological states, 

 and in the tendency to react to these derived and 

 complex states instead of to the primitive and 

 simple ones. . . . 



4. Progress in behavior may take place through 

 increased variety and precision of the movements 

 brought about by stimulation. 



New movements, even new organs, such as 

 flagella, might be acquired by the selection of 

 overproduced movements and of overdeveloped 

 structures whose movements become advan- 

 tageous. 



Thus through development in accordance with 

 the two principles mentioned, the organism comes 

 to react no longer by trial, by the overproduction 

 of movements — ^but by a single fixed response, ap- 

 propriate to the occasion. . . . Such fixed responses 

 are the general rule in the behavior of higher 

 organisms, and are found to a certain extent in 

 all organisms. In the higher organisms we speak 

 of some of these fixed responses as reflexes, trop- 

 isms, habits, instincts. 



This is, in brief, the general course of the 

 development of the behavior of single in- 

 dividuals. 



Recognizing the dangers in attempting to 

 state adequately the position of an author in 

 this fragmentary fashion, I feel confident I do 

 him no injustice (1) in calling attention to 

 his assertion that fixed responses, such as re- 

 flexes, tropisms, habits, instincts, are developed 

 through selection from overproduced random 

 movements by means of the method of trial ; 

 (2) in noting that the substances suggested as 

 agents of stimulation all initiate reactions of 

 the tactual type. Reaction takes place on con- 

 tact, the essential stimulus being the abrupt 

 change produced by the contact and depending 

 qualitatively not at all on the character of the 

 object touched. 



No one, I am sure, would doubt that the 

 behavior of an organism rests somewhere on a 



basis of physiological change. Such changes, 

 however, may be of various kinds, involving 

 various groups of factors and giving rise to 

 various types of reaction. That definitely di- 

 rective reactions are reducible to the motor 

 reaction type, as seen in its essential features 

 in Paramecium^ is a conclusion that does not 

 appear to follow from the present accessible 

 facts of behavior. Though the selection of 

 overproduced movements by trial may account 

 for certain types of behavior, I do not see how 

 it accounts also for definitely directive or 

 tropic reactions. It is not clear that the latter 

 belong in the category of secondary, rather 

 than primary reactions, notwithstanding Pro- 

 fessor Jennings's vigorous endeavors to put 

 them there. My difiiculties are soon stated. 

 Any one who has stimulated excised muscle 

 from a freshly killed animal by means of a 

 galvanic current, is awai-e that its behavior at 

 the moment the current is made or broken is 

 quite diilerent from its behavior during the 

 continuous passage of the current. It is well 

 known that this difference is to be referred 

 first of all to the fact that the make and break 

 responses are caused by sudden changes in 

 electrical potential, which changes do not ac- 

 company the passage of the constant current. 

 Further, the reactions of many organisms 

 without muscles to galvanic stimulation ex- 

 hibit parallel differences. Indeed, it has been 

 determined that the behavior, under galvanic 

 stimulation, of such an organism as Parame- 

 cium, accords in many essential details with 

 the laws and theories originally formulated 

 with reference to the reactions of the muscles 

 of vertebrates. The passage of the constant 

 current through a muscle may produce a tonic 

 contraction at the kathode, not, however, at 

 the anode. In Paramecium, a similar con- 

 dition expresses itself in the behavior of the 

 cilia. Paramecium can be made to turn with 

 the utmost definiteness and directness until 

 its anterior end is toward the kathode. When 

 finally oriented, it is still clearly affected by 

 the stimulus. For the behavior of the cilia 

 at opposite ends of the body, normally uni- 

 form, is in this case different. 



In this brief resume of some phenomena 



