154 



SCIENCE 



[S. S. Vol. XXVIII. No. 709 



would have to be done before we could state 

 the general value to the organism of the vari- 

 ous methods of training. 



In determining the dancer's power of 

 retaining discrimination habits, the author 

 found that a white-black habit may persist 

 during a period of from two to eight weeks 

 of disuse, but that such habits are rarely per- 

 fect after an interval of four weeks. The 

 retention of the color discrimination rarely 

 persisted in perfect form foB more than two 

 weeks. 



Having determined the periods of persist- 

 ence of such habits, the author next undertook 

 to find out whether training, the results of 

 which have wholly disappeared so far as mem- 

 ory tests are concerned, influences the re- 

 acquisition of the same habit. It was found 

 that the ten dancers tested had so lost the 

 habit of the white-black discrimination at the 

 end of a rest interval of eight weeks that 

 memory tests furnished no evidence of the 

 influence of previous training; retraining 

 brought about the establishment of a perfect 

 habit far more quickly than did the original 

 training. Indices of modifiability are given 

 both for the males and for the females, for 

 the learning and for the relearning. The 

 general conclusion issuing from this study is: 

 that the effect of training is of two kinds, 

 the one constitutes the basis of a definite form 

 of motor activity, the other the basis or dis- 

 position for the acquirement of a certain type 

 of behavior. 



A chapter each is devoted to individual, age 

 and sex differences, and to the inheritance of 

 forms of behavior. Yerkes obtained satisfac- 

 tory evidence from individuals of one line of 

 descent pointing to the fact that, in their 

 case, a probable tendency to whirl to the left 

 is inherited. In regard to the inheritance of 

 individually acquired forms of behavior, the 

 author states that descent from individuals 

 which had thoroughly learned to avoid the 

 black box gives the dancer no advantage in 

 the formation of a white-black discrimination 

 habit. 



In conclusion, we may say that aside from 

 its general usefulness as a reference book for 



the research student, the book forms a val- 

 uable guide to the technique of experimenta- 

 tion upon animals. There is one defect in 

 the book which certainly makes it lose in 

 value for this latter purpose. This defect lies 

 in the over-favorable emphasis given to the 

 method which employs punishment rather 

 than some form of reward (food, etc.) as an 

 incentive. The reviewer feels that Yerkes has 

 not fully justified its claims to priority even 

 for use with the dancer, much less its value 

 as a substitute for other forms of incentive in 

 experiments upon higher mammals. 



John B. Watson 

 The XjNivERSinr of Chicago 



SPECIAL ARTICLES 



THE ESSENTIAL MEANING OP d'aLEMBERt's 

 PRINCIPLE 



Newton's second law of motion is expressed 

 in the fundamental form, using C.G.S. units, 



2(AX)B=2(^'ni)- 



(1) 



The necessary range of the two summations is 

 determined without ambiguity, by the condi- 

 tions of the problem selected for discussion. 

 The first sum must include every element of 

 external force parallel to a fixed line brought 

 to bear upon any portion of mass within the 

 system, either by a process equivalent to sur- 

 face distribution at the boundary, or by 

 volimie distribution. The second sum covers 

 every part of the system's mass, and no mass 

 external to the system. Equation (1) pre- 

 sents Newton's thought that the physical 

 agencies active (forces) are measurable in 

 terms of one particular result — accelerations 

 produced in masses — other effects, if any, be- 

 ing ignored in the equation. What d'Alem- 

 bert put into clear relief, when he announced 

 his principle covering " lost forces," is the 

 unimpaired validity of the equality, after 

 eliminating all self-canceling elements from 

 the force-sum. This removes from considera- 

 tion all inner forces always, and items of ex- 

 ternal force in certain cases. The second 



