TRIAL AND ERROR REACTIONS IN MAMMALS 65 



in terms of sense-physiology and of associative habit formation, 

 but in terms of reactive tendencies as well. Evidence has also 

 been adduced to support the view that the elaboration of ex- 

 perience involves an interplay of conflicting reactive tendencies, 

 and that these are experimentally isolable according to criteria 

 legitimate to genetic psychology. 



A final and still more general conclusion to be drawn from 

 the above is as follows : Since the ideal development of genetic 

 psychology demands the unravelling of long and intricately 

 interwoven mental complexes, not only as they appear in a 

 given species or at a given age, but throughout extensive phyletic 

 and age series, our intensive studies of behavior are apt to be- 

 come irrelevant to the broader issues in which they seek justi- 

 fication if we do not explore, from time to time, for the general 

 patterns according to which the threads of fact are arranged. 

 The satisfaction derived from the accurate and conclusive 

 determination of the quality and dimensions of a single thread 

 as it appears in a carefully delimited part of its course is apt to 

 blind the genetic psychologist to the historical significance of his 

 own attitude. Darwin (ii), Spencer (12), Romanes (13), and 

 Baldwin (14), among others, have sought to deduce from ex- 

 tensive ranges of facts the more general principles of mental 

 development. Their statements concerning general principles 

 have led to the formulation of many detailed problems, and to 

 methods appropriate to the investigation of these problems, 

 with the result that the observations of behavior recorded 

 by the older w'riters are now shown to have been made under 

 insufficiently controlled conditions, and that their interpre- 

 tations were often at fault. The obvious need of detailed in- 

 vestigation as thus disclosed, everywhere finds recognition in 

 the work of the younger students of behavior. But the equally 

 obvious need of seeking direction anew from a general survey 

 of comprehensive collections of facts at the expense, if need be, 

 of some sacrifice of accuracy of detail, seems to have gained 

 nothing more substantial than verbal recognition. 



The writer begs to acknowledge his indebtedness to Professor 

 Robert M. Yerkes, Professor Adolf Meyer, and Mr. George R. 

 Agassi z for many helpful suggestions and criticisms. 



