I. HISTORICAL 



IN the present thesis an effort has been made to combine in a 

 single study three main points of investigation; first, the genetic 

 study of behavior; secondly, the subject of individual differences; 

 and thirdly, a consideration of the exact method of habit formation 

 employed by the mice that have been tested in the experiments that 

 are now to be reported. 



Practically no experimental work has been done upon individual 

 differences and family resemblances in animal behavior. In most 

 cases, the behaviorist has been content to study the mass reaction of 

 a group of animals to external stimuli, and in the main, has not at- 

 tempted to treat the variability of his group because of the relatively 

 small number of animals tested. Professor J. McKeen Cattell, about 

 fifteen years ago, began to apply the methods of genetics to the study 

 of conduct, but the results obtained by him and his students were 

 not published, and the problem has been given to me. Yerkes devotes 

 a chapter of his book on "The Dancing Mouse," 1 to differences in 

 behavior, and there brings together results for variability in ". . . 

 general behavior, rapidity of learning, memory, and discrimination." 

 His results showed the existence of a considerable amount of indi- 

 vidual differences in the behavior of the dancing mouse, and no 

 family resemblance in the litters he obtained. He does not give 

 quantitative results, but confines himself to a general discussion of 

 the individual peculiarities of the animals he had tested. The fol- 

 lowing quotation from his text expresses this point: "I noted, in this 

 test of the animals ' ability to learn, that while one individual would 

 be scurrying about trying all ways of escape, investigating its sur- 

 roundings, looking, sniffing, and dancing by turns, another would 

 devote all its time to whirling, circling, or washing itself. One in 

 the course of its activity would happen upon the way of escape, the 

 other by reason of the limited scope of its activity, not the lack of it, 

 would fail hour after hour to discover even the simplest way of get- 

 ting back to its nest, to food, and to its companions." Concerning 

 the "inheritance of forms of behavior," Yerkes found that certain 

 lines of descent exhibited a pronounced tendency to whirl to the left, 

 while others reacted in the opposite direction. When two such strains 

 were crossed the offspring showed an equal frequency of left and 



i Yerkes, Chapter 17, "The Dancing Mouse." 

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