Differences in Behavior 271 



To test the same individuals month after month would be 

 the ideal way of obtaining an answer to our question, but I 

 could devise no satisfactory way of doing this. The effects 

 of training last so long, as the results of the previous chapter 

 proved, and the uncertainty of their entire disappearance is 

 so serious, that the same training process cannot be used at 

 successive ages. The use of different methods of training is 

 even more unsatisfactory because it is extremely difficult 

 to make accurate quantitative comparison of their results. 

 It was these considerations that forced me to attempt to dis- 

 cover the relation of docility to age by carrying out the same 

 experiments with groups of individuals of different ages. 



As my plan involved the execution of precisely the same 

 set of tests with at least seventy individuals whose age, his- 

 tory, and past experience were accurately known, and of which 

 some had to be kept for nineteen months before they could 

 be trained, the amount of labor and the risk of mishap which 

 it entailed were great. To make possible the completion 

 of the investigation within two years, I accumulated healthy 

 individuals for several months without training any of them. 

 In March, 1907, I had succeeded in completing the tests for 

 the age of one month, and I had on hand for the remaining 

 tests almost a hundred individuals, whose ages ranged from 

 a few days to eighteen months. Had everything gone well, 

 the work would have been finished within six months. Sud- 

 denly, and without discoverable external cause, my mice 

 began to die of an intestinal trouble, and despite all my 

 efforts to check the disease by changing food supply and en- 

 vironment, all except a single pair died within a few weeks. 

 Thus ended a number of experiments whose final results 

 I had expected to be able to present in this volume. How- 

 ever, the work which I have done is still of value, for the 

 single pair of survivors have made possible the continuance 



