As my plan involved the execution of precisely the same set of tests with at least seventy individuals whose age, history, 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. Suddenly, 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 environment, 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. However, the work which I have done is still of value, for the single pair of survivors have made possible the continuance of my tests with other individuals of the same line of descent as those which perished, and I have to regret only the loss of time and labor. As I have on hand results for ten individuals of the age of one month, and for four individuals of the age of four months, it has seemed desirable to state the problem, method, and incomplete results of this study of the relation of modifiability to age. The indices of modifiability for these two groups of dancers differ so strikingly that I feel justified in persisting in my efforts to obtain comparable data for the seven ages which have been mentioned. TABLE 52 PLASTICITY (RELATION or MODIFIABILITY TO AGE) Number of Errors in Successive Daily Series of Ten White-Black Tests, with Dancers Four Months Old SERIES MALES FEMALES NO. 76 NO. 78 AV. NO. 75 NO. 77 AV. GENERAL AV. A 7 7 7.0 4 8 6.0 6.50 B 8 6 7.0 6 5 5.5 6.25