In the selection of these men there were several essentials to be observed. These may be outlined as follows:

(1) They Should Be In Good Health

It has been our custom to consider subjects "presumably in good health" as normal individuals, from whom general deductions could be drawn. In this case we felt it desirable to qualify the clause "presumably in good health" with a certification by a responsible physician after careful clinical examination. With these precautions we are able to state that our subjects were normal young men and in good health. While it was the intention to avoid the age of growth and we supposed that our young men were all of legal age, i. e., 21 years or over, in certain instances men as young as 19 years were inadvertently selected. Still we think this does not vitiate our general contention that in this study we are not dealing with the growth factor in any sense.

1 Lindhard, Arch. f. d. ges. Physiol., 1915, 161, p. 345.

(2) They should be responsible, cooperating, and truthful men, for in a study continuing over a period of 3 or 4 months and involving the strictest fidelity to general plan, especially in regard to dietetic habits, the whole success of the venture would, in the last analysis, depend upon absolute veracity.

(3) They should be volunteers, lor men assigned to such a research would not enter into it whole-heartedly. For this reason soldiers detailed for such duty, even those belonging to a medical ward, would not be likely to give the cooperation we desired. For many physiological studies prisoners or other institutional inmates would prove ideal subjects, but would not be suitable in this case, if the service were compulsory; if prisoners volunteered in response to such rewards as an abatement of sentence, their use would then be justifiable.

(4) The subjects should not consider themselves obligated to volunteer, or if they felt they were forced into a test of this character, they could not cooperate to the extent that the investigations demanded.

(5) They should be willing to undergo a certain amount of privation and discomfort, for there would be more or less restriction as to the usual habits, dietetic customs, social environment, etc.

(6) They should preferably be living under community conditions, such as dormitory life, with regularity of daily routine. This would assist materially in the dietetic control and the collection of urine and feces required in the daily routine.

(7) They should be uniting to serve as subjects for several months, since the time, labor, and money investment for each man would increase as the research progressed and his loss as a subject be more serious if his individual observations were incomplete.

(8) They Should Have A Unity Of Interest

In military phraseology, the "squad" system was emphasized. The personal influence was made a feature of the entire research, with the idea that each man would not only perform his own part in the observations successfully, but that his moral support would render material assistance to each of his associates.

(9) The Subjects Should Be As Varied In Type As Practicable

While not all types of physical and intellectual activity could be studied in the research, yet since the average man is more or less intellectual, and possesses a fair degree of physical development, it was desirable to select as nearly as possible a group equally divided between those who paid special attention to physical development and those whose activities were more exclusively intellectual.