The central topic about which this doctrine is elaborated is that of adolescence—that specialized, reconstructive period fraught with the highest possibilities and the severest risks for the future of the individual, and representing nature's point of emphasis in the perspective of human and racial development. The volumes accordingly constitute in the first place an encyclopedia of adolescence. They bring together with a degree of completeness quite unattempted hitherto, the main data concerning the natural history of this period of unfoldment, on its physical, physiological and psychological sides. The normal growth of the body, the special development of different portions thereof concerned with adolescence, the variations from such normal development and their aberrations in disease and crime, the great rôle that periodicity plays in human growth—these constitute the physical basis which conditions through and through the nature of mental growth, as well as determines the spirit of educational progressions. The considerable aggregate of special studies that have been made in all parts of this wide field are here carefully summarized; so that as a book of reference the work takes a commanding position. There will be a variety of opinion as to the specific and permanent value of much of this evidence; and there may be some who will call into question the validity of the question-sheet system by which much of the psychological portion of the material has been gathered. Yet those who weigh evidence in a fair spirit of criticism will find that on the whole the weakness of certain types of evidence, for which unfortunately we have no better substitute, is quite as clearly recognized by the author as by themselves. It is easy enough to discredit the results obtained by a somewhat miscellaneous set of answers to questions, many of them rather difficult to answer with conscience and pertinence. But here, as everywhere in statistical investigation, all depends upon the temper and discretion that are used in the interpretation of the results. If the method used for the extraction of the result is adapted to the nature of the material, then it is likely that the investigation, however deficient, has served a useful purpose. It is not necessary to defend the questionnaire system in itself or the use which Dr. Hall and his pupils have made of it; the real point of issue is how far such material will stand the strain of the conclusions which are based thereupon. Allowing for a wide divergence of opinion in this respect, there remains a very considerable body of evidence which is tangible and well classified, and in the aggregate has a significance in that it suggests the trend of emphasis of the growth and distribution of mental traits. It is difficult, indeed, to understand by what other means an equally adequate conception of the contents, the impulses and the modes of feeling of young minds could have been ascertained, or the larger conclusions involved therein more saliently suggested. Nor must it be supposed that Dr. Hall has limited himself to this form of evidence.