The term physiological age refers to the stage of development in contradistinction to age in years and months, which is the usual method of designating age. Various evidences of physiological age are tooth appearance, pubescence, change of voice, menstruation, menopause, etc. Pubescence is an evidence of sexual ripening and the beginning of adolescence. For the purpose of record, three physiological groups are distinguished corresponding to three successive stages of development (1) prepubescence, (2) pubescence, (3) post-pubescence. The first group is characterized by an absence of hair upon the pubis, the second is an intermediate stage, the third group have hair upon the pubis. The data of the results below are taken from 4,500 New York High School boys, and are divided into half year age groups designated by the middle age value. The following table shows the percentage composition of each chronological age: It gives also the relative size of the subgroups. These facts have been hitherto disregarded and the chronological age group treated as if it were homogeneous. These subgroups in each age exhibit characteristic differences in physical measurements which differences are far greater than the difference between contiguous year groups. At the age of 15.75 the postpubescent group (85 per cent. of all) are 34 per cent. heavier, 32 per cent. stronger and 9 per cent. taller than the prepubescent group (forming 5 per cent, of all at the same age) as indicated on the next page: The differences in weight, height and strength between prepubescents and postpubescents of the same age are equal to the differences between age groups that are 6 to 8 years apart. Conclusion. — Age groups are heterogeneous and cannot serve as a unit for reference and experiment. We must substitute groups based upon physiological age.