The first is a precise study of the effects of age upon the features. Many series of portraits taken at short intervals through life, studied carefully side by side, will probably show to some acute observer that Nature is very exact in the tallies that mark the years of human life. The second is to result from a course of investigations which we would rather indicate than follow out; for, if the student of it did not fear the fate of Phalaris,--that he should find himself condemned as unlifeworthy upon the basis of his own observations,--he would very certainly become the object of eternal hatred to the proprietors of all the semi-organizations which he felt obliged to condemn. It consists in the study of the laws of physical degeneration,--the stages and manifestations of the process by which Nature dismantles the complete and typical human organism, until it becomes too bad for her own sufferance, and she kills it off before the advent of the reproductive period, that it may not permanently depress her average of vital force by taking part in the life of the race. There are many signs that fall far short of the marks of cretinism,--yet just as plain as that is to the _visus eruditus_,--which one meets every hour of the day in every circle of society. Many of these are partial arrests of development. We do not care to mention all which we think may be recognized, but there is one which we need not hesitate to speak of from the fact that it is so exceedingly common. The vertical part of the lower jaw is short, and the angle of the jaw is obtuse, in infancy. When the physical development is complete, the lower jaw, which, as the active partner in the business of mastication, must be developed in proportion to the vigor of the nutritive apparatus, comes down by a rapid growth which gives the straight-cut posterior line and the bold right angle so familiar to us in the portraits of pugilists, exaggerated by the caricaturists in their portraits of fighting men, and noticeable in well-developed persons of all classes. But in imperfectly grown adults the jaw retains the infantile character,--the short vertical portion necessarily implying the obtuse angle. The upper jaw at the same time fails to expand laterally: in vigorous organisms it spreads out boldly, and the teeth stand square and with space enough; whereas in subvitalized persons it remains narrow, as in the child, so that the large front teeth are crowded, or slanted forward, or thrown out of line. This want of lateral expansion is frequently seen in the jaws, upper and lower, of the American, and has been considered a common cause of caries of the teeth.