The evolution of the jaw is toward shortening in both directions. This shortening will continue so long as the jaw must be adjusted to a varying environment. The jaw of man having originally contained more teeth than at present, lack of adjustment to environment produces, from the shortening, degeneracy of the jaw and atavism of the teeth. While this may coincide with general advances of the individual, it indicates that he is not yet adjusted to his new environment. The shortening of the upper jaw causes supernumerary, cone-shaped teeth to erupt, in mass, at the extreme ends of the jaw, as shown in the following figures. Fig. 57 illustrates a cone-shaped tooth between the two central incisors, forcing them out of position. Fig. 58 shows three supernumerary teeth--a cone-shaped tooth between the central laterals, and the cuspids out of position. The left permanent lateral is at the median line; another cone-shaped tooth remains in the vault, while the supernumerary left lateral is in place. As many as eight are at times to be observed in the anterior vault. Posteriorly these teeth are most often noticed in connection with the third molars, usually on a line with other teeth posterior to the last molar. Fig. 59 shows two supernumerary teeth in the anterior and two in the posterior part of the left arch; the molars have been extracted. Supernumerary teeth are not confined to these localities, but may be observed at any point in the dental arch (Figs. 60 and 61). The primitive cone-shaped tooth is rarely observed in the lower jaw. In twenty-six years' practice I have not seen a case. The mobility of the lower jaw prevents that mal-adjustment to environment present in the upper. The continual shortening, in both directions, of the jaw causes the third molars frequently to wedge in between the angle of the jaw and the second molars, so that eruption, if possible, is difficult. [Illustration: FIG. 62.] [Illustration: FIG. 63.] [Illustration: FIG. 64.]