What is true of these leading characteristics of the skull, holds good, as may be imagined, of all minor features; so that for every constant difference between the Gorilla's skull and the Man's, a similar constant difference of the same order (that is to say, consisting in excess or defect of the same quality) may be found between the Gorilla's skull and that of some other ape. So that, for the skull, no less than for the skeleton in general, the proposition holds good, that the differences between Man and the Gorilla are of smaller value than those between the Gorilla and some other Apes. In connection with the skull, I may speak of the teeth--organs which have a peculiar classificatory value, and whose resemblances and differences of number, form, and succession, taken as a whole, are usually regarded as more trustworthy indicators of affinity than any others. (FIGURE 17.--Lateral views, of the same length, of the upper jaws of various Primates (Man, Gorilla, Cynocephalus, Cebus, Cheiromys). 'i', incisors; 'c', canines' 'pm', premolars; 'm', molars. A line is drawn through the first molar of Man, 'Gorilla', 'Cynocephalus', and 'Cebus', and the grinding surface of the second molar is shown in each, its anterior and internal angle being just above the 'm' of 'm2'.) Man is provided with two sets of teeth--milk teeth and permanent teeth. The former consist of four incisors, or cutting teeth; two canines, or eyeteeth; and four molars, or grinders, in each jaw--making twenty in all. The latter (Figure 17) comprise four incisors, two canines, four small grinders, called premolars or false molars, and six large grinders, or true molars, in each jaw--making thirty-two in all. The internal incisors are larger than the external pair, in the upper jaw, smaller than the external pair, in the lower jaw. The crowns of the upper molars exhibit four cusps, or blunt-pointed elevations, and a ridge crosses the crown obliquely, from the inner, anterior cusp to the outer, posterior cusp (Figure 17 m2). The anterior lower molars have five cusps, three external and two internal. The premolars have two cusps, one internal and one external, of which the outer is the higher. In all these respects the dentition of the Gorilla may be described in the same terms as that of Man; but in other matters it exhibits many and important differences (Figure 17).