APES, MONKEYS, AND LEMURS. 13 



defined transverse ridges extending right across the crown. Vhen, again, 

 whereas in the man-like apes the last molar, or •' wisdom-tooth," in the lower 

 jaw is similar in form to the two teeth in front of it, in the great majority of 

 the Old World monkeys this tooth has a large projecting heel at its hinder 

 end. _ These dental characters afford Very important evidence of the close 

 kinship of the man-like apes to man himself, and undoubtedly outweigh the 

 difference in the form of the whole dental series now to be noticed, which is 

 largely due to adaptation. In both the upper and lower jaws of man the 

 teeth are arranged in a regular horse-shoe series, with scarcely any interrup- 

 tion to the continuity by the tusks, which are but little taller than the other 

 members of the series. On the other hand, in the adults (and especially the 

 males of the larger species) of the man-like apes the cheek-teeth are arranged 

 in a nearly straight line, and form a more or less angulated junction with the 

 line of the incisors ; the large canines, or tusks, occupying the angle between 

 the two series, and thus forming a marked break m continuity. Jn these 

 respects the man-like apes resemble their inferior kindred. If, however, a 

 young individual of the larger man-like apes, and especially the chimpanzee, 

 be examined, it will be found that the teeth, owing partly to the imperfect 

 protrusion of the tusks, form a less interrupted and more regularly curved 

 series. Indeed, with the exception that the whole jaw is longer and narrower, 

 and the partially-protruded tusks are proportionately larger, the characters 

 of such specimens make a marked approximation to the human type ; and 

 the jaw of a chimpanzee at this stage may be regarded as almost intermediate 

 in structure between that of man and that of an adult male gorilla. More- 

 over, in this juvenile state the bony union of the two branches of the lower 

 jaw partakes of the short and rounded form characterizing that of man ; 

 whereas in the adult it becomes longer and more deeply channelled, like that 

 of the lower monkeys. In many respects the teeth and jaws of the gibbons, 

 or smallest representatives of the present group, conform to the intermediate 

 type. Not only are the human characteristics most developed in the teeth 

 and jaws of the young of the larger man-like apes and the gibbons at all 

 ages, but the same is true with regard to the skull of the former. The skull 

 of the young gorilla, for instance, lacks the beetling crests over the eyes and 

 the prominent ridge down the middle of the crown, which give such a for- 

 bidding and repulsive aspect to the cranium of the full-grown male. This 

 loss of human resemblances is due to specialisation taking two dilBculty lines 

 in man on the one hand, and in the larger man-like apes on the other ; the 

 development in the one case tending to increased size of brain, coupled wirh 

 no marked increase in the size of the tusks, while in the other the brain 

 grows at a less rapid rate, and the skull and tusks (more especially in the 

 male) assume characters approximating them to those cf the inferior animals. 

 Both in men and apes the young condition may accordingly be regarded as 

 the most generalised. 



Among the other features in which the man-like apes differ from the lower 

 monkeys and resemble man, are the absence of dilatable pouches in the 

 cheeks for the storage of food, and the total loss of the tail, as well as the 

 flattened, instead of laterally compressed, form cf the breast-bone ; the 

 . gibbons alone retaining the naked patches on the buttocks so characteristic 

 of the monkeys, but only in a much reduced condition. The gorilla and 

 chimpanzees further differ from the other mem.bers of the group, and thereby 

 resemble m.an alone, in the loss of the so-called central bone of the wrist— a 

 bone occupying a nearly central position between the upper and lower rows 



