Fig. 127.—The Tarsier (Lemur).
The history of the group has been very completely made out from fossils, and it is possible to work back to forms which, apart from their known subsequent evolution, could not be definitely separated from the ancestors of other mammal groups. The Lemurs are illustrated in Figs. 125 to 127.
The monkeys, to which we now turn, are divided into two groups, the American or Western and the Eastern or Old-World types, each of which is definitely confined to the regions indicated by the names. The Western apes (Figs. 128 and 129) are by much the lower group of the two, and in fact they lead back to fossil forms which cannot be definitely distinguished from the Lemurs. They are a side branch of the monkey stem, but are in all probability the nearest living representatives of the first of our ape ancestors. They are characterised by a wide septum between the nostrils, which makes the latter open in an outward direction, a feature which enables them to be distinguished at a glance from the other group. They have further a prehensile tail, which is of much use in climbing, and generally thirty-six teeth, or four more than the Old-World group. The lowest of them are the well-known Marmosets, which have claws on all the digits except the great toe, which last has a nail. These are further characterised by the presence of four nipples, all other monkeys having only two. Correlated with this feature, the Marmosets normally bear two or three young at a birth, whereas in all other apes one is the usual number.
Fig. 128.—A Marmoset (Hapale jacchus).
The Eastern monkeys have thirty-two teeth, the same number as in man; the septum between the nostrils is narrow, so that these open downwards and forwards; the tail is never prehensile and is frequently absent. They include such well-known forms as the baboons, the Gibraltar ape, the sacred Hanuman of India, the Diana monkey (Fig. 130), and the comical-looking Nasalis (Fig. 131). These represent a further step towards the final climax of the primate group.
Fig. 129.—Squirrel Monkey (Chrysothrix sciurca).
Fig. 130.—The Diana Monkey (Cercopithecus Diana).