The lemurs are small animals very much like squirrels in their general form and in their tree-climbing habits.  They live now almost exclusively on the island of Madagascar, but palaeontology shows that they were more widely spread at an earlier time.  Their teeth are exactly like our own, except that there is one more premolar on each side of each jaw.  The “fingers” and “toes” bear nails like ours, again with an exception in the case of the second digits of the hind limbs, which bear claws.  The details of structure that set these animals apart from all the rest of the primates are too small to deserve comment in the present connection.

Passing to the true anthropoids, or man-like primates and man himself, the first forms encountered are the little marmosets, which are like the lemurs in some ways, but in other respects they resemble the familiar tailed monkeys.  They are peculiar in having three premolars and two molars on either side of both upper and lower jaws, and also in the fact that the “thumb” is not opposable to the other fingers, while all the digits except the “great toes” bear claws instead of manlike nails.  The proportion of brain-case and face does not differ much from that in the lemurs and even lower forms like cats, for the brain has not increased greatly in total mass, though the cerebrum is more convoluted than in the lower forms.

The true monkeys, or Cebidae, are more interesting, and at the same time they are much more familiar to every one, as they are the commonest anthropoids of the menagerie and circus.  Their wonderful agility and sureness in climbing about is partly due to the perfect grasping power of the lower limb.  To all intents and purposes the foot is a hand; the first toe is shorter than the others, and its free motion is unrestricted as in the thumb of the hand.  These animals usually possess a long tail which they can use as a prehensile organ, curling it about the branch of a tree with hand-like ease and grasp.  When they run on all fours, they plant the palms and soles flat upon the ground.  The feature of primary importance in a comparative sense is the advanced structure of the skull.  These anthropoids are much more intelligent than the lower forms, which is a correlate of their larger and more convoluted brains.  The increase in the total bulk of the brain has wrought considerable change, not only in the head, but also in the relation of head to the trunk.  The cranium, or brain-case of bone, is relatively larger than the “face,” and it bulges upward so as to lie no longer behind the latter as it does in the lower mammalia.  In consequence of this cranial enlargement, the face and eyes are swung downward, as it were, so that the line of vision is not straight ahead, but depressed below the horizontal.  In order to look to the front and to the immediate foreground to which it is progressing or to where its food or enemies may be, the monkey must bend back its head; if it is still, it finds greater ease in the upright sitting posture which it assumes readily and naturally.