This section is from "Scientific American Supplement Volumes 275, 286, 288, 299, 303, 312, 315, 324, 344 and 358". Also available from Amazon: Scientific American Reference Book.
In the general term, monkeys, considered as equivalent to the order Primates, or the Quadrumana of naturalists, we have to include another sub-type, that of the Lemurs. These animals are of a lower grade than the true monkeys, from which they differ in so many points of structure that they are considered to form a distinct sub-order, or, by some naturalists, even a separate order. They have usually a much larger head and more pointed muzzle than monkeys; they vary considerably in the number, form, and arrangement of the teeth; their thumbs are always well developed, but their fingers vary much in size and length; their tails are usually long, but several species have no tail whatever, and they are clothed with a more or less woolly fur, often prettily variegated with white and black. They inhabit the deep forests of Africa, Madagascar, and Southern Asia, and are more sluggish in their movements than true monkeys, most of them being of nocturnal and crepuscular habits. They feed largely on insects, eating also fruits and the eggs or young of birds.
The most curious species are--the slow lemurs of South India, small tailless nocturnal animals, somewhat resembling sloths in appearance, and almost as deliberate in their movements, except when in the act of seizing their insect prey; the Tarsier, or specter lemur, of the Malay islands, a small, long tailed nocturnal lemur, remarkable for the curious development of the hind feet, which have two of the toes very short, and with sharp claws, while the others have nails, the third toe being exceedingly long and slender, though the thumb is very large, giving the feet a very irregular and outré appearance; and, lastly, the Aye-aye, of Madagascar, the most remarkable of all. This animal has very large ears and a squirrel like tail, with long spreading hair. It has large curved incisor teeth, which add to its squirrel like appearance, and caused the early naturalists to class it among the rodents. But its most remarkable character is found in its fore feet or hands, the fingers of which are all very long and armed with sharp curved claws, but one of them, the second, is wonderfully slender, being not half the thickness of the others. This curious combination of characters shows that the aye-aye is a very specialized form--that is, one whose organization has been slowly modified to fit it for a peculiar mode of life. From information received from its native country, and from a profound study of its organization, Professor Owen believes that it is adapted for the one purpose of feeding on small wood-boring insects. Its large feet and sharp claws enable it to cling firmly to the branches of trees in almost any position; by means of its large delicate ears it listens for the sound of the insect gnawing within the branch, and is thus able to fix its exact position; with its powerful curved gnawing teeth it rapidly cuts away the bark and wood till it exposes the burrow of the insect, most probably the soft larva of some beetle, and then comes into play the extraordinary long wire-like finger, which enters the small cylindrical burrow, and with the sharp bent claw hooks out the grub. Here we have a most complex adaptation of different parts and organs, all converging to one special end, that end being the same as is reached by a group of birds, the woodpeckers, in a different way; and it is a most interesting fact that, although woodpeckers abound in all the great continents, and are especially common in the tropical forests of Asia, Africa, and America, they are quite absent from Madagascar. We may, therefore, consider that the aye-aye really occupies the same place in nature in the forests of this tropical island, as do the woodpeckers in other parts of the world.