The changes of bodily form and proportions noticeable when we compare man with the gorilla or the chimpanzee are precisely those which fit in with the supposition of a gradual change of form and habits favoured by natural selection in the struggle for existence of ape-like creatures living originally in tropical forests, but gradually spreading beyond the special conditions of tropical life into other conditions and seeking to hold their own and to nourish themselves and their young. They have had to contend with one another for food and safety and to defend themselves either by violence or by craft against predatory animals and competitors of all kinds.
There are certain notions still current dating from Roman times as to differences between man and apes, which are simply erroneous and fanciful in origin. Thus, at one time the possession of a tail was supposed to separate animals, including monkeys and apes, from man. The rare abnormal cases in which the end of the vertebral column of man is free and projects as a tail, were, a couple of hundred years ago, cited with wonder and head-shakings as a proof that there was, after all, a real similarity in man's structure to that of animals, and pictures of the "Homo caudatus," or tailed man, were to be found in ancient books dealing with marvels and mysteries. As a matter of fact, three or four small insignificant vertebræ, almost immovable, are always present in man attached to the great bone called the sacrum, formed by the union of five vertebræ. These small vertebræ, to which the name "coccyx" is applied, are sunk beneath the skin and fat, at the end of the backbone, and though they correspond to bones of the tail of other animals, they are, in normal mankind, thus concealed from view. Precisely the same atrophy and concealment of the bones of the tail is found in the gorilla, chimpanzee, orang, and gibbons. They are all of them, seen in the flesh, as tail-less as man is, and seen in skeleton have precisely the same number of minute tail bones forming a "coccyx." This is true not only of the higher apes mentioned, but of the Barbary ape--which lives at Gibraltar--whilst others, such as the mandrill, have very short tails. In fact, the tail is a very variable appendage in monkeys, and, as the Manx breed shows, also in cats. It is mainly "decorative" in the old-world monkeys, and is probably maintained by sexual selection. It is only in the new-world monkeys that it has acquired obvious mechanical value. In them it is prehensile, and is used with great effect in swinging among the trees from branch to branch, whilst the hands and feet are left free to grasp any new support.