The pieces of the back-bone (or vertebræ), as they pass between the hip-bones behind, unite them together, and degenerate until they form the curious little tail-end of the back-bone, which in us, and in the Apes, is curled slightly, with the concave part of the bend forward. The pieces unite strongly to each other above and below, and form really one bone, which is called the sacrum. Now, if these pieces were nearly or quite as stout and broad as those higher up, the hips would be wide apart; but if they become narrow, the hips will be all the closer together. In man, the pieces are broad, and the sacrum, as a whole, is so also, and the hips are widely separate; but the reverse is the case in the Apes. [Illustration: BRAIN OF CHIMPANZEE.] This difference in the breadth of the bone and the width of the hip has evidently to do with the maintenance of the erect posture in man, and the inability to keep erect for long, and comfortably, by these great Apes. The larger the surface of the sacrum, the greater is the mass of muscle passing to the back and downwards; and this is small in comparison in the Chimpanzee. Where the proper vertebræ of the sacrum end--that is to say, the pieces of the back-bone which are placed between the hip (ilium) bones--the tail begins. It is made up of three stunted bones, which are something like ill-made back-bone pieces (vertebræ); they are usually inseparably joined together to make a special bone, which is broad above, and tapering below. This bone, the rudiment of the tail, which, from some fancied resemblance to a Cuckoo’s back, has been called the cuckoo-bone (_os coccygis_), is covered by skin and embedded in muscles, which do not allow it to stick out visibly even as a stump; for its tip is curled inwards. This apology for the member which is so vastly important in many Monkeys is narrow in the man-like Apes, the black Chimpanzee included; but it is a little wider in man, although the general construction is the same. Could these bones--which, by their being united, form this rudiment of a tail--be disunited and increased in number, stuck out, and covered with skin and muscles, something like the very Monkey-like appendage would be formed. But noble tails are not the gifts of the higher Apes, as they are called, from their many points of resemblance in structure with man, and even in the smaller Monkeys they are extremely variable belongings, being given to one kind and not to another in a manner far beyond our philosophy.