Near the base of the spinal column is a large compound bone, known as the _sacrum_. It is a constituent part of the column, but in its singular form and structure it differs slightly from the corresponding bone in man. The general outline of this bone has the form of an isosceles triangle. It fits in between the two large bones that spread out towards the hips and articulate with the thigh bones. In man, about halfway between the center and the edge along each side is a row of four nearly round holes. Across the surface of the bone is a dim, transverse line, or seam, between each pair of holes, from which it is seen that five smaller sections of the spinal column have anchylosed, or grown together, to form the sacrum. The holes coincide with the open spaces between the transverse processes, or lateral projections, of the other bones of the spinal column above this. In the chimpanzee this bone has the same general form as in man, except that instead of four holes in each row it has five. They are connected by transverse seams the same as in man, thus indicating that six of the vertebræ, instead of five, are united. In compensation for this, the ape has one vertebra less in the portion of the spinal column just above, which is called the lumbar. In man there are five free lumbar vertebræ and five united sections of the sacrum, while in the ape there are only four free lumbar vertebræ and six united sections forming the sacrum. But regarding each section of the sacrum as a separate bone and counting the whole number of vertebræ in the spinal column there are found to be exactly the same number in each. [Illustration: Pelvis of the Chimpanzee _A, sacrum; B, fourth lumbar vertebra; C, coccyx; D, ilium or hip bone; E, femur or thigh bone._] Some writers have put great stress upon the difference in the structure of this bone, and have pointed out as impossible a common origin for man and ape; but one fact remains to be explained, and that is, that while these appear to be fixed and constant characteristics of man and ape there are many exceptions known in human anatomy. In the splendid collection of human spinal columns in the museum of the Harvard Medical School are no less than eighteen specimens of the human sacrum having six united segments; and I have found in the collections of various museums a total of more than thirty others. These facts show that this characteristic is not confined to the ape. It is true that in some of these abnormal specimens there remain five _lumbar_ vertebræ. This seems to indicate that this portion of the spinal column is the most susceptible to variation. I have never seen an instance, however, of variation in the sacrum of the chimpanzee. In this respect he appears to be, in his structural type, more constant than man.