In Man the force exerted on the pelvis from above is greatest among all animals and is greater when he is in active motion. The force is directed from above to the sacroiliac joint, the iliac bodies and the acetabular region while from below the pressure is directly exerted upon the last named. The force from above rotates the sacrum downward at its upper end, the attached sacroiliac ligaments at the same time pulling the alæ inward and throwing outward the lower end, the acetabular part. This tends to widen the pelvic box and to reduce its anteroposterior diameter. But the force acting from the legs and the adductors of the thigh push the lower parts of the ilia and the ischia and pubes apart, thus counterbalancing the effort of the force from above. The combined forces tend to bend the iliac bodies, thereby producing the curved lateral margins of the superior strait and making the lateral axis long. As between these two forces that from above is certainly the greater. The effect of these forces can be followed by comparing a quadruped pelvis, a human fetal pelvis and an adult human one. The first two are similar chiefly in the shape of the inlet and the flat character of the ilia. This is strongly corroborative of the evolutionary development of the pelvis to meet the demand of the upright position. The effects of this evolution are as follows: First, to develop a forward inclination of the sacrum and a concavity in its surface anteriorly, second to increase greatly the iliopubic and ilioischiatic angles, third to cause the acetabula to move forward of the lumbosacral axis, fourth to shorten the bodies of the ilia and to develop in them a regularly curved surface, the concavity of which faces forward and inward, fifth to decrease the interpubic angle and accordingly to remove the triangular quality of the pelvic contour, sixth to increase the transverse diameter at the expense of the anteroposterior, and seventh to decrease greatly the sacrosymphyseal angle with the result that the entire pelvic cavity lies in one plane. The effect of forces in alterations of the pelvic architecture may be seen in their several stages by the observation of the mammalian pelvis in the course of its evolution and development. The transitions in form are very gradual, but their gradations are well shown in the characteristic forms which have been described; the quadruped, monkey, anthropoid ape, human fetal and adult human type.