THE FEMALE REPRODUCTIVE ORGANS 303 
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 compar- 
ing 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 lumbo- 
sacral axis, fourth to shorten the bodies of the ilia and 
to develop in them a regularly curved surface, the con- 
cavity 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 archi- 
tecture 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 quad- 
ruped, monkey, anthropoid ape, human fetal and adult 
human type. 
By an examination of the adult pelvis and fetal skulls 
it will be seen that the shape of the quadruped birth canal 
accommodates the fetal head nicely when in extension 
since there is no large posterior cranial development, the 
head and neck being of nearly the same thickness. In the 
monkey, where the facial angle increases, the head does 
not advance as a pointed presentation but as an irregular 
