204 ROYAL NORTON CHAPMAN 
sufficient room for the passage of the fetus at the time of birth, 
and there has, therefore, been no necessity for its loss. 
The pocket gophers evidently represent a stage in which the 
room within the pelvis is greatly restricted, and the symphysis 
is in the process of being lost. The pelvis has attained a position 
nearly parallel to the sacrum and the caudal vertebrae, the 
sacrum and the ischium are codssified, forming a closed box. 
At the same time, the pubic bones have diverged caudally, and 
the symphysis, when it is present, is maintained by a rod of 
bone which seems to be unnecessary otherwise, for when the 
symphysis is absent, a portion of the bone on either side is also 
absent (compare figs. 4 and 6). 
Having arrived at this condition, it would seem to te a distinct 
advantage for the female to lose the symphysis and thus pro- 
vide for the easier passage of the fetus at the time of birth. No 
advantage can be seen in the retention of the symphysis for the 
attachment of the rectus abdominis muscles, since they have 
been. found to accomodate themselves to all of the modifications 
of the pelvis. There seems to be no reason, therefore, why the 
female should retain the symphysis. 
The males, as far as can be seen, have no call for the loss of 
the symphysis. On the other hand, they have well-developed 
levator muscles of the penis attaching to the bones forming the 
symphysis which, in a mechanical way, would seem to justify 
its retention. 
From this consideration of the pocket gophers, it is concluded 
that, when the mechanical necessities of propulsion have brought 
the pelvis to a horizontal position, caused it to fuse with the 
sacrum, and the pubic bones to diverge posteriorly in such a 
way that a portion of bone, otherwise unnecessary, must be 
present in order to maintain a symphysis, other forces appear 
which are different in the two sexes. Thus, when the symphysis 
has been greatly reduced and is on the verge of being lost, the 
step seems to be hastened in the female associated with the 
necessity of more room in the pelvis and retarded in the male, 
an association with the necessity of a support for the levator 
muscles of the penis. 
