n the trees inferior to
that of the long-armed apes; while, as has just been said, it unfitted
it to walk on the ground either as a quadruped or in the jumping method
of its fellow anthropoids. In short, the biped attitude was much the
best suited to its organization and the one it was most likely to
assume. This once adopted as its habitual posture, efficiency in walking
would be gained by practice.

When once this animal became a ground walker, its facility of motion in
the trees was in a measure lost. When the feet became accustomed to the
flat surface of the ground, they became less capable of grasping the
rounded surface of the bough. Fitness to the one situation entailed loss
of fitness to the other. The feet of the apes can clasp the bough
firmly, by curving around its opposite sloping sides, and to this these
animals doubtless owe their bowed legs and their disposition to walk on
the outer edge of the foot. This disposition the man-ape lost as its
foot fitted itself to the surface of the ground. It was probably
retained in a measure by the young, after it had been lost by the mature
form, and is still manifested in the position of the foot in the human
embryo.

These considerations bring us to an important question: Why did the
man-ape gain a length of arm not the best suited to its arboreal
habitat? Why, in fact, do changes in physical structure ever take place?
How does an animal succeed in passing from one mode of life to another,
when during the transition period it is imperfectly adapted to either,
and therefore at a seeming disadvantage in the struggle for existence?
The study of animal development has given rise to certain difficult
problems of this character, some of which have been solved by showing
that the supposed disadvantage did not arise, or that it was balanced by
some equal advantage. In this way a considerable gap in life conditions
has perhaps occasionally been crossed. Small gaps have doubtless been
frequently passed over in the same manner.

In the case of the anthropoid apes, we perceive a considerable variation
in the length of the arms, from the very long arms of the gibbon to the
comparatively short ones of the chimpanzee. These differences are
probably the result of some difference in their life habits, and accord
with the possibility of a still shorter arm in the man-ape. There is,
however, some reason to believe, as we shall show later on, that the arm
of this animal was longer and