How slowly and surely the circulatory system improved! From the
cold-blooded animal to the warm-blooded is a great advance. In the
warm-blooded is developed the capacity to maintain a fixed
temperature while that of the surrounding medium changes. The brain
and nervous system display the same progressive ascent from the
brainless acrania, up through the fishes, batrachia, reptiles, and
birds to the top in mammals. The same with the skeletons in the
invertebrates, from membrane to cartilage, from cartilage to bone,
so that the primitive cartilage remaining in any part of the
skeleton is considered a mark of inferiority.

According to Cope, there has been progressive improvement in the
mechanism of the body--it has become a better and better machine.
The suspension of the lower jaw, so as to bring the teeth nearer the
power,--the masseter and related muscles,--was a slow evolution and
a great advance. The fin is more primitive than the limb; the limbs
themselves display a constantly increasing differentiation of parts
from the batrachian to the mammalian. There was no good ankle joint
in early Eocene times. The model ankle joint is a tongue and groove
arrangement, and this is a later evolution. In Eocene times they
were nearly all flat. The arched foot, too, comes in; this is an
advance on the flat foot. The bones of the palms and soles are not
locked until the later Tertiary. The vertebral column progressed in
the same way, from flat to the double curve and the interlocking
process, thus securing greatest strength with greatest mobility. In
the earliest life locomotion was diffused, later it became
concentrated. The worm walks with its whole body.