Consciousness implies perpetual discrimination, or the recognition of
likenesses and differences, and this is impossible unless impressions
persist long enough to be compared with one another. The physical organs
in connection with whose activity consciousness is manifested are the
upper and outer parts of the brain,--the cerebrum and cerebellum. These
organs never receive impressions directly from the outside world, but
only from lower nerve-centres, such as the spinal cord, the medulla, the
optic lobes, and other special centres of sensation. The impressions
received by the cerebrum and cerebellum are waves of molecular
disturbance sent up along centripetal nerves from the lower centres, and
presently drafted off along centrifugal nerves back to the lower
centres, thus causing the myriad movements which make up our active
life. Now there is no consciousness except when molecular disturbance is
generated in the cerebrum and cerebellum faster than it can be drafted
off to the lower centres.[5] It is the surplus of molecular disturbance
remaining in the cerebrum and cerebellum, and reflected back and forth
among the cells and fibres of which these highest centres are composed,
that affords the physical condition for the manifestation of
consciousness. Memory, emotion, reason, and volition begin with this
retention of a surplus of molecular motion in the highest centres. As we
survey the vertebrate sub-kingdom of animals, we find that as this
surplus increases, the surface of the highest centres increases in area.
In the lowest vertebrate animal, the amphioxus, the cerebrum and
cerebellum do not exist at all. In fishes we begin to find them, but
they are much smaller than the optic lobes. In such a highly organized
fish as the halibut, which weighs about as much as an average-sized man,
the cerebrum is smaller than a melon-seed. Continuing to grow by adding
concentric layers at the surface, the cerebrum and cerebellum become
much larger in birds and lower mammals, gradually covering up the optic
lobes. As we pass to higher mammalian forms, the growth of the cerebrum