al remains, and advancing to the most recent, we recognize a
continual improvement in construction, indicated by the degree of
advancement of the nervous system. The earliest fishes did not proceed
beyond that condition of the spinal column which is to be considered as
embryonic. The Silurian and Devonian rocks do not present it in an
ossified state. Fishes, up to the Carboniferous epoch, had a
heterocercal tail, just as the embryos of osseous fishes of the present
time have up to a certain period of their life. There was, therefore, an
arrest in the old extinct forms, and an advance to a higher point in the
more modern. The buckler-headed fishes of the Devonian rocks had their
respiratory organs and much of their digestive apparatus in the head,
and showed an approximation to the tadpoles or embryos of the frog. The
crocodiles of the oolite had biconcave vertebrae, like the embryos of the
recent ones which have gained the capability of making an advance to a
higher point. In the geological order, reptiles make their appearance
next after fishes, and this is what we should expect on the principle of
an ascending nervous development. Not until long after come birds, later
in date and higher in nervous advancement, capable not only of instinct,
but also of intelligence. Of mammals, the first that appear are what we
should have expected--the marsupials; but among the tertiary rocks, very
many other forms are presented, the earlier ones, whether herbivorous or
carnivorous, having a closer correspondence to the archetype than the
existing ones, save in their embryonic states, the analogies occurring
in such minor details as the possession of forty-four teeth. [Sidenote:
Absolute necessity of admitting transmutation of forms.] The biography
of the earth is thus, on the great scale, typical of individual life,
even that of man, and the succession of species in the progress of
numberless ages is the counterpart of the transmutation of an individual
from form to form. As in a dissolving view, new objects emerge from old
ones, and new forms spontaneously appear without the exercise of any
periodical creative act.

[Sidenote: Life of man from infancy to maturity in accordance with his
anatomy.] For some days after birth the actions of the human being are
merely reflex. Its cranio-spinal axis alone is in operation, and thus
far it is only an automaton. But soon the impressions of external
objects begin to be registered or preserved