orders of a class, succeeding one another, as Cope says, in the
relative order of their zoological rank. Thus the sponges are later
than the protozoa, the corals succeed the sponges, the sea-urchins
come after the corals, the shell-fish follow the sea-urchins, the
articulates are later than the shell-fish, the vertebrates are later
than the articulates. Among the former, the amphibian follows the
fish, the reptile follows the amphibian, the mammal follows the
reptile, and non-placental mammals are followed by the placental.

It almost seems as if nature hesitated whether to produce the mammal
from the reptile or from the amphibian, as the mammal bears marks of
both in its anatomy, and which was the parent stem is still a
question.

The heart started as a simple tube in the Leptocardii; it divides
itself into two cavities in the fishes, into three in the reptiles,
and into four in the birds and mammals. So the ossification of the
vertebral column takes place progressively, from the Silurian to the
middle Jurassic.

The same ascending series of creation as a whole is repeated in the
inception and development of every one of the higher animals to-day.
Each one begins as a single cell, which soon becomes a congeries of
cells, which is followed by congeries of congeries of cells, till
the highly complex structure of the grown animal with all its
intricate physiological activities and specialization of parts, is
reached. It is typical of the course of the creative energy from the
first unicellular life up to man, each succeeding stage flowing out
of, and necessitated by, the preceding stage.