Can we fail to see the significance of the order in which life has
appeared upon the globe--the ascending series from the simple to the
more and more complex? Can we doubt that each series is the outcome
of the one below it--that there is a logical sequence from the
protozoa up through the invertebrates, the vertebrates, to man? Is
it not like all that we know of the method of nature? Could we
substitute the life of one period for that of another without doing
obvious violence to the logic of nature? Is there no fundamental
reason for the gradation we behold?
All animal life lowest in organization is earliest in time, and vice
versa, the different classes of a sub-kingdom, and the different
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.
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.