All life on this globe is divided into two great classes, vegetable
and animal. All animals belong to one or the other of two grand
divisions: invertebrates (those without backbones) and vertebrates
(the backboned). The latter class, comprising more than 30,000 known
species, is subdivided into five great groups: fishes, frogs, reptiles,
birds and mammals. At the top of the highest group, mammals (those
which bring forth their young alive), is man. Birds and mammals were
evolved from the reptiles--both offshoots of the same stock--and are
contemporaneous in development and descent.
The various classes and groups of animals, both in the order of
development from the simple to the complex and in the time of their
arrival, are in the order named: first the small, one-celled animals
(many species of which are found today) then more complex organisms of
the invertebrate division, later the fishes, amphibians, reptiles, and
mammals, including man.
The story told by the geologist is in perfect agreement with those laws
of development which we call evolution. The lowest, simplest forms
of life are the oldest as is shown by their fossil remains (bones,
shells, etc.) found in stratified or water-laid rock. These fossils
were deposited in sediment as it formed in sea and lake thousands
and millions of years ago. In the movements of the earth’s crust the
sediment of ancient sea and lake bottoms was raised above the water and
became land, the sediment hardening into rock. All sediment is formed
on a level, but in its up-heaval--generally slow, sometimes violent--it
is often tilted at various angles exposing its edges. It is from this
out-cropping, stratified rock (the sum of which is often several miles
in depth) that forms the outer part of the earth’s crust that the
geologist reads the story of creation.
The oldest or lowest water-laid rock is the archaean in which there
are no fossils. Above this, in the stratum formed in a later period
are found evidences of the beginnings of life. After millions of
years of growth and development the shell fish, at the top of the
invertebrate group, is produced. Then comes a period of uncounted
millions of years in which the fishes, the lowest of the vertebrates,
are being developed--millions of years to bridge the chasm between
the two main divisions of the animal kingdom; millions of years to
produce a backbone! Other long periods of time, filled with change and
development, come and go--the age of the coal plant and of the frogs,
succeeded by the age of reptiles, giant monsters, cold-blooded and
of small brain, swarming sea and land. Long ages pass; the reptilian
monsters have become extinct, leaving as their representatives only a
few dwarfed species--the crocodile, the lizard, the snake, the turtle.
Of mammals the lowest orders arrive first, followed by the more highly
developed until finally man appears.
Naturally, fossils of land animals are few compared with those of
marine animals. Practically all the remains of the latter sank and
were covered with the slowly accumulating sediment, while the bones of
the former only rarely were swept out to sea and lake. Rarest of all
fossils are those of man. A few have been found antedating history by
several thousand years, but we get a far more complete knowledge of
our primitive ancestors from the tools and weapons that they left in
sediment and drift before the dawn of the present era. We can trace
his progress upward through all degrees of culture from the rude old
stone age of a hundred thousand years ago, through the new stone age,
the copper and bronze age, and the iron age to the beginning of written
records.
Yet, so old is our planet and so long ago since life dawned on it--so
long even since the first mammals appeared--that man’s arrival a
hundred-or-so thousand years ago is but as yesterday. To paraphrase
an illustration by Slade and Ferguson: “Suppose we take the earth as
365 million years old, and consider this period as a year, one million
years being taken as a day.” Then, on this scale the vertebrates came
into existence late in the summer or early fall, the mammals not
earlier than the end of November, and “the whole period of man is not
likely to have been further back than the evening of December 31st, and
the earliest historic evidence (in Egypt) is not more than ten minutes
before the last midnight.”--“the last midnight,” of course, being the
present.