If we know anything of the earth's past history, we know that the
continents were long in forming, that they passed through many
vicissitudes of heat and cold, of fire and flood, of upheaval and
subsidence--that they had, so to speak, their first low, simple
rudimentary or invertebrate life, that they were all slow in getting
their backbones, slower still in clothing their rock ribs with soil
and verdure, that they passed through a sort of amphibian stage, now
under water, now on dry land, that their many kinds of soils and
climes were not differentiated and their complex water-systems
established till well into Tertiary times--in short, that they have
passed more and more from the simple to the complex, from the
disorganized to the organized. When man comes to draw his sustenance
from their breasts, may they not be said to have reached the
mammalian stage?
The fertile plain and valley and the rounded hill are of slow
growth, immensely slow. But any given stage of the earth has
followed naturally from the previous stage, only more and more and
higher and higher forces took a hand in the game. First its elements
passed through the stage of fire, then through the stage of water,
then merged into the stage of air. More and more the aerial
elements--oxygen, carbon, nitrogen--have entered into its
constituents and fattened the soil. The humanizing of the earth has
been largely a process of oxidation. More than disintegrated rock
makes up the soil; the air and the rains and the snows have all
contributed a share.
The history of the soil which we turn with our spade, and stamp with
our shoes, covers millions upon millions of years. It is the ashes
of the mountains, the leavings of untold generations of animal and
vegetable life. It came out of the sea, it drifted from the heavens;
it flowed out from the fiery heart of the globe; it has been worked
over and over by frost and flood, blown by winds, shoveled by ice,
--mixed and kneaded and moulded as the house-wife kneads and moulds
her bread,--refining and refining from age to age. Much of it was
held in solution in the primordial seas, whence it was filtered and
used and precipitated by countless forms of marine life, making a
sediment that in time became rocks, that again in time became
continents or parts of them, which the aerial forces reduced to
soil. Indeed, the soil itself is an evolution, as much so as the
life upon it.
We probably have little conception of how intimate and cooperative
all parts of the universe are with one another,--of the debt we owe
to the farthest stars, and to the remotest period of time. We must
owe a debt to the monsters of Mesozoic and Caenozoic time; they
helped to fertilize the soil for us, and to discipline the ruder
forces of life. We owe a debt to all that has gone before: to the
heavens above and to the earth-fires beneath, to the ice-sheets that
ground down the mountains, and to the ocean currents. Just as we owe
a debt to the men and women in our line of descent, so we owe a debt
to the ruder primordial forces that shaped the planet to our use,
and took a hand in the game of animal life.
The gods of evolution had served a long apprenticeship; they had
gained proficiency and were master workmen. Or shall we say that the
elements of life had become more plastic and adaptable, or that the
life fund had accumulated, so to speak? Had the vast succession of
living beings, the long experience in organization, at last made the
problem of the origin of man easier to solve?
One fancies every living thing as not only returning its mineral
elements to the soil, but as in some subtle way leaving its vital
forces also, and thus contributing to the impalpable, invisible
store-house of vital energy of the globe.
At first among the mammalian tribes there was much muscle and little
brains. But in the middle Tertiary the mammal brain began suddenly
to enlarge, so that in our time the brain of the horse is more than
eight times the size of the brain of his progenitor, the dinoceras
of Eocene times.
Nature seems to have experimented with brains and nerve ganglia, as
she has with so many other things. The huge reptilian creatures of
Mesozoic time--the various dinosaurs--had ridiculously small heads
and brains, but they had what might be called supplementary brains
well toward the other end of the body,--great nervous masses near
the sacrum, many times the size of the ostensible brain, which no
doubt performed certain brain functions. But the principle of
centralization was at work, and when in later time we reach the
higher mammalian forms, we find these outlying nervous masses called
in, so to speak, and concentrated in the head.
Nature has tried the big, the gigantic, over and over, and then
abandoned it. In Carboniferous times there was a gigantic
dragon-fly, measuring more than two feet in the expanse of wings.
Still earlier, there were gigantic mollusks and sea scorpions, a
cephalopod larger than a man; then gigantic fishes and amphibians
and reptiles, followed by enormous mammals. But the geologic record
shows that these huge forms did not continue. The mollusks that last
unchanged through millions of years are the clam and the oyster of
our day. The huge mosses and tree-ferns are gone, and only their
humbler types remain. Among men giants are short-lived.
On the other hand, the steady increase in size of certain other
species of animals during the later geologic ages is a curious and
interesting fact. The first progenitors of the elephant that have
been found show a small animal that steadily grew through the ages
till the animal as we now find it is reached. Among the
invertebrates this same progressive increase in size has been noted,
a small shell in the Devonian becoming enormous in the Triassic.
Certain species of sharks of medium size in the lower Eocene
continue to increase till they attain the astounding dimensions in
the Miocene and Pliocene of over one hundred feet long. A certain
fish appearing in the Devonian as a small fish of seven centimetres
in length, becomes in the Carboniferous era a creature twenty-seven
centimetres in length. Among the mammals of Tertiary times this same
law of steady increase in size has been operative, as seen in the
Felidae, the stag, and the antelope. Man himself has, no doubt, been
under the same law, and is probably a much larger animal than any of
his Tertiary ancestors. In the vegetable world this process, in many
cases, at least, has been reversed, and the huge treelike
club-mosses and horsetails of Carboniferous times have dwindled in
our time to very insignificant herbaceous forms.
Animals of overweening size are handicapped in many ways, so that
nature in most cases finally abandons the gigantic and sticks to the
medium and the small.