Distribution of Early Rocks
The distribution of the early rocks suggests that there was
comparatively little dry land showing above the surface of the
Archaean ocean. Our knowledge of these rocks is not at all
complete, and we must remember that some of this primitive land
may be now under the sea or buried in unsuspected regions. It is
significant, however, that, up to the present, exploration seems
to show that in those remote ages only about one-fifth of our
actual land-surface stood above the level of the waters. Apart
from a patch of some 20,000 square miles of what is now
Australia, and smaller patches in Tasmania, New Zealand, and
India, nearly the whole of this land was in the far North. A
considerable area of eastern Canada had emerged, with lesser
islands standing out to the west and south of North America.
Another large area lay round the basin of the Baltic; and as
Greenland, the Hebrides, and the extreme tip of Scotland, belong
to the same age, it is believed that a continent, of which they
are fragments, united America and Europe across the North
Atlantic. Of the rest of what is now Europe there were merely
large islands--one on the border of England and Wales, others in
France, Spain, and Southern Germany. Asia was represented by a
large area in China and Siberia, and an island or islands on the
site of India. Very little of Africa or South America existed.
It will be seen at a glance that the physical story of the earth
from that time is a record of the emergence from the waters of
larger continents and the formation of lofty chains of mountains.
Now this world-old battle of land and sea has been waged with
varying fortune from age to age, and it has been one of the most
important factors in the development of life. We are just
beginning to realise what a wonderful light it throws on the
upward advance of animals and plants. No one in the scientific
world to-day questions that, however imperfect the record may be,
there has been a continuous development of life from the lowest
level to the highest. But why there was advance at all, why the
primitive microbe climbs the scale of being, during millions of
years, until it reaches the stature of humanity, seems to many a
profound mystery. The solution of this mystery begins to break
upon us when we contemplate, in the geological record, the
prolonged series of changes in the face of the earth itself, and
try to realise how these changes must have impelled living things
to fresh and higher adaptations to their changing surroundings.
Imagine some early continent with its population of animals and
plants. Each bay, estuary, river, and lake, each forest and marsh
and solid plain, has its distinctive inhabitants. Imagine this
continent slowly sinking into the sea, until the advancing arms
of the salt water meet across it, mingling their diverse
populations in a common world, making the fresh-water lake
brackish or salt, turning the dry land into swamp, and flooding
the forest. Or suppose, on the other hand, that the land rises,
the marsh is drained, the genial climate succeeded by an icy
cold, the luscious vegetation destroyed, the whole animal
population compelled to change its habits and its food. But this
is no imaginary picture. It is the actual story of the earth
during millions of years, and it is chiefly in the light of these
vast and exacting changes in the environment that we are going to
survey the panorama of the advance of terrestrial life.
|
