5. The terrestrial haunt has been invaded age after age by contingents
from the sea or from the freshwaters. We must recognise the worm
invasion, which led eventually to the making of the fertile soil, the
invasion due to air-breathing Arthropods, which led eventually to the
important linkage between flowers and their insect visitors, and the
invasion due to air-breathing Amphibians, which led eventually to the
higher terrestrial animals and to the development of intelligence and
family affection. Besides these three great invasions, there were minor
ones such as that leading to land-snails, for there has been a
widespread and persistent tendency among aquatic animals to try to
possess the dry land.
Getting on to dry land had a manifold significance.
It implied getting into a medium with a much larger supply of oxygen
than there is dissolved in the water. But the oxygen of the air is more
difficult to capture, especially when the skin becomes hard or well
protected, as it is almost bound to become in animals living on dry
ground. Thus this leads to the development of _internal surfaces_, such
as those of lungs, where the oxygen taken into the body may be absorbed
by the blood. In most animals the blood goes to the surface of
oxygen-capture; but in insects and their relatives there is a different
idea--of taking the air to the blood or in greater part to the area of
oxygen-combustion, the living tissues. A system of branching air-tubes
takes air into every hole and corner of the insect's body, and this
thorough aeration is doubtless in part the secret of the insect's
intense activity. The blood never becomes impure.
The conquest of the dry land also implied a predominance of that kind of
locomotion which may be compared to punting, when the body is pushed
along by pressing a lever against a hard substratum. And it also
followed that with few exceptions the body of the terrestrial animal
tended to be compact, readily lifted off the ground by the limbs or
adjusted in some other way so that there may not be too large a surface
trailing on the ground. An animal like a jellyfish, easily supported in
the water, would be impossible on land. Such apparent exceptions as
earthworms, centipedes, and snakes are not difficult to explain, for the
earthworm is a burrower which eats its way through the soil, the
centipede's long body is supported by numerous hard legs, and the snake
pushes itself along by means of the large ventral scales to which the
lower ends of very numerous ribs are attached.