Leaving the water for dry land implied a loss in freedom of movement,
for the terrestrial animal is primarily restricted to the surface of the
earth. Thus it became essential that movements should be very rapid and
very precise, needs with which we may associate the acquisition of fine
cross-striped, quickly contracting muscles, and also, in time, their
multiplication into very numerous separate engines. We exercise
fifty-four muscles in the half-second that elapses between raising the
heel of our foot in walking and planting it firmly on the ground again.
Moreover, the need for rapid precisely controlled movements implied an
improved nervous system, for the brain was a movement-controlling organ
for ages before it did much in the way of thinking. The transition to
terra firma also involved a greater compactness of body, so that there
should not be too great friction on the surface. An animal like the
jellyfish is unthinkable on land, and the elongated bodies of some land
animals like centipedes and snakes are specially adapted so that they do
not "sprawl." They are exceptions that prove the rule.
Getting on to dry land meant entering a kingdom where the differences
between day and night, between summer and winter are more felt than in
the sea. This made it advantageous to have protections against
evaporation and loss of heat and other such dangers. Hence a variety of
ways in which the surface of the body acquired a thickened skin, or a
dead cuticle, or a shell, or a growth of hair, and so forth. In many
cases there is an increase of the protection before the winter sets in,
e.g. by growing thicker fur or by accumulating a layer of fat below the
skin.
But the thickening or protection of the skin involved a partial or total
loss of the skin as a respiratory surface. There is more oxygen
available on dry land than in the water, but it is not so readily
captured. Thus we see the importance of moist internal surfaces for
capturing the oxygen which has been drawn into the interior of the body
into some sort of lung. A unique solution was offered by Tracheate
Arthropods, such as Peripatus, Centipedes, Millipedes, and Insects,
where the air is carried to every hole and corner of the body by a
ramifying system of air-tubes or tracheæ. In most animals the blood goes
to the air, in insects the air goes to the blood. In the Robber-Crab,
which has migrated from the shore inland, the dry air is absorbed by
vascular tufts growing under the shelter of the gill-cover.
The problem of disposing of eggs or young ones is obviously much more
difficult on land than in the water. For the water offers an immediate
cradle, whereas on the dry land there were many dangers, e.g. of
drought, extremes of temperature, and hungry sharp-eyed enemies, which
had to be circumvented. So we find all manner of ways in which land
animals hide their eggs or their young ones in holes and nests, on herbs
and on trees. Some carry their young ones about after they are born,
like the Surinam toad and the kangaroo, while others have prolonged the
period of ante-natal life during which the young ones develop in safety
within their mother, and in very intimate partnership with her in the
case of the placental mammals. It is very interesting to find that the
pioneer animal called Peripatus, which bridges the gap between worms and
insects, carries its young for almost a year before birth.
Enough has been said to show that the successive conquests of the dry
land had great evolutionary results. It is hardly too much to say that
the invasion which the Amphibians led was the beginning of better
brains, more controlled activities, and higher expressions of family
life.
[Illustration: ALBATROSS: A CHARACTERISTIC PELAGIC BIRD OF THE SOUTHERN
SEA
It may have a spread of wing of over 11 feet from tip to tip. It is
famous for its extraordinary power of "sailing" round the ship without
any apparent strokes of its wings.]