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 tracheae. 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.]

VI. THE AIR

There are no animals thoroughly aerial, but many insects spend much of
their adult life in the free air, and the swift hardly pauses in its
flight from dawn to