A great restriction attendant on the invasion of the dry land is that
locomotion becomes limited to one plane, namely, the surface of the
earth. This is in great contrast to what is true in the water, where the
animal can move up or down, to right or to left, at any angle and in
three dimensions. It surely follows from this that the movements of land
animals must be rapid and precise, unless, indeed, safety is secured in
some other way. Hence it is easy to understand why most land animals
have very finely developed striped muscles, and why a beetle running on
the ground has far more numerous muscles than a lobster swimming in the
sea.
Land animals were also handicapped by the risks of drought and of frost,
but these were met by defences of the most diverse description, from the
hairs of woolly caterpillars to the fur of mammals, from the carapace of
tortoises to the armour of armadillos. In other cases, it is hardly
necessary to say, the difficulties may be met in other ways, as frogs
meet the winter by falling into a lethargic state in some secluded
retreat.
Another consequence of getting on to dry land is that the eggs or young
can no longer be set free anyhow, as is possible when the animal is
surrounded by water, which is in itself more or less of a cradle. If the
eggs were laid or the young liberated on dry ground, the chances are
many that they would be dried up or devoured. So there are numerous ways
in which land animals secure the safety of their young, e.g. by burying
them in the ground, or by hiding them in nests, or by carrying them
about for a prolonged period either before or after birth. This may mean
great safety for the young, this may make it possible to have only a
small family, and this may tend to the evolution of parental care and
the kindly emotions. Thus it may be understood that from the conquest of
the land many far-reaching consequences have followed.
[Illustration: _Photo: Rischgitz._