the
earthworms began their worldwide task of forming vegetable mould,
opening up the earth with their burrows, circulating the soil by means
of their castings, and bruising the particles in their
gizzard--certainly the most important mill in the world.

Another important idea is that littoral haunts, both on the seashore and
in the freshwaters, afforded the necessary apprenticeship and
transitional experience for the more strenuous life on dry land. Much
that was perfected on land had its beginnings on the shore. Let us
inquire, however, what the passage from water to dry land actually
implied. This has been briefly discussed in a previous article (on
Evolution), but the subject is one of great interest and importance.

Difficulties and Results of the Transition from Water to Land

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