Over and over again in the history of animal life there have been
attempts to get out of the water on to terra firma, and many of these
have been successful, notably those made (1) by worms, (2) by
air-breathing Arthropods, and (3) by amphibians.
In thinking of the conquest of the dry land by animals, we must
recognise the indispensable rôle of plants in preparing the way. The dry
ground would have proved too inhospitable had not terrestrial plants
begun to establish themselves, affording food, shelter, and humidity.
There had to be plants before there could be earthworms, which feed on
decaying leaves and the like, but how soon was the debt repaid when 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.