We saw in a previous chapter how the process of evolution led to a
mastery of all the haunts of life. But it is necessary to return to
these haunts or homes of animals in some detail, so as to understand the
peculiar circumstances of each, and to see how in the course of ages of
struggle all sorts of self-preserving and race-continuing adaptations or
fitnesses have been wrought out and firmly established. Living creatures
have spread over all the earth and in the waters under the earth; some
of them have conquered the underground world and others the air. It is
possible, however, as has been indicated, to distinguish six great
haunts of life, each tenanted by a distinctive fauna, namely, the shore
of the sea, the open sea, the depths of the sea, the freshwaters, the
dry land, and the air. In the deep sea there are no plants at all; in
the air the only plants are floating bacteria, though there is a sense
in which a tree is very aerial, and the orchid perched on its branches
still more so; in the other four haunts there is a flora as well as a
fauna--the two working into one another's hands in interesting and often
subtle inter-relations--the subject of a separate study.