intercrossing between members
of a species, e.g. by geographical barriers. Interbreeding of similar
forms tends to make a stable stock; out-breeding among dissimilars tends
to promote variability. But for an outline like this it is enough to
suggest the general method of organic evolution: Throughout the ages
organisms have been making tentatives--new departures of varying
magnitude--and these tentatives have been tested. The method is that of
testing all things and holding fast that which is good.

BIBLIOGRAPHY

(The following short list may be useful to readers who desire to have
further books recommended to them.)

CLODD, _Story of Creation: A Plain Account of Evolution._
DARWIN, _Origin of Species, Descent of Man._
DEPERET, _Transformation of the Animal World_ (Internat. Sci. Series).
GEDDES AND THOMSON, _Evolution_ (Home University Library).
GOODRICH, _Evolution_ (The People's Books).
HEADLEY, _Life and Evolution._
HUTCHINSON, H. NEVILLE, _Extinct Monsters_ (1892).
LULL, _Organic Evolution._
MCCABE, _A B C of Evolution._
METCALF, _Outline of the Theory of Organic Evolution._
OSBORN, H. F., _The Evolution of Life_ (1921).
THOMSON, _Darwinism and Human Life._
WALLACE, _Darwinism._

III

ADAPTATIONS TO ENVIRONMENT

ADAPTATIONS TO ENVIRONMENT

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--