occurs, under favourable conditions. We have to prove, further, that
variations of all kinds can be increased and accumulated by selection;
and that the struggle for existence to the extent here indicated
actually occurs in nature, and leads to the continued preservation of
favourable variations.

These matters will be discussed in the four succeeding chapters, though
in a somewhat different order--the struggle for existence and the power
of rapid multiplication, which is its cause, occupying the first place,
as comprising those facts which are the most fundamental and those which
can be perfectly explained without any reference to the less generally
understood facts of variation. These chapters will be followed by a
discussion of certain difficulties, and of the vexed question of
hybridity. Then will come a rather full account of the more important of
the complex relations of organisms to each other and to the earth
itself, which are either fully explained or greatly elucidated by the
theory. The concluding chapter will treat of the origin of man and his
relations to the lower animals.

FOOTNOTES:

[Footnote 1: _Geography and Classification of Animals_, p. 350.]

[Footnote 2: These expressions occur in Chapter IX. of the earlier
editions (to the ninth) of the _Principles of Geology_.]

[Footnote 3: L. Agassiz, _Lake Superior_, p. 377.]