_Proposed Mode of Treatment of the Subject_. It is evidently of the most vital importance to any theory that its foundations should be absolutely secure. It is therefore necessary to show, by a wide and comprehensive array of facts, that animals and plants _do_ perpetually vary in the manner and to the amount requisite; and that this takes place in wild animals as well as in those which are domesticated. It is necessary also to prove that all organisms _do_ tend to increase at the great rate alleged, and that this increase actually 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.] CHAPTER II THE STRUGGLE FOR EXISTENCE Its importance--The struggle among plants--Among animals--Illustrative cases--Succession of trees in forests of Denmark--The struggle for existence on the Pampas--Increase of organisms in a geometrical ratio--Examples of great powers of increase of animals--Rapid increase and wide spread of plants--Great fertility not essential to rapid increase--Struggle between closely allied species most severe--The ethical aspect of the struggle for existence.