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 rapid 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 CHAPTER III THE VARIABILITY OF SPECIES IN A STATE OF NATURE Importance of variability--Popular ideas regarding it--Variability of the lower animals--The variability of insects--Variation among lizards--Variation among birds--Diagrams of bird-variation--Number of varying individuals--Variation in the mammalia--Variation in internal organs--Variations in the skull--Variations in the habits of animals--The variability of plants--Species which vary little--Concluding remarks CHAPTER IV VARIATION OF DOMESTICATED ANIMALS AND CULTIVATED PLANTS The facts of variation and artificial selection--Proofs of the generality of variation--Variations of apples and melons--Variations of flowers--Variations of domestic animals--Domestic pigeons--Acclimatisation--Circumstances favourable to selection by man--Conditions favourable to variation--Concluding remarks CHAPTER V NATURAL SELECTION BY VARIATION AND SURVIVAL OF THE FITTEST Effect of struggle for existence under unchanged conditions--The effect under change of conditions--Divergence of character--In insects--In birds--In mammalia--Divergence leads to a maximum of life in each area--Closely allied species inhabit distinct areas--Adaptation to conditions at various periods of life--The continued existence of low forms of life--Extinction of low types among the higher animals--Circumstances favourable to the origin of new species--Probable origin of the dippers--The importance of isolation--On the advance of organisation by natural selection--Summary of the first five chapters CHAPTER VI DIFFICULTIES AND OBJECTIONS Difficulty as to smallness of variations--As to the right variations occurring when required--The beginnings of important organs--The mammary glands--The eyes of flatfish--Origin of the eye--Useless or non-adaptive characters--Recent extension of the region of utility in plants--The same in animals--Uses of tails--Of the horns of deer--Of the scale-ornamentation of reptiles--Instability of non-adaptive characters--Delboeuf's law--No "specific" character proved to be useless--The swamping effects of intercrossing--Isolation as preventing intercrossing--Gulick on the effects of isolation--Cases in which isolation is ineffective CHAPTER VII ON THE INFERTILITY OF CROSSES BETWEEN DISTINCT SPECIES AND THE USUAL STERILITY OF THEIR HYBRID OFFSPRING