Origin of Species, by Charles Darwin

Chapter III:
Struggle For Life

Its bearing on natural selection — The Term used in a wide sense — Geometrical Ratio of Increase — Rapid increase of naturalised animals and plants — Nature of the Checks to Increase — Competition Universal; Effects of Climate; Protection from the number of individual — Complex relations of all animals and plants throughout nature — Struggle for life most severe between individuals and varieties of the same species — often severe between species of the same genus — The relation of organism to organism the most important of all relations