Darwin’s Natural Selection Theory changed the way we see life and the world for ever and so many discussions and books have been written about it since … Darwin wrote a book explaining it but needed just one paragraph to describe it. Here is Darwin himself describing Natural Selection in 1859 in a few words:

“If, during the long course of ages and under varying conditions of life, organic beings vary at all in the several parts of their organization, and I think this cannot be disputed; if there be, owing to the high geometric powers of increase of each species, at some age, season or year, a severe struggle for life, and this certainly cannot be disputed; then, considering the infinite complexity of the relations of all organic beings to each other and to their conditions of existence, causing an infinite variety in structure, constitution, and habits, to be advantageous to them, I think it would be a most extraordinary fact if no variation ever had occurred useful to each being’s own welfare, in the same way as so many variations have occurred useful to man. But if variations useful to any organic being do occur, assuredly individuals thus characterized will have the best chance of being preserved in the struggle for life; and from the strong principle of inheritance they will tend to produce offspring similarly characterized. This principle of preservation, I have called, for the sake of brevity, Natural Selection.”

Darwin, Charles (1859)On the Origin of Species in From So Simple a Beginning: The Four Great Books of Charles Darwin. Edited, with introductions, by Edward O. Wilson. W. W. Norton & Company. New York, 2006.