Charles Darwin 
On the Origin of Species by Means of Natural Selection [EPUB ebook] 
The Cornerstone of the Evolutionary Biology

The Origin of Species introduced the scientific theory that populations evolve over the course of generations through a process of natural selection. It presented a body of evidence that the diversity of life arose by common descent through a branching pattern of evolution.
Contents:
Variation Under Domestication
Variation Under Nature
Struggle for Existence
Natural Selection
Laws of Variation
Difficulties on Theory
Instinct
Hybridism
On the Imperfection of the Geological Record.
On the Geological Succession of Organic Beings
Geographical Distribution
Mutual Affinities of Organic Beings: Morphology
Recapitulation and Conclusion