On The Origin of Species

Written in 1859 Darwin here puts forward his theory of evolution which stats that species derive from other species over time through a gradual evolutionary process, and the most adaptable to environmental changes, the "fittest" were the ones who survived through this process.

It is also the origin of the term "survival of the fittest", essentially meaning that whatever is most successful in a given environment at adaptation to that environment, will over time pass on it's genes more succesfully and survive as a species.