In 1859, Charles Darwin published On the Origin of Species.  This text would clearly indicate the path by which organisms evolve over time.  Key to the theory of evolution are the concepts of natural selection (the development of characteristics in members of a population which enable them to successfully compete and reproduce) and adaptation (structural or physiological changes in those members of a population which allows survival during sometimes rapid and unpredicable changes in the environment).

 

Alfred Russell Wallace developed many of the same ideas as Darwin, and in 1858 sent Darwin a manuscript outlining his ideas.  This manuscript prompted Darwin to complete Origin and arrange a joint presentation between himself and Wallace at a seminar in London.