Darwin's analogy with artificial selection to show that natural selection occurs in our environment.

On the Origin of Species presented Darwin's contemporaries with two major proposals. The first proposal was that of "decent with modification", which says that all life originated from a common ancestor by divergence among lineages of species. The second proposal was that natural selection is the main, but not exclusive, mechanism for this divergence of lineages. Proof for Darwin's first argument of "decent with modification" came from his analysis in patterns of morphology, embryology, geography, …

…natural selection was an inference from a mass of indirect evidence; he argued that it should occur, but he did not claim to have directly observed natural selection acting to produce changes in free living populations (Dobzhansky, 1958). Darwin himself believed that natural selection occurs at a very slow rate, and that it would be difficult to observe in action. Presently, there is ample evidence that supports Darwin's hypothesis in a wide range of scientific fields.