In previous sections of this book, basic principles of genetics have been discussed In organisms ranging from viruses and bacteria to higher plants and animals. Another phenomenon which attracted the attention of geneticists, is organic evolution, which means the process by which simple organisms gave rise to complex organisms. However, much before the science of genetics took birth in 1900 A.D., various theories of organic evolution were put forward. However, obstacles like the notion of 'fixity of species' -stood in the way of rational scientific appreciation of evolution. This notion was supported, by biologists like C. Linnaeus apart from philosophers and religious leaders and implied that (i) the species could be described as types and (ii) these types were sharply distinct and therefore could not have arisen from each other. It was difficult to accept evolutionary change, until J.B. Lamarck, after extensive studies on animals, concluded that evolution is a general fact covering all forms of life and recognized the fact that a species is a 'dynamic' entity. Later, in 1859, Charles Darwin also supported evolution and argued in his book 'The Origin of Species' and in his other works that although there were morphological species types, they arose by natural selection among the variable members of previous species. Some of the popular theories of organic evolution are discussed in this section in brief. The readers are advised to consult a text book on evolution for a more detailed information.