The earlier organisms were simpler. They gradually changed into complex organisms. They are interrelated because of common origin. They have common shelter and environment. Evolution is generally slow, continuous, irreversible and progressive. The most important requirement for the evolution is variation. The ultimate source of organic evolution is the mutation. One of the most important factors of the development of a new species is the geographical isolation. The ultimate source of organic variation is not natural selection, sex reproduction and hormonal action. Thus, option D is correct and other options are wrong.