Darwin claimed that all living things on Earth are descended, with modification, from one or a small number of simple original forms.2.3 He further claimed that the major cause of evolutionary change was natural selection, i.e. those modifications which help the evolving entities to survive and reproduce will be maintained, and those which do not help will not. While Darwin himself was willing to accept that evolutionary changes may also be brought about by other causes (such as the effects of use and disuse), the neo-Darwinist position sees natural selection as the only relevant mechanism by which evolution leads to adaptation. The position does not rule out other causes of evolutionary change, but it does contend that any cause other than natural selection will result in essentially random changes.
An extreme neo-Darwinist definition of life might therefore be ``any entity with the properties of multiplication, variation and heredity''. There are a number of complications to this picture, which, while not contradicting the neo-Darwinist position, at least suggest other things to be taken into account in our efforts to build an artificial evolutionary system. Some of these (self-organisation and development, sex, and symbiogenesis) are briefly described below, and will be returned to in later chapters.