edition he adds, "But the expression often used by Mr. Herbert Spencer, of the Survival of the Fittest, is more accurate, and is sometimes equally convenient." Owing to this struggle for life, any variation, however slight and from whatever cause proceeding, if it be in any degree profitable to an individual of any species, in its infinitely complex relations to other organic beings and to external nature, will tend to the preservation of that individual, and will generally be inherited by its offspring ... I have called this principle, by which each slight variation, if useful, is preserved, by the term of Natural Selection, in order to mark its relation to man's power of selection. He notes that both A. P. de Candolle and Charles Lyell had stated that all organisms are exposed to severe competition. Darwin emphasizes that he used the phrase "struggle for existence" in "a large and metaphorical sense, including dependence of one being on another"; he gives examples ranging from plants struggling against drought to plants competing for birds to eat their fruit and disseminate their seeds. He describes the struggle resulting from population growth: "It is the doctrine of Malthus applied with manifold force to the whole animal and vegetable kingdoms." He discusses checks to such increase including complex ecological interdependencies, and notes that competition is most severe between closely related forms "which fill nearly the same place in the economy of nature". Chapter IV details natural selection under the "infinitely complex and close-fitting ... mutual relations of all organic beings to each other and to their physical conditions of life". Darwin takes as an example a country where a change in conditions led to extinction of some species, immigration of others and, where suitable variations occurred, descendants of some species became adapted to new conditions. He remarks that the artificial selection practised by animal breeders frequently produced sharp divergence in character between breeds, and suggests that natural selection might do the same, saying: But how, it may be asked, can any analogous principle apply in nature? I believe it can and does apply most efficiently, from the simple circumstance that the more diversified the descendants from any one species become in structure, constitution, and habits, by so much will they be better enabled to seize on many and widely diversified places in the polity of nature, and so be enabled to increase in numbers.