Homologous organ and vestigial organs provide an important evidence in favour of organic evolution (process by which changes in the genetic composition of populations of organisms occur in response to environmental changes). For example, human appendix is a developmental derivative and evolutionary vestige of the end of the much larger herbivorous caecum found in our primate ancestors. They both are structurally homologous and have different functions. In most vertebrates, the caecum is a large, complex gastrointestinal organ, enriched in mucosal lymphatic tissue and specialized for digestion of plants. The caecum varies in size among species, but in general the size of the caecum is proportional to the amount of plant matter in a given organism's diet. It is largest in obligate herbivores, animals whose diets consist entirely of plant matter. However, even though humans are herbivorous, the small human caecum does not house cellulose-digesting bacteria, and lost an essential function of cellulose digestion. This shows the close evolutionary relationships between homologous and vestigial organs.