Extinction is an integral part of the process of natural selection. Species have a finite life span and, since life first appeared on this planet, plants and animals have evolved into different forms. While some have vanished without issue or have not adapted quickly enough to changing environmental or climatic conditions, other have died out for such reasons as over specialization, inability to compete with other more successful species for the necessities of life, or because of some natural cataclysm such as the advance or retreat of glaciation. The best way to safeguard rare species of animals and plants is to ensure the conservation of the biotic community of which they are a part. Rarity is not in itself a cause for concern. Some species are inherently rare, often because ecological niche. But few species can survive outside their natural habitat, and if the habitat is threatened - or illegal hunting, introduced predators, or disease cannot be adequately controlled - there may in the final resort be alternative but to capture the last survivors and bring them into captivity. While this last-ditch solution is tantamount to an admission of failure, it provides the opportunity to build up stocks until conditions are right for the species to be reintroduced back into its natural habitats.