 Colouration across a species is often uniform when it is a camouflage against predation. The more specific the ecological niche occupied, the more uniform will colouration and other features be. Deviations will lead to decreased fitness and will be selected against. Obligate mutualistic symbiosis often provides very specialized ecological niches; consequently certain key properties must be present in order for an individual to survive. Certain beard worms, such as Riftia pachyptila, live at ocean floor thermal vents and have no digestive tract; instead they supply hydrogen sulphide and methane to bacteria in their trophosomes, which oxidise the hydrogen sulphide and methane on behalf of the worm. As a result the worm cannot lack any property essential for the survival of the bacteria to which it is host. Such homeostatic mechanisms do not preclude deviations from the standard combinations of properties—mutations do occur. Homeostasis just means that usually such mutations will not persist. The rare benevolent, or even merely neutral, mutations may persist and flourish, in which case there will be variation and change. Environments may change, perhaps locally, also leading to variation and change (e.g. the famous peppered moth). Consequently, it will not always be the case that property P in a homeostatic property cluster F is a necessary condition of an individual belonging to the relevant kind.