Gravitational life is hypothetical life that has evolved under and adapted to conditions of extreme gravity, either very high or very low. Many examples of microscopic barophiles are known and the prodigious strength-to-mass ratio of some more advanced organisms (the rhinoceros beetle, for example, can manipulate objects with up to 850 times its own body mass), suggests that even complex mobile creatures might occur on worlds where the surface gravity is significantly higher than Earth's, providing that other basic biological requirements are not compromised. However, some scientists and science fiction writers have speculated on the prospects for life where the gravitational pull is far beyond the normal planetary range. Hal Clement's A Mission of Gravity, and its sequels, considers the nature of life on a world where the gravity pull varies between twice and 700 times that on Earth. Even more extraordinary are the conditions facing organisms which reside on the surface of a neutron star, as described by Frank Drake and Robert Forward (see neutron star, life on). At the opposite extreme, in The Black Cloud, Fred Hoyle considers how freedom from planetary gravity in interstellar space might allow intelligence to evolve to a much higher order.