Hermit crab wood and stone are used by some species). The tip of the hermit crab's abdomen is adapted to clasp strongly onto the columella of the snail shell. Most hermit crabs are nocturnal. Hermit crabs can be divided into two groups: As hermit crabs grow, they require larger shells. Since suitable intact gastropod shells are sometimes a limited resource, vigorous competition often occurs among hermit crabs for shells. The availability of empty shells at any given place depends on the relative abundance of gastropods and hermit crabs, matched for size. An equally important issue is the population of organisms that prey
On the Tendency of Species to form Varieties; and on the Perpetuation of Varieties and Species by Natural Means of Selection On the Tendency of Species to form Varieties; and on the Perpetuation of Varieties and Species by Natural Means of Selection On the Tendency of Species to form Varieties; and on the Perpetuation of Varieties and Species by Natural Means of Selection is the title of a joint presentation of two scientific papers to the Linnean Society of London on 1 July 1858: "On The Tendency of Varieties to Depart Indefinitely from the Original Type" by Alfred Russel Wallace and an "Extract from an unpublished Work on Species" from Charles Darwin's "Essay" of 1844, together with an "Abstract of a