Hermit crab gives rise to the popular name "hermit crab", by analogy to a hermit who lives alone. Several hermit crab species, both terrestrial and marine, have been observed forming a vacancy chain to exchange shells. When an individual crab finds a new empty shell it will leave its own shell and inspect the vacant shell for size. If the shell is found to be too large, the crab goes back to its own shell and then waits by the vacant shell for up to 8 hours. As new crabs arrive they also inspect the shell and, if it is too big,
On the Origin of Species an incipient species" and that "species are only strongly marked and permanent varieties". He argues for the ubiquity of variation in nature. Historians have noted that naturalists had long been aware that the individuals of a species differed from one another, but had generally considered such variations to be limited and unimportant deviations from the archetype of each species, that archetype being a fixed ideal in the mind of God. Darwin and Wallace made variation among individuals of the same species central to understanding the natural world. In Chapter III, Darwin asks how varieties "which I have called incipient species"