'Every species has come into existence coincident both in space and time with a pre-existing closely allied species.'
Alfred Russel Wallace
This is a reprint of the first paper in which Wallace publicly discusses evolution. The title is 'On the Law which has Regulated the Introduction of New Species', but it is generally called the Sarawak Law paper since Wallace wrote it in 1855 while in the Sarawak region of Borneo.
In the paper, he asked why species similar to each other in appearance are often located near one another, both geographically and in the fossil record: 'the most closely allied species [are] found in geographical proximity. The question forces itself upon every thinking mind - why are these things so?'
To answer this question, Wallace proposed the Sarawak Law: 'Every species has come into existence coincident both in space and time with a pre-existing closely allied species.'
In other words, new species evolve from existing ones, rather than simply appearing where they are. This explains why similar species are found near one another in both space and time.
More broadly, Wallace suggested that the distribution of animals and plants is related to gradual geological changes over time. For example, the formation of an island isolating two groups of a species from one other leads them to evolve into separate species. These ideas are part of a concept we now call biogeography.