This paper is sometimes referred to as the Sarawak Law paper since it was written while Wallace was on a specimen collecting expedition in the province of Sarawak (East Malayasian States) on the great island of Borneo. The paper has been misrepresented by certain historians as presenting a portion of the theory of natural selection. That is false; Wallace did not publish on natural selection until the Darwin-Wallace papers published in 1858 (No. 219).
The "law" states "The following law may be deduced from these facts: — Every species has come into existence coincident both in space and time with a pre–existing closely allied species."
About this Malcolm Jay Kottler wrote to me in April, 2023, "Darwin did not see that Wallace was thinking in evolutionary terms in this paper. In his paper Wallace used the word 'created' a number of times--such as 'It is evidently possible that two or three distinct species may have had a common antitype, and that each of these may again have become the antitypes from which other closely allied species were created"--which Darwin interpreted as creationist, and not evolutionary, in meaning.
"But Lyell saw Wallace's paper totally differently. Wallace's paper prompted Lyell to begin his Species Journal in 1855, and it was Lyell telling Darwin in April 1856--when Darwin revealed natural selection to Lyell for the first time--that Wallace was thinking along similar lines to Darwin and that Darwin had better put his views in print before Wallace beat him to it. Darwin listened to Lyell and began to write for publication."
See John van Wyhe, "The impact of A. R. Wallace's Sarawak Law paper reassessed," Studies in History and Philosophy of Science Part: Studies in History and Philosophy of Biological and Biomedical Sciences, 60 (2016) 56-66.