My Dear Sir,-- The accompanying papers, which we have the honour of communicating to the Linnean Society, and which all relate to the same subject, viz. the Laws which affect the Production of Varieties, Races, and Species, contain the results of the investigations of two indefatigable naturalists, Mr. Charles Darwin and Mr. Alfred Wallace.

These gentlemen having, independently and unknown to one another, conceived the very same very ingenious theory to account for the appearance and perpetuation of varieties and of specific forms on our planet, may both fairly claim the merit of being original thinkers in this important line of inquiry; but neither of them having published his views, though Mr. Darwin has for many years past been repeatedly urged by us to do so, and both authors having now unreservedly placed their papers in our hands, we think it would best promote the interests of science that a selection from them should be laid before the Linnean Society.

So begins the introduction to pair of papers by Darwin and Wallace entitled On the Tendency of Species to form Varieties; and on the Perpetuation of Varieties and Species by Natural Means of Selection, presented to the Linnean Society of London on July 1, 1858, by Darwin's friends and colleagues Charles Lyell and Joseph Dalton Hooker

The Linnean Society has provided the full text of the papers online and in the form of a PDF file.  They also have a press release about the papers to celebrate the 150th anniversary of what at the time was a small step but in retrospect enabled a great leap forward in science.