On July 1st, 1858, an essay by Alfred Russell Wallace and two unpublished excerpts from a book by Charles Darwin were presented to a meeting of the Linnean Society of London. These writings, which were to be published in the Society's journal a month later as "On the Tendency of Species to Form Varieties; And On the Perpetuation of Varieties and Species by Natural Means of Selection", was the first public presentation of what has come to be known as the theory of evolution by natural selection, a foundational cornerstone of modern biology.
It is impossible to overestimate the impact of Darwin and Wallace's idea on science or on history in general. Even today it remains at the forefront of not only scientific exploration, but also in our culture; as a cause of tension between reason and faith (or rationality and irrationality, if you will).
Obviously, the theory as it was then limned and presented has not survived unaltered these one hundred and fifty years, but the spark of that genius shines brightly even now. Happy Birthday, Evolution!