The Linnaean Society hosts a joint presentation of the ideas of natural evolution of Charles Darwin and Alfred Russel Wallace, called On the Tendency of Species to form Varieties; and on the Perpetuation of Varieties and Species by Natural Means of Selection.
It's a combination of Wallace's paper "On the Law which has Regulated the Introduction of New Species" and portions of Darwin's long-withheld manuscript of On the Origin of Species by Means of Natural Selection.
This is the first public announcement of Darwin's theory, and it's prompted by Darwin's receipt the previous month of Wallace's paper. Wallace's ideas aren't exactly the same as Darwin's, but they are close enough to startle him and force him to move his own ideas into the public in order to establish precedence.
Charles Darwin is himself unable to attend this presentation because his infant son has just died of scarlet fever. Not much attention is paid to the presentation in any event, and no one immediately recognizes the significance of Darwin's and Wallace's ideas.