The presentation was called “On the Tendency of Species to form Varieties; and on the Perpetuation of Varieties and Species by Natural Means of Selection”.
The rest, as they say, is history.
To commemorate this event, here is an extract … not from the 1st July presentation because that would be obvious and, indeed, relevant, but from Wallace's The Malay Archipelago (1869).
In early 1858, Wallace was in the Moluccas, between Sulawesi and New Guinea. While there, he wrote his not-quite-as-famous-essay on evolution and sent it to Darwin for comment. From Ternate, Wallace travelled to Bird's Head, at the NW tip of New Guinea. He remained there from mid-April to late July.
His stay didn't quite go the way he had planned.
On the 22nd of July the schooner Hester Helena arrived, and five days afterwards we bade adieu to Dorey, without much regret, for in no place which I have visited have I encountered more privations and annoyances. Continual rain, continual sickness, little wholesome food, with a plague of ants and flies, surpassing anything I had before met with, required all a naturalist's ardour to encounter; and when they were uncompensated by great success in collecting, became all the more insupportable. This long thought-of and much-desired voyage to New Guinea had realized none of my expectations. Instead of being far better than the Aru Islands, it was in almost everything much worse. Instead of producing several of the rarer Paradise birds, I had not even seen one of them, and had not obtained any one superlatively fine bird or insect. I cannot deny, however, that Dorey was very rich in ants. One small black kind was excessively abundant. Almost every shrub and tree was more or less infested with it, and its large papery nests were everywhere to be seen. They immediately took possession of my house, building a large nest in the roof, and forming papery tunnels down almost every post. They swarmed on my table as I was at work setting out my insects, carrying them off from under my very nose, and even tearing them from the cards on which they were gummed if I left them for an instant. They crawled continually over my hands and face, got into my hair, and roamed at will over my whole body, not producing much inconvenience till they began to bite, which they would do on meeting with any obstruction to their passage, and with a sharpness which made me jump again and rush to undress and turn out the offender. They visited my bed also, so that night brought no relief from their persecutions; and I verily believe that during my three and a half months' residence at Dorey I was never for a single hour entirely free from them. They were not nearly so voracious as many other kinds, but their numbers and ubiquity rendered it necessary to be constantly on guard against them.
The flies that troubled me most were a large kind of blue-bottle or blow-fly. These settled in swarms on my bird skins when first put out to dry, filling their plumage with masses of eggs, which, if neglected, the next day produced maggots. They would get under the wings or under the body where it rested on the drying-board, sometimes actually raising it up half an inch by the mass of eggs deposited in a few hours; and every egg was so firmly glued to the fibres of the feathers, as to make it a work of much time and patience to get them off without injuring the bird. In no other locality have I ever been troubled with such a plague as this.
Ah, yes, the romance of the tropics ...