On this date, Charles Darwin wrote a letter to the botanist Joseph Hooker, revealing for the first time to another human being his theory of evolution by natural selection:
I am almost convinced (quite contrary to opinion I started with) that species are not (it is like confessing a murder) immutable…I think I have found out (here’s presumption!) the simple way by which species become exquisitely adapted to various ends…
Darwin’s description of himself as “confessing a murder” was not entirely tongue-in-cheek – he genuinely feared that Hooker might be appalled by his ideas. Prior to the nineteenth century, naturalists had divided the study of nature into descriptive natural history and explanatory natural philosophy, and the prevailing natural philosophy had been “natural theology.” According to natural theology, organisms had been separately and instantaneously created in their modern forms by an Intelligent Designer, or God (the terms were synonymous).
However, in Hooker’s day, this distinction between catalogues and causes was breaking down – the “natural” system of classifying organisms suggested a genealogical, not theological, explanation for the origin of species. Hooker replied to Darwin, saying that there might well have been “a gradual change of species,” adding, “I shall be delighted to hear how you think that this change may have taken place, as no presently conceived opinions satisfy me on the subject.” Darwin was pleasantly surprised.