Adding it Up
By the late summer of 1842 Darwin felt ready to commit an outline of his theory to paper. The main points were clear: Plants and animals with useful--and heritable--variations were likely to live longer. That meant they could leave more offspring, some of which would carry the new trait. Over time, species could change through this process of "natural selection," a term Darwin first uses here.
Darwin's rough sketch of his argument is almost a miniature version of his future masterwork, the Origin of Species. Yet he kept his ideas under wraps for nearly two decades more. Why? For one thing, Darwin wasn't finished thinking: Some critical details were still to come. And he knew he needed to amass a great deal of evidence to convince others of such a radical idea. But equally important, the time wasn't right. Still establishing his career, Darwin feared the ridicule of respected scientists such as Charles Lyell and Richard Owen. And he knew his ideas would be seen as an attack on religion and established society. It was better to wait.
Putting It All Together
In the summer of 1842, Darwin and his wife Emma spent several months visiting their families in the quiet English countryside. While at Emma's family estate, Darwin took up a pencil and covered 35 sheets of rough paper with an outline of his theory. This draft, which Darwin revised and expanded until its ideas found a home in the Origin, was for his eyes only.
Darwin wrote in a quick, intense burst: As one of his sons would later write, his outline is "more like a hasty memoranda of what is clear to himself, than material for the convincing of others."
In this passage Darwin considers the connection between geographic isolation and the formation of species.
". . .barriers of every kind seem to separate regions in a greater degree than proportionally to the difference of climates on each side. Thus great chains of mountains, spaces of sea between islands and continents, even great rivers and deserts."
He is emphasizing the importance of physical barriers--"spaces of sea between islands" like the Galpagos--in the origin of new species.