The Expression was an original and, for many contemporaries, a controversial book. It formed the final part of a series that had started with On the Origin of Species and had controversially peaked the previous year with the Descent of Man. The former, published in 1859, laid out Darwin's theory of descent with modification through natural selection in animals and plants: the notion that randomly occurring variation within a population, if conferring a breeding or survival advantage, tends to be preserved, leading over time to divergence. The Descent, in which he extended the theory to humans, appeared more than a decade later in 1871, its publication delayed by a reticent Darwin.