n 1852, Spencer anticipated natural selection. But this anticipation cannot be found in "The Development Hypothesis", which is an advocacy against creationism and for a progressive (evolutionary) view. While in "The Development Hypothesis", Spencer simply turns the creationists' arguments against themselves, he also published "A Theory of Population, Deduced from the General Law of Animal Fertility" in 1852. Herein, an early formulation of the mechanism of natural selection can be found.
And here it must be remarked, that the effect of pressure of population [...] is not a uniform effect, but an average one. [...] For as those prematurely carried off must, in the average of cases, be those in whom the power of self-preservation is the least, it unavoidably follows, that those left behind to continue the race are those in whom the power of self-preservation is the greatest — are the select of their generation. [pp. 499ff.]
As this passage demonstrates, Spencer did formulate a mechanism of natural selection. The weird part of Spencer's argument, from our perspective, is the context, which I omitted above. First, this context is Lamarckian (inheritance of acquired characters), and second, Spencer referred exclusively to the human population. He believed that there was a trade-off between fertility and intellectual power. Contrary to a flat neo-Darwinian reasoning, he proposed that those humans who invested less in reproduction and more in education were better off and would therefore eventually become the select members of their generation. In times when genes were thought to be the only kind of replicator that can be inherited, this must have seemed folly, of course. Considering memes or other forms of non-genetic inheritance, however, even Spencer's Lamarckism might not seem so egregious anymore. The problem with these interpretations and re-interpretations is, I think, that they are all retrospective Whig-histories. We do not know what Spencer really thought, but he surely did not discriminate between genetic and non-genetic forms of inheritance as he even rejected the separation of germ-line and soma.
Apart from this retrospective interpretation, however, the above passage proves, I think, that Spencer indeed anticipated the mechanism of natural selection in print. So did many others. Darwin's deed is then not the "discovery" of natural selection, but its integration with other common parts of reasoning (e.g. Malthusian growth) into a unique combination that is still thought to be valid today, while other combinations of the same parts (e.g. Spencer's explanatory system) are not.
Spencer, Herbert. "A theory of population, deduced from the general law of animal fertility." Westminster Review 57 (1852): 468-501.
Last modified 20 August 2003