Crawfurd dedicated considerable effort to a critique of Darwin's theories of human evolution; as a proponent of polygenism, who believed that human races did not share common ancestors, Crawfurd was an early and prominent critic of Darwin's ideas. Right at the end of his life, in 1868, Crawfurd was using a "missing link" argument against Sir John Lubbock, in what Ellingson describes as a misrepresentation of a Darwinist viewpoint based on the idea that a precursor of humans must still be extant.