wrote the book Darwinism, which explained and defended natural selection. In it, he proposed the hypothesis that natural selection could drive the reproductive isolation of two varieties by encouraging the development of barriers against hybridisation. Thus it might contribute to the development of new species. He suggested the following scenario: When two populations of a species had diverged beyond a certain point, each adapted to particular conditions, hybrid offspring would be less adapted than either parent form and so natural selection would tend to eliminate the hybrids. Furthermore, under such conditions, natural selection would favour the development of barriers to hybridisation, as individuals that avoided hybrid matings would tend to have more fit offspring, and thus contribute to the reproductive isolation of the two incipient species. This idea came to be known as the Wallace effect, later called reinforcement. Wallace had suggested to Darwin that natural selection could play a role in preventing hybridisation in private correspondence as early as 1868, but had not worked it out to this level of detail. It continues to be a topic of research in evolutionary biology today, with both computer simulation and empirical results supporting its validity. Application of theory to humans, and role of teleology in evolution In 1864, Wallace published a paper, "The Origin of Human Races and the Antiquity of Man Deduced from the Theory of 'Natural Selection, applying the theory to humankind. Darwin had not yet publicly addressed the subject, although Thomas Huxley had in Evidence as to Man's Place in Nature. Wallace explained the apparent stability of the human stock by pointing to the vast gap in cranial capacities between humans and the great apes. Unlike some other Darwinists, including Darwin himself, he did not "regard modern primitives as almost filling the gap between man and ape". He saw the evolution of humans in two stages: achieving a bipedal posture that freed the hands to carry out the dictates of the brain, and the "recognition of the human brain as a totally new factor in the history of life". Wallace seems to have been the first evolutionist to see that the human brain effectively made further specialisation of the body unnecessary. Wallace wrote the paper for the Anthropological Society of London to address the debate between the supporters of monogenism, the belief that all human races shared a common ancestor and were one species, and the supporters of