Philosophically Haeckel advocated a ‘neutral monism’ rejecting free will and theism; also, following the idealist strand in German thinking, he advocated the fundamental unity of organic and inorganic nature in ‘a sort of pantheism’. He is famous for the (incorrect) statement that ‘ontogeny recapitulates phylogeny’. Here ontogeny is the the development of the embryo, and phylogeny the evolution of the species; the claim is that the embryo of every species repeats the evolutionary development of that species, which is not true.