collapse [with Malthus's pessimism and the shock of the French revolution] of the foundations of the past." The Baltic German biologist Karl Ernst von Baer (1792–1876) argued for an orthogenetic force in nature, reasoning in a review of Darwin's 1859 On the Origin of Species that "Forces which are not directed—so-called blind forces—can never produce order." In 1864, the Swiss anatomist Albert von Kölliker (1817–1905) presented his orthogenetic theory, heterogenesis, arguing for wholly separate lines of descent with no common ancestor. In 1884, the Swiss botanist Carl Nägeli (1817–1891) proposed a version of orthogenesis involving an "inner perfecting principle". Gregor Mendel died that same year; Nägeli, who proposed that an "idioplasm" transmitted inherited characteristics, dissuaded Mendel from continuing to work on plant genetics. According to Nägeli many evolutionary developments were nonadaptive and variation was internally programmed. Charles Darwin saw this as a serious challenge, replying that "There must be some efficient cause for each slight individual difference", but was unable to provide a specific answer without knowledge of genetics. Further, Darwin was himself somewhat progressionist, believing for example that "Man" was "higher" than the barnacles he studied. Darwin indeed wrote in his 1859 Origin of Species: In 1898, after studying butterfly coloration, Theodor Eimer (1843–1898) introduced the term orthogenesis with a widely read book, On Orthogenesis: And the Impotence of Natural Selection in Species Formation. Eimer claimed there were trends in evolution with no adaptive significance that would be difficult to explain by natural selection. To supporters of orthogenesis, in some cases species could be led by such trends to extinction. Eimer linked orthogenesis to neo-Lamarckism in his 1890 book Organic Evolution as the Result of the Inheritance of Acquired Characteristics According to the Laws of Organic Growth. He used examples such as the evolution of the horse to argue that evolution had proceeded in a regular single direction that was difficult to explain by random variation. Gould described Eimer as a materialist who rejected any vitalist or teleological approach to orthogenesis, arguing that Eimer's criticism of natural selection was common amongst many evolutionists of his generation; they were searching for alternative mechanisms, as they had come to believe that natural selection could not create new species. Nineteenth and twentieth centuries Numerous versions of orthogenesis (see table) have been proposed. Debate centred on whether such theories were scientific, or whether orthogenesis was inherently vitalistic or essentially theological. For example, biologists