Evolution was widely accepted in scientific circles within a few years of the publication of Origin, but the acceptance of natural selection as its driving mechanism was much less widespread. The four major alternatives to natural selection in the late 19th century were:
Theistic evolution was the idea that God intervened in the process of evolution to guide it in such a way that the living world could still be considered to be designed. However, this idea gradually fell out of favour among scientists, as they accepted the idea of methodological naturalism and came to believe that direct appeals to supernatural involvement were scientifically unproductive. By 1900, theistic evolution had largely disappeared from professional scientific discussions, although it retained a strong popular following.
In the late 19th century, the term neo-Lamarckism came to be associated with the position of naturalists who viewed the inheritance of acquired characteristics as the most important evolutionary mechanism. Cope looked for, and thought he found, patterns of linear progression in the fossil record. Inheritance of acquired characteristics was part of Haeckel’s recapitulation theory of evolution, which held that the embryological development of an organism repeats its evolutionary history.
Critics of neo-Lamarckism, such as the German biologist August Weismann and Alfred Russell Wallace pointed out that no one had ever produced solid evidence for the inheritance of acquired characteristics.
Orthogenesis was the hypothesis that life has an innate tendency to change, in a unilinear fashion, towards ever-greater perfection. Orthogenesis was popular among some palaetologists, who believed that the fossil record showed a gradual and constant unidirectional change.
Saltationism was the idea that new species arise as a result of large mutations. It was seen as an alternative to the Darwinian concept of a gradual process of small random variations being acted on by natural selection, and was popular with early geneticists. It became the basis of the mutation theory of evolution.