Using uniformitarianism, which states that one cannot make an appeal to any force or phenomenon which cannot presently be observed (see catastrophism), Darwin theorized that the evolutionary process must occur gradually, not in saltations, since saltations are not presently observed, and extreme deviations from the usual phenotypic variation would be more likely to be selected against.
Usando o uniformitarismo, que establece que non se pode apelar para explicar os cambios no pasado a unha forza ou fenómeno que non se observa no presente (ver catastrofismo), Darwin teorizou que o proceso evolutivo debe ocorrer gradualmente, non a saltos, xa que os saltos non se observan no presente, e desviacións extremas das variacións fenotípicas usuais serían probablemente seleccionadas en contra.