Berg claimed that the variation of characters in species is confined within certain limits due to both internal and external factors. The limitation of the variability, Berg argued, left hardly any space for natural selection; he claimed this was supported by the paleontological record because all the phylogenetic branches look more or less like straight lines. Berg distanced himself from both Darwinism and Lamarckism. Instead he proposed the mutationist concept of directed mass mutations as the main mechanism for directing evolution.