Species are fixed, separately created forms that do not change into new species over time.

Early naturalists could document variation within species, but many still treated species themselves as fixed natural units. In the dominant creationist framework of European natural history, each species had an independent origin and could vary only within limits.

Darwin and Wallace challenged the boundary rather than merely rearranging classifications. Biogeography, fossils, selective breeding, and variation in natural populations suggested that related species could share common ancestors and diverge over time.

Natural selection supplied a mechanism by which inherited variation could accumulate under different environmental conditions.

The evidence did not stop with the nineteenth century. Genetics explained inheritance, population biology measured evolutionary change, and molecular sequences revealed nested patterns of common descent across the tree of life.

Modern biology therefore treats species as historical populations that can arise, change, split, hybridize, and sometimes disappear.

Not every pre-Darwinian thinker believed species were absolutely immutable, and evolutionary proposals existed before 1859. The superseded claim concerns the dominant fixed-species framework, not a universal belief across every culture or naturalist.