Final answer:
Unilineal evolution theories suggested a fixed sequence of cultural progression, while neoevolutionary theories recognized individual cultural paths and interactions between societies.
Step-by-step explanation:
The principal differences between 19th-century unilineal evolution and 20th-century neoevolutionary theories revolve around the interpretation and progression of cultures and societies. Proponents of the 19th-century unilineal evolution, such as Edward Burnett Tylor and Lewis Henry Morgan, proposed that societies evolve through a fixed sequence of stages towards civilization. In contrast, 20th-century neoevolutionary theories, championed by figures like Franz Boas, suggest that each culture evolves along its own path, responding to unique historical, social, and environmental influences. Neoevolutionists acknowledged the interconnectedness and interaction between cultures, pointing out the diffusion of technological, material, and ideological aspects rather than a universal evolutionary path.