Final answer:
Franz Boas criticized nineteenth-century anthropologists for relying on 'armchair anthropology' instead of conducting their own fieldwork and collecting firsthand data. He emphasized the importance of obtaining ethnographic information directly from the people being studied to understand the unique historical trajectories of different cultures.
Step-by-step explanation:
One of Franz Boas's principal criticisms of nineteenth-century anthropologists was that their theories were based on 'armchair anthropology' and not their own fieldwork. Boas argued that anthropologists should obtain ethnographic information directly from the people they aim to write about, rather than relying on information from published sources. By conducting his own fieldwork and collecting data firsthand, Boas showed that cultures do not develop in isolation along a common trajectory toward 'civilization,' but rather each culture has its own unique historical trajectory.