Final answer:
Sacagawea and her husband are referred to as links of a chain because they acted as vital connectors between different cultural groups during the Lewis and Clark expedition, thereby aiding mutual understanding and communication.
Step-by-step explanation:
The author compares Sacagawea and her French husband to the links of a chain to emphasize their role in connecting different groups of people during the Lewis and Clark expedition. Sacagawea, as a Shoshone woman, and her husband, Toussaint Charbonneau, a French fur trapper, served as crucial intermediaries. Their unique positions as both members of and liaisons between different cultures allowed them to help groups of people understand each other.
This comparison suggests that they were indispensable in facilitating communication and cooperation between the American explorers and various Native American tribes. This invaluable assistance led to the success of the expedition, as they were able to establish relationships with many Native American tribes and collect important information about the region's geography, flora, and fauna.