Darwin is describing the inhabitants of Tiera del Fuego, and their food:
"Whenever it is low water, winter or summer, night or day they must rise to pick shell-fish from the rocks; and the women either dive to collect sea-eggs, or sit patiently in their canoes, and with a baited hair-line without any hook, jerk out little fish."
What can "sea-eggs" be? Being underwater, they can't be birds' eggs.