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.

Sea urchins

http://www.barbadosmarinetrust.com/sea_eggs.htm

Dear Faldage: I couldn't make your link work.And it wouldn't let me copy it. But I found another link, that says yes, they are edible, but also mentions their having very painful prickers. How do you suppose the Fuegian women could collect them underwater in rough surf, withoug getting their hands badly hurt?
http://search.yahoo.com/bin/search?p=sea+urchins+edible&ei=UTF-8
Dear Faldage: they must be a lot better eating than I would have imagined, for it to be necessary to have a law to protect them. On the second try, the URL worked.
And apparently they are not as hard to handle as I thought
from previous reading about them.