Why is a batsman who make no runs at cricket said to be out 'for a duck'?

When a batsman has scored no runs, a zero is written in the scorebook against his/her name. This zero resembles the look of a duck's egg hence the term "out for a duck". When a batsman is out for a duck in both innings of a match, the batsman is said to be "out for a pair". This is not because he has a pair of ducks but because the two zeroes side by side look like a pair of spectacles hence "out for a pair".