Same-name first cousin marriages 142
-------------------------------- = --- = .57
All same-name marriages 249

He is inclined to think that the ratio should be lower and perhaps .50
instead of .57. By a similar line of reasoning he obtains this
proportion:

Same-name first cousin marriages 1
------------------------------------- = ---
Different-name first cousin marriages 3

Here too, he fears that the denominator is too small, for by
theoretical calculation he obtains by one method the ratio 2/7, and
by another 1/1. He finally takes 1/4 for this factor. To express the
proportion in another form:

Same-name first cousin marriages 1
-------------------------------- = ---
All first cousin marriages 5

The completed formula then becomes:

All same-name marriages 100 1
-------------------------- = ----- X --- = .35 (nearly)
All first cousin marriages 57 5

Applying this formula to the English statistics, Mr. Darwin computes
the percentages of first cousin marriages in England with the