This is rather higher than the average for urban districts, according
to official figures, but Dally seems to consider it as typical. He
gives examples of the carelessness and incompetency of the rural
record keepers, and insists that the percentage is really much higher
than the official figures would indicate. He estimates the
consanguineous marriages in France not including second cousins, at
from four to five per cent.

A very ingenious method of determining the approximate number of
first-cousin marriages was devised by Mr. George H. Darwin.[24]
Noticing that in marriage announcements, some were between persons of
the same surname, it occurred to him that there might be a constant
ratio between same-name marriages and first cousin marriages. Some
same-name marriages would of course be purely adventitious; so, to
eliminate this element of chance, he obtained from the Registrar
General's Report the frequency of occurrence of the various surnames
in England. The fifty commonest names embraced 18 per cent of the
population. One person in every 73 was a Smith, one in every 76 a
Jones and so on. Then the probability of a Smith-Smith marriage due to
mere chance would be 1/73^2 and of a Jones-Jones marriage 1/76^2.
The sum of fifty such fractions he found to be .0009207 or .9207 per
thousand. After the fiftieth name the fractions were so small as to
have comparatively little effect upon the total. He therefore
concluded that about one marriage in a thousand takes place, in which
the parties have the same surname and have been uninfluenced by any
relationship between them bringing them together.

[Footnote 24: "Marriages between First Cousins in England and their
Effects," in _Journal of the Statistical Society_, June, 1875. pp. 154