parents will be born blind is ten times as great (632 per million vs.
63 per million) as when the parents are not related, the numbers are
so small that there seems to be very little basis for a belief that
consanguinity does more than to intensify an inherited tendency,
especially since over one half of the congenitally blind of
consanguineous parentage are known to have blind relatives.

[Footnote 80: From 1-1/2 to 2 per cent of all marriages were found to
be between cousins within the degree of second cousins, and cousin
marriages were found to be normally fertile.]

2. The Deaf. The extent to which the connection between consanguineous
marriage and deaf-mutism has been studied is indicated by a table
given by Mr. Huth, in which are set forth the results of fifty
distinct investigations.[81] In this table the percentages of
deaf-mute offspring of consanguineous marriage to the total number of
deaf-mutes investigated, varies from 30 per cent to none at all. Of
these studies not more than ten or eleven have the slightest
statistical value, and four of these--the most reliable--are from the
reports of the Census of Ireland in the years 1851, 1861, 1871 and
1881.

[Footnote 81: _Marriage of Near Kin_, p. 229.]

The Irish censuses of 1891 and 1901 give similar data, though not so
detailed as in 1871 and 1881. Thus we have in these reports a census
inquiry into a phase of the consanguineous marriage problem extending
over the period of six successive censal years. Although we can hardly
suppose that these figures are accurate in all respects, they throw a
great deal of light upon the problem, and are worth quoting in some
detail. The tables as given by Mr. Huth contain a number of errors of
detail, the correction of which changes the results materially.[82]

[Footnote 82: In a subsequent article Mr. Huth corrects some of these
errors. See: "Consanguineous Marriage and Deaf-mutism," _The Lancet_,
Feb. 10, 1900.]

TABLE XXIV.
------------------------------------------------------------------
| | Congenital deaf-mutes
| |-----------------------------------------------
| | | | | Parents cousins
| | | | |----------------------
| | | |Average | | |Average
| | |Number |number | | |number
Censal | Total | |per