It will be noted that the greatest proportion is in the case of scrofula.

Since it is probable that a part of those who did answer the question as to consanguinity are in fact the offspring of cousins, the percentage in each case should be somewhat increased.  Allowing for these the same proportion as for those who did answer the question we should have of all the blind 4.47 per cent as the offspring of cousins; of the totally blind 4.14 per cent and of the partially blind 4.88.  While of the congenitally blind we should have 14.7 per cent as offspring of cousins.

It is interesting to note in this connection that in 1900, Dr. Lee Wallace Dean, of the University of Iowa examined the 181 blind children in the Iowa College for the Blind, and found that 9 or nearly 5 per cent were the offspring of first cousin marriages.[78] Dr. Dean continues,

If we exclude from the list those blind children who were blind because of blennorrhea neonatorum, sympathetic opthalmia, trachoma, etc., and consider only those who suffered because of congenital conditions, we should find that 14 per cent were the result of consanguineous marriage of the first degree....  Among the pupils who have entered the college since 1900 the percentage is about the same.

[Footnote 78:  Effect of Consanguinity upon the Organs of Special Sense, p. 4.]

This was written in 1903, three years before the publication of Dr. Bell’s report.

Statistics from foreign sources give even larger percentages of the blind as the offspring of consanguineous marriage.  Dr. Feer quotes fourteen distinct investigations of the etiology of retinitis pigmentosa, embodying in all 621 cases, of which 167 or 27 per cent were the offspring of consanguineous parents.[79] Retinitis pigmentosa is perhaps more generally attributed to consanguineous marriage than any other specific disease of the eye, and it is to be regretted that the Census report does not give any data in regard to this cause.  Retinitis pigmentosa in known to be strongly inheritable, as is albinism and congenital cataract.

[Footnote 79:  Der Einfluss der Blutsverwandschaft der Eltern auf die Kinder, p. 14.]

Looking now at the other side of the problem, that of the probability of consanguineous marriages producing blind offspring, we have as our data the 2527 blind whose parents were cousins, and a conservative estimate which may be made from the data in Chapter II that 1,000,000 persons in continental United States are the offspring of cousins within the degrees included in the Census report.[80] In the general population 852 per million are reported as blind, and 63 per million as congenitally blind.  The actual figures for the offspring of cousin marriages are 2527 per million for all blind and 632 per million for the congenitally so.  In other words only 0.25 per cent of the offspring of cousin marriages are blind and only 0.05