The most important source for this chapter is the special report on the Blind and the Deaf in the Twelfth Census of the United States.[[75]] This report was prepared under the direction of Dr. Alexander Graham Bell, as Expert Special Agent of the Census Office.
The enumerators of the Twelfth Census reported a total of 101,123 persons as blind, and to each of these Dr. Bell addressed a circular of inquiry. By this method he obtained verified returns of 64,763 cases of blindness in continental United States or 85.2 per 100,000 of the total population. In the same way he obtained data in regard to 89,287 persons with seriously impaired powers of hearing, or 117.5 Per 100,000 of the total population.
In each case the following questions among others were asked: "Were his (or her) parents first cousins? If not first cousins were they otherwise related by blood to each other, before their marriage? Were any of his relatives blind? If yes, what relatives? (Father, mother, grandparents, brothers, sisters, uncles, aunts, and how many of each, so far as known)." The results of this inquiry give us the best and most reliable statistical material which has ever been compiled on any phase of the problem of consanguineous marriage. The investigation of the deaf was similar to that of the blind, but even more complete.
I. The Blind. The question as to the relationship of the parents was answered in 56,507 cases, in 2,527 or 4.47 per cent of which the parents were reported as cousins. Of the 57,726 who answered the question in regard to blind relatives, 10,967 or 19 per cent replied in the affirmative.[[76]] The blind relatives were divided into two groups: (a) blind brothers, sisters or ancestors, and (b) blind collateral relatives or descendants. Table XXII concisely expresses the results most fundamental for this study.
| TABLE XXII. | |||||
|---|---|---|---|---|---|
| Consanguinity of Parents. | Totals | Having blind relatives Class (a).[A] | Having blind relatives Class (b).[A] | Having no blind relatives. | Not Stated. |
| The blind | 64,763 | 8,629 | 2,338 | 46,759 | 7,037 |
| Totally blind | 35,645 | 4,378 | 1,215 | 26,349 | 3,703 |
| Partially blind | 29,118 | 4,251 | 1,123 | 20,410 | 3,334 |
| Parents cousins.--The blind | 2,527 | 844 | 149 | 1,456 | 78 |
| Parents cousins.--Totally blind | 1,291 | 435 | 78 | 739 | 39 |
| Parents cousins.--Partially blind | 1,236 | 409 | 71 | 717 | 39 |
| Parents not cousins.--The blind | 53,980 | 7,395 | 2,095 | 43,368 | 1,122 |
| Parents not cousins.--Totally blind | 29,892 | 3,720 | 1,090 | 24,541 | 541 |
| Parents not cousins.--Partially blind | 24,088 | 3,675 | 1,005 | 18,827 | 581 |
| Consanguinity not stated.--The blind | 8,256 | 390 | 94 | 1,935 | 5,837 |
| Consanguinity not stated.--Totally blind | 4,462 | 223 | 47 | 1,069 | 3,123 |
| Consanguinity not stated.--Partially blind | 3,794 | 167 | 47 | 866 | 2,714 |
| [A] Symbols for Blind Relatives--(a) blind brothers, sisters or ancestors; (b) blind collateral relatives or descendants. | |||||
Of the 2527 blind persons whose parents were cousins, 993 or 39.3 per cent have blind relatives, 33.4 per cent having blind brothers, sisters or ancestors, and 3.9 per cent having blind collateral relatives or descendants. And 9 per cent of the blind who have blind relatives are of consanguineous parentage, while but 3.1 per cent of the blind who have no blind relatives are the offspring of cousins. These figures alone indicate a decided intensification of blindness through consanguinity, although it should be remembered that a relationship "works both ways," so that when a brother has a blind sister, the sister would have a blind brother. This fact has probably diminished the apparent number of sporadic cases of blindness.
Considered with reference to the degree of blindness the table shows that 1291 or 51.1 per cent of the blind of consanguineous parentage are totally blind, and 1236 or 48.9 per cent are partially blind. Among those whose parents were not cousins, 55.4 per cent were totally and 44.6 per cent were partially blind.
Of the 2527 blind of consanguineous parentage, 632 or 25.0 per cent were congenitally blind, of whom 350 or 55.4 per cent also had blind relatives of the degrees specified. Not counting those who did not answer the question in regard to blind relatives, we have 615 cases of which 51.5 per cent had blind relatives of class (a), and 5.4 per cent blind relatives of class (b). Taking the 53,980 blind whose parents were not so related the number of congenitally blind was 3666 or but 6.8 per cent, of whom 1023 or 27.9 per cent had blind relatives. Omitting as before the "blind relatives not stated," we have 23.4 per cent who had blind relatives of class (a), and 4.3 per cent relatives of class (b).
On the hypothesis that consanguinity in the parents intensifies a tendency toward blindness we should expect to find among the congenitally blind a larger proportion of consanguineous parentage than among those blind from specific causes. In Table XXIII a general classification of the causes of blindness is given together with the consanguinity of parents. Specific causes in which the percentage of consanguinity differs in a marked degree from the average, are given parenthetically.
| TABLE XXIII. | |||||||
|---|---|---|---|---|---|---|---|
| Cause of Blindness. | Total. | Consanguinity of Parents | Percentages | ||||
| Cousins | Not Cousins | Not stated | Cousins | Not Cousins | Not stated | ||
| Total | 64,763 | 2,527 | 53,980 | 8,256 | 3.9 | 83.4 | 12.7 |
| Opacity of the eye | 33,930 | 1,000 | 28,797 | 4,133 | 2.9 | 84.9 | 12.2 |
| a. Causes affecting cornea | 11,380 | 444 | 10,016 | 920 | 3.9 | 88.0 | 8.1 |
| (1) Measles | 1,451 | 73 | 1,267 | 111 | 5.0 | 87.4 | 7.6 |
| (2) Scrofula | 1,165 | 71 | 1,026 | 68 | 6.1 | 88.1 | 5.8 |
| b. Causes affecting iris | 1,307 | 33 | 1,093 | 181 | 2.5 | 83.6 | 13.9 |
| c. Causes affecting lens | 11,769 | 228 | 9,467 | 2,074 | 1.9 | 80.4 | 17.7 |
| d. Other causes | 9,474 | 235 | 8,221 | 1,018 | 2.5 | 86.8 | 10.7 |
| Nervous apparatus affected | 7,944 | 276 | 6,980 | 688 | 3.5 | 87.8 | 8.7 |
| Unclassified | 14,885 | 938 | 12,463 | 1,484 | 6.3 | 83.7 | 10.0 |
| (1) Congenital | 4,728 | 632 | 3,666 | 430 | 13.4 | 77.5 | 9.1 |
| (2) Other causes | 10,157 | 306 | 8,797 | 1,054 | 3.0 | 86.6 | 10.4 |
| Unknown | 8,004 | 313 | 5,740 | 1,951 | 3.9 | 71.7 | 24.4 |