Consanguineous Marriages in the American Population
by
George B. Louis Arner
Part 2 out of 2
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.
-------------------------------------------------------------------------------
| |Consanguinity of |
| | Parents | Percentages
| |---------------------------------------------
| | | Not | Not | | Not | Not
Cause of Blindness. |Total.|Cousins|cousins|stated|Cousins|cousins|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
-------------------------------------------------------------------------------
To quote from the Report:
The only specific causes, other than congenital, to which is
due a greater proportion of the total cases of blindness among
those whose parents were cousins than among those whose parents
were not related, are: Catarrh (parents cousins 28.1, parents
not cousins 8.7 per 1,000), scarlet fever (parents cousins
10.7, parents not cousins 10.1 per 1,000), scrofula (parents
cousins 28.9, parents not cousins 19 per 1,000), and measles
(parents cousins 28.9, parents not cousins 23.5 per 1,000). The
difference in these proportions is but slight, and the relative
number of cases of blindness attributed to each of the other
causes is greater among those whose parents were not
related.[77]
[Footnote 77: U.S. Census, 1900, op. cit., p. 17.]
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 per cent are
congenitally blind. Although the probability that a child of related
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 | to a | |Per | to a
year. |population|Number|million|family[A]|Number|cent.|family[A]
------------------------------------------------------------------
1851[B]| 6,574,278| 4,127| 628 | ---- | 242 | 5.86| 1.66
1861 | 5,798,967| 4,096| 706 | 1.22 | 362 | 8.84| 1.72
1871 | 5,412,377| 3,503| 647 | 1.30 | 287 | 7.35| 1.76
1881 | 5,174,836| 3,163| 611 | 1.32 | 191 | 6.04| 1.69
1891 | 4,706,448| 2,570| 546 | 1.40 | 297 |11.56| 1.92
1901 | 4,456,546| 2,179| 489 | 1.40 | 249 |11.43| 1.73
------------------------------------------------------------------
[A] From Table XXV.
[B] 1851 data from Huth, "Consanguineous Marriage and
Deaf-mutism." _The Lancet_, 1900.
Table XXIV summarizes the most important points in the Irish data. It
will be seen that while there has been an absolute diminution in the
number of deaf-mutes in Ireland with the decrease in population, there
has been a relative increase of deaf-mutism. There are two possible
explanations for this phenomenon, both of which may have operated in
part; first that in the great emigration the deaf-mutes have been left
behind, and second that with the introduction of improved methods of
census taking, the returns are more complete than a half century ago.
Mr. Huth believes that there is still room for improvement in Irish
census methods, and thinks there is reason to believe that in the
enumeration of the deaf all children born deaf in a family are
included whether living or not.
Since Ireland is strongly Roman Catholic, the proportion of
consanguineous marriages is probably small, so that the percentage of
deafmutes derived from consanguineous marriages, varying from 5.86 to
11.56 is very much greater than the percentage of these marriages in
the general population. The average number of deaf children to a
family in Table XXIV varies less than any other part of the table, and
clearly shows a much higher average number of deaf children where the
parents were cousins. They reveal the interesting fact that the
occurrence of two or more deafmutes in a family is more than twice as
probable where the parents are related as where they are not. Table
XXV still better illustrates this point. Of the families where there
was but one deaf-mute, only 4.3 per cent were the offspring of cousin
marriages; where there were two in a family 12.9 per cent were of
consanguineous parentage; three in a family, 13.3 per cent; four in a
family, 19.0 per cent; more than four in a family, 21.1 per cent.
TABLE XXV.
_Number of Congenital Deaf mutes to a Family in Ireland._
----------------------------------------------------------------
| | Families in which deaf-mutes numbered.
| |----------------------------------------
Year.| Parentage. | 1. | 2. | 3. |4.|5.|6.|7.|8.|9.|10.|11.
----------------------------------------------------------------
1851 | Parents cousins | 127| 45 | 20 |10| 5| 2|..| 1|..|.. |..
1871 | Parents cousins | 91| 38 | 24 | 5| 3| 1| 1|..|..|.. |..
1881 | Parents cousins | 63| 30 | 13 | 6| 1|..|..|..|..|.. |..
1891 | Parents cousins | 82| 38 | 19 | 9| 1| 3| 1| 2|..|.. |..
1901 | Parents cousins | 79| 34 | 23 | 7| 1|..| 1|..|..|.. |..
1851 | All families[A] |2963|347 |158 |35|13| 5|..| 1|..|.. |..
1871 | All families[A] |2460|305 |167 |47|20| 5| 1|..|..|.. |..
1881 | All families[A] |2080|281 |162 |39|18| 6|..|..|..| 1 |..
1891 | All families[A] |1473|273 |134 |40|12| 6| 1| 2|..|.. | 1
1901 | All families[A] |1219|231 |122 |34|10| 4| 2|..|..|.. |..
----------------------------------------------------------------
[A] Number of the "Deaf and Dumb" to a family, "as far as
could be ascertained."
In 1871 and 1881 the inquiry was more minute and the degrees of
consanguinity were specified. Mr. Huth quotes some of the figures for
these years, probably derived from the same sources as Table XXVI, and
comments as follows: "An examination of this table will show that the
statistics so much relied upon as proving the causation of deaf-mutism
by consanguineous marriages show nothing of the sort. In 1871 fourth
cousins produced more deaf-mutes per marriage than any nearer
relationship. In 1881 third cousins produced more than any nearer
relationship."[83] Mr. Huth forgets that he is basing these statements
on five and nine families respectively, and does not take into
consideration the probability that if the returns are biased, as he
suspects, this bias would affect the more distantly related,
relatively more than the first cousin marriages, for the same reason
that this would be true of the cases collected by Dr. Bemiss.[84]
Combining the figures of the two censal years helps to correct these
averages, and the distantly related show approximately the same
average as the first cousin marriages in spite of the vastly greater
selection which must have obtained in the distantly related cases.
[Footnote 83: Huth, _Marriage of Near Kin_, p. 227.]
[Footnote 84: _Cf. supra_, p. 42.]
In Table XXVI it will be seen that 52.5 per cent of the deaf-mute
offspring of consanguineous parents were the offspring of first cousin
marriages. On the assumption that this percentage is fairly typical of
each set of returns we may say that from three to six per cent of the
Irish deaf-mutes are the offspring of first cousin marriages. If,
then, the proportion of first cousin marriages is no greater than in
England, the percentage of deaf-mute offspring is several times as
great as in the average non-related marriage.
TABLE XXVI.
--------------------------------------------------------------------------
| 1871 | 1881 | 1871 and 1881
|-----------------------------------------------------------
| |Number| | |Number| | |Number|
| | of |Aver | | of |Aver | | of |Aver
|Number|conge-|age |Number|conge-|age |Number|conge-|age
Consanguinity | of |nital |per | of |nital |per | of |nital |per
of |mar- |deaf- |mar- |mar- |deaf- |mar- |mar- |deaf- |mar-
Parents. |riages|mutes |riage|riages|mutes |riage|riages|mutes |riage
--------------------------------------------------------------------------
First cousins | 72 | 128 | 1.78| 74 | 123 | 1.66| 146 | 251 | 1.72
Second cousins| 50 | 89 | 1.78| 29 | 46 | 1.58| 79 | 135 | 1.71
Third cousins | 24 | 40 | 1.67| 9 | 21 | 2.33| 33 | 61 | 1.85
Fourth cousins| 5 | 11 | 2.20| 1 | 1 | -- | 6 | 12 | 2.00
Fifth and | | | | | | | | |
sixth cousins | 12 | 19 | 1.58| not stated | 12 | 19 | 1.58
--------------------------------------------------------------------------
Total | 163 | 287 | 1.76| 113 | 191 | 1.69| 276 | 478 | 1.73
No | | | | | | | | |
relation- | | | | | | | | |
ship[A] |2,842 |3,609 | 1.27|2,474 |3,229 | 1.31|5,316 |6,838 | 1.29
--------------------------------------------------------------------------
Grand total |3,005 |3,896 | 1.30|2,587 |3,420 | 1.32|5,592 |7,316 | 1.31
--------------------------------------------------------------------------
[A] See Table XXV.
In Scotland Dr. Arthur Mitchell made inquiry of the superintendents of
a number of deaf-mute asylums, and found that of 544 deaf-mutes, 28
were the offspring of 24 consanguineous marriages.[85] There were 504
families represented in all, so that the average per family was 1.17
among the consanguineous to 1.07 among the non-consanguineous.
[Footnote 85: Huth, op. cit., p. 226.]
In Norway, according to Uchermann, while 6.9 per cent of all marriages
are consanguineous within and including the degree of second cousins,
and in single cantons the percentages range as high as 31.0, only in
one single district does the number of the deaf-mutes harmonize with
that of the marriage of cousins. The district of Saeterdalen has the
greatest number of consanguineous marriages (201 out of 1250), but not
a single case of deaf-mutism. Hedemarken, which has the fewest
consanguineous marriages has a great many deaf-mutes. Where
deaf-mutism exists it seems to be intensified by consanguinity, but
where it is not hereditary it is not caused by consanguinity. Of the
1841 deaf-mutes in Norway, 919 were congenitally deaf, and of these
212 or 23 per cent were of consanguineous parentage.[86]
[Footnote 86: _Les Sourds-muets en Norvege_. Quoted by Feer, _Der
Einfluss der Blutsverwandschaft der Eltern auf die Kinder_, p. 22.]
Dr. Feer gives a table containing the results of a number of studies
of deaf-mutism, which shows an average of 20 per cent as of
consanguineous origin. Four investigations give the number of children
to a family. Table XXVII from Feer seems to indicate that the Irish
census is fairly accurate at this point.[87]
[Footnote 87: Feer, op. cit., p. 22.]
TABLE XXVII.
_Average Number of Children to a Family._
-------------------------------------------------
|Consanguineous|"Crossed"
Observer. | marriages. |marriages.
-------------------------------------------------
Huth (Irish Census) | 1.68 | 1.17
Wilhelmi | 1.71 | 1.26
Mygind | 1.53 | 1.20
Uchermann | 1.41 | 1.19
-------------------------------------------------
In the American Census the instructions to enumerators have been so
diverse that statistics of the deaf have been very poor until recent
years. Not until the Twelfth Census was the inquiry put upon a really
scientific basis.
This reform, as also the more intelligent attitude of the American
people in general towards the affliction of deafness, is due largely
to the work of Dr. Alexander Graham Bell. An enumeration of Dr. Bell's
services directly, and through the agency of the Volta Bureau, in this
cause, cannot be given here. For our purpose the most important of his
contributions is embodied in the Special Report of the Twelfth Census
of the United States already referred to.
As in the investigation of the Blind, the circular letter sent to each
person reported by the enumerators as deaf contained questions in
regard to parentage and the existence of deaf relatives. It is
unfortunate that in these returns it is impossible to distinguish
between degrees of relationship, but in such an extensive compilation
it was doubtless impracticable to attempt to unravel the intricacies
of consanguinity. Judging from the returns of the Census of Ireland we
may assume that about half of the cases returned as "cousins" were
first cousins.
The replies to the inquiry as to deaf relatives were more carefully
analyzed, and were divided into four groups, which are referred to
throughout as (a), (b), (c) and (d) relatives. These groups are: (a),
deaf brothers, sisters or ancestors; (b), deaf uncles, aunts, cousins
or other relatives not (a), (c) or (d); (c), deaf children, (sons or
daughters); (d), deaf husbands or wives. Thus a large proportion of
the hereditary cases would be included in the first two categories,
(a) and (b).[88]
[Footnote 88: U.S. Census _Report on the Blind and the Deaf_, p. 127.]
The causes of deafness are given in detail, but as might be expected
the returns are not as definite or as accurate as we should desire.
The causes given have been grouped under five main heads; these again
are subdivided, often into divisions numerically too minute for real
statistical value. Table XXVIII includes the main groups and those
specific causes which number more than 3000 cases. The extreme
variation in the percentages of those who are the offspring of
consanguineous marriages cannot be attributed to mere chance. There is
clearly some fundamental connection between consanguinity and
congenital deafness if 11.8 per cent of all the congenitally deaf are
the offspring of consanguineous marriages, while of the adventitiously
deaf but 3.1 per cent are the offspring of such marriages. In fact we
are tempted to jump at the conclusion that consanguinity is in itself
a cause of deaf-mutism. Furthermore 42.1 per cent of the deaf whose
parents were cousins were congenitally deaf, while this was true of
but 15 per cent of those whose parents were unrelated.
TABLE XXVIII.
-------------------------------------------------------------------------
| | |
| | Consanguinty |
| | of Parents. | Per cent.
| |--------------------|-----------------
| | | | | | |
Cause of Deafness. | | | Not | Not | | Not | Not
|Total.|Cous- |Cous- | Sta- |Cous-|Cous-| Sta-
| | ins. | ins. | ted. | ins.| ins.| ted.
---------------------------|------|------|------|------|-----|-----|-----
| | | | | | |
Total |89,287| 4,065|75,530| 9,692| 4.5| 84.6| 10.9
---------------------------|------|------|------|------|-----|-----|-----
Affections of external ear | 871| 29| 760| 82| 3.3| 87.3| 9.4
Affections of middle ear |34,801| 1,238|30,824| 2,739| 3.5| 88.6| 7.9
Affections of internal ear |12,295| 343|11,121| 831| 2.8| 90.4| 6.8
Unclassified |31,205| 2,183|25,281| 3,741| 7.0| 81.0| 12.0
Unknown |10,115| 272| 7,544| 2,299| 2.7| 74.6| 22.7
| | | | | | |
Scarlet fever | 7,424| 285| 6,647| 492| 3.9| 89.5| 6.6
Disease of ear | 4,210| 222| 3,683| 305| 5.3| 87.5| 7.2
Catarrh |11,702| 304|10,450| 948| 2.6| 89.3| 8.1
Colds | 3,074| 81| 2,666| 327| 2.6| 86.7| 10.7
Meningitis | 3,991| 83| 3,741| 167| 2.1| 93.7| 4.2
Old age | 3,361| 38| 2,369| 954| 1.1| 70.5| 28.4
Military service | 3,242| 40| 2,897| 305| 1.2| 89.4| 9.4
Congenital |14,472| 1,710|11,322| 1,440| 11.8| 78.2| 10.0
-------------------------------------------------------------------------
But on the other hand, 53.4 per cent of the deaf whose parents were
cousins had deaf relatives of the (a) and (b) groups, while of those
whose parents were not cousins, only 29.9 per cent in these groups had
deaf relatives. In Table XXIX the close connection between deaf
relatives of these groups and consanguinity is shown. For the sake of
simplicity no account is taken of (c) relatives (deaf children), and
(d) relatives (deaf husbands or wives), for in the first case only 370
deaf are reported as having deaf children and at the same time no (a)
or (b) relatives, and in the Second case (d) relatives are not
ordinarily blood relatives at all.
TABLE XXIX.
-----------------------------------------------------------------------
| | |
| | Consanguinty |
| | of Parents. | Per cent.
| |--------------------|-----------------
| | | | | | |
Class of Deaf | | | Not | Not | | Not | Not
Relative.[A] |Total.| Cous-| Cous-| Sta- |Cous-|Cous-| Sta-
| | ins.| ins.| ted. | ins.| ins.| ted.
-----------------------------------------------------------------------
| | | | | | |
Total |89,287| 4,065|75,530| 9,692| 4.5| 84.6| 10.6
-----------------------------------------------------------------------
Stated |80,481| 3,911|73,639| 2,931| 4.9| 91.5| 3.6
Not stated | 8,806| 154| 1,891| 6,761| 1.7| 21.5| 76.8
| | | | | | |
(a) relatives |21,660| 1,850|18,838| 972| 8.5| 87.0| 4.5
No (a) relatives |58,821| 2,061|54,801| 1,959| 3.5| 93.2| 3.3
| | | | | | |
(a) or (b) relatives |25,851| 2,171|22,552| 1,128| 8.4| 87.2| 4.4
(a) and (b) relatives | 4,117| 412| 3,587| 118| 10.0| 87.1| 2.9
(a) but no (b) relatives |17,543| 1,438|15,251| 854| 8.2| 86.9| 4.2
(b) but no (a) relatives | 4,191| 321| 3,714| 156| 7.7| 88.6| 3.7
No (a) or (b) relatives |54,630| 1,740|51,087| 1,803| 3.2| 93.5| 3.3
-----------------------------------------------------------------------
[A] Symbols for deaf relatives: (a) deaf brothers, sisters and
ancestors; (b) deaf uncles, aunts, cousins, etc.
Table XXIX shows unmistakably that the connection between
consanguinity and hereditary deafness is very close. Where there is
the largest amount of deafness in the family the percentage of
consanguinity is the highest. That is, of those who had both (a) and
(b) relatives ten per cent were the offspring of cousins, while of
those who had neither (a) nor (b) relatives only three per cent were
the offspring of cousins. It is natural to assume that as a rule where
the deaf have either (a) or (b) deaf relatives, deafness is
hereditary, for the probability of two cases of deafness occurring in
the same family, uninfluenced by heredity would be very small. It is
likely also that a great many of the deaf who stated that they had no
deaf relatives were mistaken, for few people are well enough informed
in regard to their ancestry to answer this question definitely. Not
one man in thousands can even name all of his great-grandparents, to
say nothing of describing their physical or mental traits. Others may
have understood the inquiry to refer only to living relatives and
therefore have omitted almost all reference to their ancestors. These
possible errors might easily explain all the excess of the percentage
of consanguinity among those reported as having no deaf relatives over
the probable percentage of consanguineous marriage in the general
population. But this very probability that comparatively few deaf
ancestors have been reported increases the probability that the
greater part of the (a) relatives were brothers and sisters rather
than ancestors. Now of the 26,221 deaf having deaf relatives, 17,345
have only (a) relatives, and if these are largely living brothers and
sisters the relationship would "work both ways," so that if there were
two deaf children in a family, each would have an (a) deaf relative.
In the Census of Ireland figures above quoted it will be remembered
that among families which were the offspring of cousins the proportion
having two or more deaf children was three times as great as among
those who were not the offspring of consanguineous unions. If this
follows in America, it largely accounts for the high percentage of the
congenitally deaf who are the offspring of cousin marriages, and
especially of those who have (a) deaf relatives.
TABLE XXX.
------------------------------------------------------------------------
| | Consanguinity |
| | of Parents. | Per Cent.
| |----------------------------------------------
Class of Deaf | | | Not | Not | | Not | Not
Relatives.[A] |Total. |Cousins|Cousins|stated |Cousins|Cousins|stated
------------------------------------------------------------------------
Total |14,472 | 1,710 | 11,322| 1,440 | 11.8 | 78.2 |10.0
------------------------------------------------------------------------
Stated |13,428 | 1,647 | 11,110| 671 | 12.3 | 82.7 | 5.0
Not stated | 1,044 | 63 | 212| 769 | 6.0 | 20.3 |76.7
(a) relatives | 5,295 | 986 | 3,961| 48 | 18.6 | 74.8 | 6.6
(b) and (c) but | | | | | | |
no (a) relatives| 860 | 126 | 686| 48 | 14.6 | 79.8 | 5.6
No (a), (b) or | | | | | | |
(c) relatives | 7,273 | 535 | 6,463| 275 | 7.3 | 88.9 | 3.8
------------------------------------------------------------------------
[A] Symbols for deaf relatives: (a) deaf brothers, sisters or
ancestors; (b) deaf uncles, aunts, cousins, etc.; (c) deaf children.
A further analysis of the congenitally deaf according to consanguinity
of parents and deaf relatives, as in Table XXX, helps to determine to
what extent the greater number of deaf children to a family among the
offspring of consanguineous marriages has influenced the totals. From
the report it cannot be determined how many of the congenitally deaf
had (a), (b) or (c) relatives alone, but the existence of (b) and (c)
relatives would almost certainly indicate that the deafness was
hereditary. Of these 14.6 per cent were the offspring of cousins,
while of those having (a) relatives 18.6 per cent were the offspring
of consanguineous unions. Thus it would seem to be a more reasonable
conclusion that where two or more deaf-mutes appear in the same
family, at least a tendency toward deaf-mutism is hereditary in the
family and is intensified by the marriage of cousins, rather than that
consanguineous marriage is in itself a cause. The fact that in many
cases the relationship would "work both ways" would not greatly affect
the percentage of the offspring of cousins having (b) and (c)
relatives, for the chance would be slight that the (b) or (c) relative
would be himself the offspring of a consanguineous marriage. Among the
congenitally deaf who reported no deaf relatives, the percentage of
consanguineous parentage is still high, (7.3 per cent), but this
excess can easily be accounted for by the ignorance of deaf relatives
on the part of the informant, without contradicting the hypothesis of
heredity.
Basing now our percentages on the totals of consanguineous and
non-consanguineous parentage respectively, and including only those
who answered the inquiry as to deaf relatives, it will be seen (Table
XXXI) that while of all the deaf less than one third are returned as
having deaf relatives, of the deaf who were the offspring of cousins
over one half (55.5 per cent) were returned as having (a) or (b) deaf
relatives.
Again taking into consideration only the congenitally deaf the results
are still more striking. Table XXXII shows that 66.5 per cent of the
congenitally deaf who are of consanguineous parentage are known to
have deaf relatives.
TABLE XXXI.
----------------------------------------------------------------------
| |Consanguinity |
| | of Parents. | Per cent.
| |-------------------------------------
| | |Not | | |Not
Class of Deaf Relatives. |Total.|Cousins|Cousins|Total|Cousins|Cousins
----------------------------------------------------------------------
Deaf relatives stated |80,481| 3,911 |73,639 |100.0|100.00 |100.00
----------------------------------------------------------------------
(a) relatives |21,660| 1,850 |18,838 | 26.9| 47.3 | 25.5
No (a) relatives |58,821| 2,061 |54,801 | 73.1| 52.7 | 74.5
| | | | | |
(a) or (b) relatives |25,851| 2,171 |22,552 | 32.1| 55.5 | 30.6
(a) and (b) relatives | 4,117| 412 | 3,587 | 5.1| 10.5 | 4.8
(a) and no (b) relatives |17,543| 1,438 |15,251 | 21.8| 36.8 | 20.7
(b) and no (a) relatives | 4,191| 321 | 3,714 | 5.2| 8.2 | 5.1
No (a) or (b) relatives |54,630| 1,740 |51,087 | 67.9| 44.5 | 69.4
----------------------------------------------------------------------
Symbols for deaf relatives: (a) deaf brothers, sisters or ancestors;
(b) deaf uncles, aunts, cousins, etc.; (c) deaf children;
(d) deaf husbands or wives.
TABLE XXXII.
-------------------------------------------------------------------------
| |Consanguinity |
| |of Parents. | Per cent.
| |-------------------------------------
| | |Not | | |Not
Class of Deaf Relatives |Total.|Cousins|Cousins|Total|Cousins|Cousins
-------------------------------------------------------------------------
Deaf relatives stated |13,428| 1,647 |11,110 |100.0|100.0 |100.0
-------------------------------------------------------------------------
(a) relatives | 5,295| 986 | 3,961 | 39.5| 59.9 | 35.6
(b) or (c), no (a) relatives| 860| 126 | 686 | 6.4| 7.6 | 6.2
No (a), (b) or (c) relatives| 7,273| 535 | 6,463 | 54.2| 32.5 | 58.2
-------------------------------------------------------------------------
Symbols for deaf relatives: (a) deaf brothers, sisters or ancestors;
(b) deaf uncles, aunts, cousins, etc.; (c) deaf children.
The percentage having (a) relatives, including brothers, and sisters,
is nearly twice as great among the deaf of consanguineous parentage as
among the offspring of unrelated parents. This is not inconsistent
with the Irish returns which show the average number of deaf children
to a family to be so much greater where the parents were cousins, than
where they were not.
The statistics of the (c) relatives, or deaf sons and daughters of the
deaf, are not very full. Of the 31,334 married deaf who answered the
inquiry in regard to deaf relatives, 437 or 1.4 per cent reported deaf
children and 30,897 or 98.6 per cent reported no deaf children. Of the
totally deaf 2.4 per cent had deaf children, and of the congenitally
deaf 5.0 per cent. The percentage of deaf children varied greatly
according to the number and class of deaf relatives, as shown by Table
XXXIII.
TABLE XXXIII.
-------------------------------------------------------------------
| Percentage having deaf children.
|---------------------------------------
Class of Deaf Relatives. | |Totally |Partially |Congenitally
|Total.|deaf. |deaf. |deaf.
---------------------------|------|--------|----------|------------
(a), (b) or (d) | 1.4 | 2.4 | 1.1 | 5.0
(d) | 3.2 | 3.3 | 2.6 | 6.4
No (d) | 1.1 | 1.4 | 1.0 | 2.5
(a) and (d) | 6.3 | 6.7 | 4.3 | 7.8
(d), but no (a) | 2.2 | 2.2 | 2.0 | 4.9
(a), but no (d) | 1.4 | 2.3 | 1.3 | 2.6
No (a) or (d) | 0.9 | 1.0 | 0.9 | 2.3
(a), (b) and (d) | 9.5 | 9.9 | [A] | 9.0
(a), (d), but no (b) | 5.5 | 5.9 | 3.6 | 7.4
(b), (d), but no (a) | 2.5 | 2.4 | [A] | [A]
(d), but no (a) or (b) | 2.2 | 2.2 | 2.0 | 5.2
(a), (b), but no (d) | 1.9 | 3.1 | 1.7 | [A]
(a), but no (b) or (d) | 1.3 | 2.1 | 1.2 | 2.8
(b), but no (a) or (d) | 1.0 | 1.6 | 1.0 | [A]
No (a), (b) or (d) | 0.9 | 1.0 | 0.9 | 2.6
----------------------------------------------------------------
[A] Percentages not given where base is less than 100.
Symbols: (a) deaf brothers, sisters or ancestors; (b) deaf
uncles, aunts, cousins, etc.; (d) deaf husbands or wives.
The striking feature of these percentages is the regularity with
which they increase in proportion as the number of deaf relatives
increases, until among the 242 persons who have (a), (b) and (d)
relatives, 23 or 9.5 per cent also have (c) relatives. A
consanguineous marriage within a family tainted with deafness would
have the same effect as doubling the number of deaf relatives, which
as we have seen greatly increases the percentage having deaf children.
It would seem that the number of the married deaf reported as having
deaf children is much too small, especially since Dr. Fay[89] produces
statistics of 4471 marriages of the deaf of which 300 produced deaf
offspring. Counting only the 3,078 marriages of which information in
regard to offspring was available these figures show an average of a
little less than one such marriage in ten as productive of deaf
offspring. The total number of children of these marriages was 6,782,
of which 588 were deaf. These 3,078 marriages represented 5,199 deaf
married persons as compared with the 31,334 reported in the Twelfth
Census, or about one sixth. Increasing the 300 families who had deaf
children in the same ratio we have 1800 as compared with the 437
reported by the census. But as it was inevitable that Dr. Fay's cases
should be selected somewhat, he has probably collected records of more
than one sixth of all the cases where deaf children were born of deaf
parents. But we can hardly believe that he found three-fourths of such
cases. The true number therefore must be considerably greater than
437, but less than 1800.[90]
[Footnote 89: _Marriages of the Deaf in America_, chap. v.]
[Footnote 90: Of the 17 children of first cousins reported on my
circulars as either totally or partially deaf, 9 are known to have had
deaf ancestors.]
Dr. Fay found that 31 out of the 4,471 marriages of the deaf were
consanguineous, but he expresses the belief that the actual number
and percentage of consanguineous marriages of the deaf are larger. The
following table which combines several of Dr. Fay's tables sets forth
the main results of his work. In each instance one or both parties to
the marriage were deaf. The totals include only those of whom
information as to the offspring was available.
TABLE XXXIV.
----------------------------------------------------------
| | Marriages |
| | resulting |
| | in deaf |
| | offspring | Deaf children
|Number|-----------|------------------
Consanguineous |of | | | |Per
Marriages |mar- | |Per | |Number|Cent
of the Deaf. |riages|Number|Cent|Number|Deaf |Deaf
----------------------------------------------------------
First cousins | 7 | 4 | 57.| 26 | 7 | 27.
Second cousins | 5 | 3 | 60.| 25 | 10 | 40.
Third cousins | 1 | 1 | -- | 1 | 1 | --
"Cousins" | 14 | 3 | 21.| 36 | 7 | 19.
Nephew and aunt | 1 | 1 | -- | 4 | 3 | 75.
Distantly related | 3 | 2 | 67.| 8 | 2 | 25.
----------------------------------------------------------
Total consanguineous| 31 | 14 | 45.| 100 | 30 | 30.
Not consanguineous, | | | | | |
or no information |3,047 | 286 | 9.|6,682 | 558 | 8.
----------------------------------------------------------
Grand total |3,078 | 300 | 10.|6,782 | 588 | 9.
----------------------------------------------------------
Obviously percentages based on these figures are of little value of
themselves, especially since Dr. Fay's cases are not entirely typical,
but in general this table points us to the same conclusion that we
have reached by other means, namely that where a tendency toward
deafness exists, a consanguineous marriage is more likely to produce
deaf children than a non-consanguineous marriage. If more figures were
available the percentage of deaf children would probably increase with
the nearness of consanguinity and the number of deaf relatives, but
with the present data a further analysis has no significance.[91]
[Footnote 91: Mr. Edgar Schuster (_Biometrika_, vol. iv, p. 465) finds
from Dr. Fay's statistics that the average parental correlation
(parent and child) of deafness is: paternal, .54; maternal, .535.
English statistics of deafness give: paternal correlation, .515;
maternal, .535. The fraternal correlation from the American data is
.74 and from the English .70. See _infra_, p. 92.]
If, then, consanguineous marriages where relatives are deaf have a
greater probability of producing deaf offspring, and also a greater
probability of producing plural deaf offspring, than ordinary
marriages, and two thirds of the congenitally deaf offspring of
consanguineous marriages do have deaf relatives, it does not seem
necessary to look beyond the law of heredity for an explanation of the
high percentage of the congenitally deaf who are of consanguineous
parentage.
In those cases of deafness which, in the Census returns, are ascribed
to specific causes, the factor of consanguinity is still noticeable,
although the percentage of the non-congenitally deaf who are the
offspring of cousins never exceeds 5.3 (Table XXVIII). But the
influence of heredity is not removed by the elimination of the
congenitally deaf. Many instances are known where successive
generations in the same family have developed deafness in adult life,
often at about the same age and from no apparent cause. The following
case well illustrates this point. It is furnished me by a
correspondent in whom I have great confidence. The facts are these:
A---- aged 28 married B---- aged 19, his first cousin who bore the
same surname as himself. Both lived to old age and were the parents of
eight children, two of whom died in infancy. My informant further
states:
Having personally known very well all of the surviving six
children of this family, I can truthfully state that all were
unusually strong, active and vigorous people and all the
parents of healthy children. A---- was troubled with deafness
as long as I can remember, and this physical trait he
transmitted to all of his children, though some of them did not
develop the same till well along in life. C---- (the youngest
son), however, began to indicate deafness quite early. No one
of his four children is in the least deaf.
It will be noticed here that whereas in the case of the cousin
marriage the trait was so strongly inherited, it disappeared entirely
in the next generation with a non-consanguineous marriage. The
inheritance of tendencies or weaknesses may be more common than the
actual inheritance of defects. Dr. Bell's words on this point are
suggestive:
Where a tendency toward ear trouble exists in a family, it may
lie dormant and unsuspected until some serious illness attacks
some member of the family, when the weak spot is revealed and
deafness is produced. We are not all built like that wonderful
one-horse shay that was so perfectly made in all its parts that
when at last it broke down it crumbled into dust. When an
accident occurs it is the weak spot that gives way, and it
would be incorrect to attribute the damage to the accident
alone and ignore the weakness of the part; both undoubtedly are
contributing causes.
In the case, then, of a deaf person who has deaf relatives, the
assigned cause of deafness may not be the only cause involved,
or indeed the true cause at all. It may be the cause simply in
the same sense that the pulling of a trigger is the cause of
the expulsion of a bullet from a rifle, or a spark the cause of
the explosion of a gunpowder magazine; hereditary influences
may be involved.[92]
[Footnote 92: U.S. Census _Report on the Blind and the Deaf_, p. 127.]
It is thus possible to account for the large proportion of deafness
among persons of consanguineous parentage by the simple action of the
laws of heredity. Why then should we go out of our way to look for a
cause of the defect in consanguinity itself? When two explanations are
possible, the simpler explanation is the more probable, other factors
being equal; but in the present problem the factors are not equal, for
the evidence points strongly toward the simpler hypothesis of
intensified heredity, while there is little or no evidence that
consanguinity is a cause _per se_.
As to the probability then of a consanguineous marriage producing deaf
offspring, it will readily be seen to be very slight, and in those
cases where there is actually no trace of hereditary deafness in the
family, perhaps no greater than in non-related marriages. While the
census figures in regard to the deaf are not complete they probably
include a great majority of the deaf in the United States. The 89,287
deaf would mean an average of 12 deaf persons to every 10,000
inhabitants and the 14,472 congenitally deaf, 2 persons to every
10,000. Assuming then, as before[93] that 1,000,000 persons in
continental United States are the offspring of consanguineous
marriages within the limits of the term "cousins" as used in the
Census report, 41 out of every 10,000 persons of consanguineous
parentage would be deaf, and 17 congenitally so. Thus less than one
half of one per cent of the offspring of consanguineous marriages in
the United States are deaf, and only one sixth of one per cent are
deaf-mutes in the commonly accepted sense of the term.
[Footnote 93: _Supra_, p. 64.]
It is interesting here to quote an opinion given by Dr. Bell in 1891,
as to the probable results of the consanguineous marriage of deaf
persons.[94]
[Footnote 94: _Marriage--An Address to the Deaf_, second edition,
Appendix.]
1. A deaf person, not born deaf, who has no deaf relatives,
will probably not increase his liability to have deaf offspring
by marrying a blood relative.
2. A deaf person, born deaf, who has no deaf relatives, will
probably increase his liability to have deaf offspring by
marrying a blood relative.
3. A deaf person, whether born deaf or not, who has deaf
relatives, will probably increase his liability to have deaf
offspring by marrying a blood relative, especially if that
relative should happen to be on the deaf side of the family.
For example: If his father has deaf relatives and his mother
has none, he will be more likely to have deaf offspring if he
marries a relative of his father than if he marries a relative
of his mother.
The laws of heredity seem to indicate that a consanguineous
marriage increases or intensifies in the offspring whatever
peculiarities exist in the family. If a family is
characterized by the large proportion of persons who enjoy
good health and live to old age with unimpaired faculties,
then a consanguineous marriage in such a family would probably
be beneficial, by increasing and intensifying these desirable
characteristics in the offspring. On the other hand, if a
large proportion of the members of a family betray weakness of
constitution--for example: if many of the children die in
infancy, and a large proportion of the others suffer from ill
health, only a few living to old age with unimpaired
faculties--then a consanguineous marriage in such a family
would probably be hurtful to the offspring. A large
proportion of the children would probably die in infancy, and
the survivors be subject to some form of constitutional
weakness.
As there are few families entirely free from constitutional
defects of some kind, a prudent person would do well to avoid
consanguineous marriage in any case--not necessarily on account
of deafness, but on account of the danger of weakening the
constitution of the offspring. Remoteness of blood is eminently
favorable to the production of vigorous offspring, and those
deaf persons who have many deaf relatives would greatly
diminish their liability to have deaf offspring by marrying
persons very remote in blood from themselves.
Children, I think, tend to revert to the type of the common
ancestors of their parents. If the nearest common ancestors are
very far back in the line of ancestry, the children tend to
revert to the common type of the race. Deafness and other
defects would be most likely to disappear from a family by
marriage with a person of different nationality. English,
Irish, Scotch, German, Scandinavian and Russian blood seems to
mingle beneficially with the Anglo-Saxon American, apparently
producing increased vigor in the offspring.
CHAPTER VII
SUMMARY AND CONCLUSION
Having thus considered the more important problems which have been
connected with the marriage of near kin, we have only to discuss the
bearing of the conclusions thus formed upon the social aggregate, and
the effect which consanguineous marriages have upon the evolution and
improvement of the human species.
It has been shown that the frequency with which consanguineous
marriages occur varies greatly with the physical and social
environments; that such marriages are more frequent in isolated and in
rural communities than in cities; and that with the increasing range
of individual activity and acquaintance the relative frequency of
consanguineous marriage is decreasing.
Consanguinity in the parents has no perceptible influence upon the
number of children or upon their masculinity, and has little, if any,
direct effect upon the physical or mental condition of the offspring.
The most important physiological effect of consanguineous marriage is
to intensify any or all inheritable family characteristics or
peculiarities by double inheritance. The degree of intensification
probably varies with the nature of the characteristic; degenerate
conditions of the mind, and of the delicate organs of special sense
being the most strongly intensified.
It is probable also that in the absence of degenerative tendencies
the higher qualities of mind and body are similarly intensified by
marriage between highly endowed members of the same family. Dr.
Reibmayr believes that inbreeding is necessary to the higher evolution
of the race: "A settled abode, natural protection from race mixture
and the development of a closely inbred social class are the basic
conditions of every culture period." But inbreeding must not be
carried too far: "In the course of generations the ruling class begins
to degenerate mentally and physically, until not only is the class
destroyed, but for lack of capable leadership the people (Volk) itself
is subjugated and a crossing of blood again takes place."[95]
[Footnote 95: Trans. from _Insucht und Vermischung beim Menschen_, p.
46.]
In the breeding of animals the closest inbreeding is frequently
resorted to in order to improve the stock, and many examples can be
given of the closest possible inbreeding for generations without
apparent detriment, but it is universally admitted that the animals
selected for such inbreeding must be sound constitutionally, and free
from disease. After a certain number of generations however,
degeneration apparently sets in. The number of generations through
which inbreeding may be carried varies with the species, and the
purpose for which the animals are bred. Where they are bred primarily
for their flesh, as for beef, mutton or pork, it can be pursued
farther and closer than where they are bred for achievement in which a
special strength is required--for instance in the breeding of race
horses. This would indicate that the more delicate brain and nervous
system is sooner affected than the lower bodily functions.
In man, however, freedom from hereditary taint cannot so easily be
secured. Individuals cannot be selected scientifically for breeding
purposes. Furthermore, the human body is more delicately constructed
than that of the lower animals, and the nervous system is more highly
developed and specialized, so that it is reasonable to suppose that in
man degeneration would set in earlier in the process of inbreeding,
and that it would be impossible to breed as closely as with the lower
animals. Instances are well known, however, where incestuous unions
have been productive of healthy offspring, and successive generations
of offspring of incestuous connection are not unknown; but, although
statistics are lacking, it seems to be very often true that children
of such unions are degenerate. It may be that the reason for this is
that with the laws and social sentiments now prevailing in all
civilized communities, only degenerates ever contract incestuous
alliances. Desirable as it may be from a social point of view that
this strong sentiment against incest should continue, it is not yet
_proven_ that even the closest blood relationship between the parents
is directly injurious to the offspring. The "instinctive horror of
incest" is a myth, for although a horror of incest does very properly
exist in civilized, and in some tribal societies, it is purely a
matter of custom and education, and not at all a universal law.
Double heredity may account for all the observed ill effects of
consanguineous marriage, including the high youthful death-rate, the
higher percentage of idiocy, deafness and blindness, and probably also
the scrofulous and other degenerate tendencies; nevertheless, there
may be in some instances a lowering of vitality which this hypothesis
does not fully explain.
The tendency of inbreeding in animals, it is well known, is to fix the
type, the tendency of crossing, to variation. Inbreeding then, tends
to become simple repetition with no natural variations in any
direction, a stagnation which in itself would indicate a comparatively
low vitality. Variation and consequent selection is necessary to
progress. "Sex," according to Ward[96] "is a device for keeping up a
difference of potential," and its object is not primarily
reproduction, but variation.[97]
[Footnote 96: _Pure Sociology_, p. 232.]
[Footnote 97: Pearson (_Grammar of Science_, p. 373) points out that
variation does occur in asexual reproduction. But that sex is at least
a powerful stimulus to variation can hardly be questioned.]
It is organic differentiation, higher life, progress,
evolution.... But difference of potential is a social as well
as a physiological and physical principle, and perhaps we shall
find the easiest transition from the physiological to the
social in viewing the deteriorating effects of close inbreeding
from the standpoint of the environment instead of from that of
the organism. A long-continued uniform environment is more
deteriorating than similarity of blood. Persons who remain for
their whole lives, and their descendants after them, in the
same spot, surrounded by precisely the same conditions, and
intermarry with others doing the same, and who continue this
for a series of generations, deteriorate mentally at least, and
probably also physically, although there may not be any mixing
of blood. Their whole lives, physical, mental, and moral,
become fixed and monotonous, and the partners chosen for
continuing the race have nothing new to add to each other's
stock. There is no variation of the social monotony, and the
result is socially the same as close consanguineal
interbreeding. On the other hand, a case in which a man should,
without knowing it, marry his own sister, after they had been
long separated and living under widely different skies, would
probably entail no special deterioration, and their different
conditions of life would have produced practically the same
effect as if they were not related.[98]
[Footnote 98: Ward, op. cit., pp. 234-235.]
Professor Ward's idea of "difference of potential," or contrast, as
essential to the highest vigor of the race as well as to that of the
individual offspring, offers an alternative explanation of the
observed results of consanguineous marriages, and one which does not
necessarily conflict with the explanation already given. All the
phenomena of intensification are simply due to a resemblance between
husband and wife in particular characteristics, such as a common
tendency toward deafness or toward mental weakness. This resemblance,
which may or may not be the result of a common descent, renders more
probable the appearance of the trait in the offspring. If the parents
closely resembled each other in many respects they would be more
likely to "breed true" and the children would resemble one another in
their inherited traits, thus accounting for the high average of
deaf-mutes to the family, observed in the Irish statistics.[99]
[Footnote 99: _Cf. supra_, p. 66.]
The theory of contrast and resemblance supplements that of intensified
heredity where the resemblance is general, rather than in particular
traits or characteristics. In such a case the absence of the
stimulating effects of contrast might result in a lowering of
vitality, which in turn would react upon the youthful death-rate.
Where then related persons differ greatly in mental and physical
traits, and generally speaking, belong to different types, it is very
improbable that there would be any ill effects resulting from the mere
fact of consanguinity. A case in point is furnished me by a
correspondent. A first cousin marriage which turned out exceedingly
well was between strongly contrasted individuals; the husband was
"short, stocky and dark complexioned" while the wife was "tall, slight
of figure, and of exceedingly light complexion." In other cases in
which the results were not so good the husband and wife bore a close
resemblance to one another, physically and mentally.
This, however, does not agree with the results obtained by Professor
Karl Pearson. Basing his conclusions on the correlation of stature
between husband and wife, he believes that homogamy is a factor of
fertility. Taking 205 marriages from Mr. Francis Galton's _Family
Records_, Professor Pearson found the correlation between husband and
wife to be .0931 +- .0467, while weighted by their fertility the
correlation was .1783 +- .0210, practically doubling the intensity of
assortative mating.[100] The value of these correlations, however, is
impaired, as he says, by the insufficient number of observations, and
by the fact that absolutely taller mothers are the more fertile.
[Footnote 100: _Royal Society Proceedings_, vol. 66, p. 30.]
In a subsequent investigation of from 1000 to 1050 pairs of parents of
adult children, Professor Pearson found the correlation in stature to
be .2804 +- .0189; of span .1989 +- .0204; and of forearm .1977 +- .0205;
with cross coefficients varying from .1403 to .2023. If, as he
believes, "The parents of adult children are on the average more alike
than first cousins, then it follows that any evils which may flow from
first cousin marriage depend not on likeness of characters, but on
sameness of stock."[101]
[Footnote 101: _Biometrika_, vol. ii, p. 373.]
But even if it were true, as is very improbable, that parents of adult
children are more alike than first cousins, it would still be likely
to follow that first cousins who married would be more alike than
first cousins in general. A certain degree of resemblance is
undoubtedly necessary to complete fertility: husband and wife must be
physically compatible, and must both enjoy a certain degree of health
and physical strength. These facts are admitted by all, but it does
not follow that resemblance beyond a certain point is not in itself
detrimental.
Professor Pearson's own experiments in this line, however, do not
give consistent results, for in correlating eyecolor with fertility,
heterogamy seems to increase fertility. The highest average fertility
(4.57) is in those cases where the father is dark-eyed and the mother
light-eyed, while the lowest is where both parents have blue-green or
gray eyes.[102]
[Footnote 102: _Phil. Trans. of the Royal Society_, vol. 195 A, p.
150.]
In a recent study an attempt has been made to measure the coefficient
of correlation between cousins.[103] In the characteristics of health,
success, temper and intelligence the coefficients ranged between .25
and .30. These values differ but little from those found to obtain for
the resemblance between avuncular relatives for eye color (.265), or
between grandparent and grandchild for the same characteristic
(.3164).[104] Positive results were also found, with one doubtful
exception, for the occurrence of insanity and tuberculosis in cousins.
The writer concludes: "The grandparent, the uncle and aunt, and the
cousin are on practically the same footing with regard to relationship
or intensity of kinship as measured by degree of likeness of
character; and it seems probable that any scientific marriage
enactments would equally allow or equally forbid marriage between
grandparent and grandchild, uncle and niece, aunt and nephew, and
between first cousins."[105]
[Footnote 103: Elderton and Pearson, "On the Measure of the
Resemblance of First Cousins." _Eugenics Laboratory Memoirs IV._
Reviewed in _Br. Med. Journal_, Feb. 15, 1908.]
[Footnote 104: _Phil. Trans. of the Royal Society_, vol. 195 A, p.
106.]
[Footnote 105: Elderton and Pearson, op. cit.]
As we should expect the resemblance between near relatives has been
found to be much greater. From a measurement of from 4000 to 4886
pairs, the average correlation of the characteristics of stature,
span, forearm length and eyecolor between parent and child was .4695.
By similar computations and measuring the same characteristics, the
fraternal correlation was found to be .508.[106] From measurements of
a greater variety of characteristics in school children the mean
fraternal correlation was .539.[107] In athletic power the coefficient
was still higher, .72 between brothers, .75 between sisters and
.49 between brothers and sisters. Measurements of mental
characteristics--vivacity, assertiveness, introspection, popularity,
conscientiousness, temper, ability and handwriting proved to be as
easily correlated, the mean coefficients being; brothers, .52,
sisters .51, brothers and sisters .52.[108]
[Footnote 106: Pearson and Lee, "On the Laws of Inheritance in Man,"
_Biometrika_, vol. ii, p. 387.]
[Footnote 107: Ibid., p. 388.]
[Footnote 108: Pearson, "On the Laws of Inheritance in Man," part 2,
_Biometrika_, vol. iii, p. 154.]
The relative amount of degeneracy and disease among the offspring of
consanguineous marriages has been enormously exaggerated, and the
danger is by no means as great as is popularly supposed. Nevertheless,
since it is undoubtedly true that on the average such marriages do not
produce quite as healthy offspring as do non-consanguineous unions,
and since public sentiment is already opposed to the marriage of
cousins, it is perhaps just as well that existing laws on the subject
should remain in force. From the standpoint of eugenics however, it is
much more important that the marriage of persons affected with
hereditary disease should be prevented. Dr. Bell has pointed out the
danger of producing a deaf-mute race by the intermarriage of
congenitally deaf persons,[109] and this warning should be made to
apply to other congenital defects as well. Some states already
prohibit the marriage of the mentally defective, and persons under the
influence of intoxicants. Such provisions are wise, and are the most
practical means of achieving eugenic ideals--by preventing the
propagation of the unfit. The interests of society demand that the
mentally and physically defective should not propagate their kind.
[Footnote 109: "Memoir upon the Formation of a Deaf Variety of the
Human Race." _Memoirs of the National Academy of Sciences_, vol. ii,
pp. 177-262.]
From the broader viewpoint of social evolution the problems of
inbreeding or crossing of stocks merge into the discussion of the
endogamous and exogamous types of society. Whatever may have been the
origin of exogamy, the survival of the exogamous type in progressive
societies may easily be explained on the ground of superior
adaptability, variability and plasticity, which enables such societies
to survive a change of environment while the more rigid structure of
the endogamous clan brings about its extermination.
Inbreeding leads to caste formation and a rigid and stratified social
structure, which is in the end self-destructive, and cannot survive a
change of environment. The governing caste may, as Reibmayr says,
favor the growth of culture, but it is usually the culture of that
caste, and not of the people at large. The ruling caste is usually the
result of selection of the strongest and ablest, but after it becomes
a caste, the individuals are selected on account of hereditary social
position and not primarily on account of ability. Now biological
experiments show that although artificial selection may be carried to
a point where animals will breed true to a characteristic to within 90
per cent, yet if selection is stopped, and the descendants of the
selected individuals are allowed to breed freely among themselves,
they will in a very few generations revert to the original type. This
is what happens in a social caste, unless, as in the case of the
English aristocracy, it is continually renewed by selection of the
ablest of the other classes.
The superposition and crossing of cultures, the development of
secondary civilization, is necessary to social evolution in its
broadest sense, and this usually involves crossing of blood as well as
crossing of cultures. As a result of the unprecedented migrations of
the last half-century we have in the United States the greatest
variety of social types ever brought so closely together. An
opportunity is offered either for the perpetuation of each racial type
by inbreeding, with the prospect of an indefinite stratification of
society, or for the amalgamation of all cultural and racial elements
into a homogeneous whole, and the development of a race more versatile
and adaptable than any the world has yet known. The general tendency
will undoubtedly be toward amalgamation, but there are decided
tendencies in the other direction, as for instance in the "first
families of Virginia," and in that large element of the New England
population which prides itself upon its exclusively Puritan ancestry,
and which has inherited from its progenitors that intolerance which
characterized the early settlers of New England more than the pioneers
of the other colonies. The dynamic forces of modern civilization are,
however, opposed to caste--the West has long ago obliterated the
distinction between the Pennsylvania German and the Puritan, the
Scotch-Irish and the Knickerbocker Dutch. These same dynamic forces,
which have prevented the formation of caste have at the same time been
diminishing the percentage of consanguineous marriage and will
undoubtedly continue to operate in the same way for some time to come.
And when rational laws prohibit the marriage of the diseased and the
degenerate, the problem of consanguineous marriage will cease to be of
vital importance.
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GENEALOGIES CONSULTED.
* * * * *
Family. Author. Place and Date of Publication.
-----------------------------------------------------------------
Augur, E.P. Augur, Middletown, Conn. 1904.
Banta, T.M. Banta, New York, 1893.
Bent, A.H. Bent, Boston, 1900.
Bolton, H.C. and R.P. Bolton, New York, 1895.
Champion, F.B. Trowbridge, New Haven, 1891.
Dewey, L.M. Dewey, Westfield, Mass., 1898.
Faxon, G.L. Faxon, Springfield, Mass., 1880.
Foster, F.C. Pierce, Chicago, 1899.
Gates, C.O. Gates, New York, 1898.
Giddings, M.S. Giddings, Hartford, 1882.
Goodwin, J.J. Goodwin, Hartford, 1891.
Hurlbut, H.H. Hurlbut, Albany, 1888.
Kneeland, S.F. Kneeland, New York, 1897.
Lee, E.J. Lee, Philadelphia, 1895.
Mather, H.E. Mather, Hartford, 1890.
Mead, S.P. Mead, New York, 1901.
Potts, T.M. Potts, Canonsburg, Pa., 1901.
Shattuck, L. Shattuck Boston, 1855.
Tenney, M.J. Tenney, Boston, 1891.
Udall, G.B.L. Arner, In _Genealogical Exchange_,
Buffalo 1904-5.
Varnum, J.M. Varnum, Boston, 1907.
Wood, C.W. Holmes, Elmira, 1901.
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