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 environm