ee of Blindness--Causes of Blindness--Retinitis
Pigmentosa--European Data--Probability of Blind Offspring of
Consanguineous Marriages--The Deaf--Irish Census--Scotland and
Norway--United States Census--Consanguinity of Parents--Deaf
Relatives--Causes of Deafness--Degree of Deafness--Direct Inheritance
of Deafness--Intensification through Consanguinity--Dr. Fay's
Statistics--Personal Data--Probability of Deaf Offspring from
Consanguineous Marriages--Opinion of Dr. Bell

CHAPTER VII

SUMMARY AND CONCLUSION

Summary of Results--Inbreeding and Evolution--Effects of Close
Inbreeding--Crossing and Variation--"Difference of
Potential"--Resemblance and Intensification--Coefficient of
Correlation between Husband and Wife--Between Cousins--Between
Brothers and Sisters--Consanguinity and Eugenics--Consanguinity and
Social Evolution--Conclusion

CHAPTER I

INTRODUCTION

The purpose of this essay is to present in a concise form and without
bias or prejudice, the most important facts in regard to
consanguineous marriages, their effects upon society, and more
particularly their bearing upon American social evolution. The
problems to be considered are not only those which relate primarily to
the individual and secondarily to the race, such as the supposed
effect of blood relationship in the parents upon the health and
condition of the offspring; but also the effect, if any, which such
marriages have upon the birth-rate, upon the proportion of the sexes
at birth, and the most fundamental problem of all, the relative
frequency with which consanguineous marriages take place in a given
community.

No thorough and systematic study of the subject has ever been made,
and could not be made except through the agency of the census. The
statistical material here brought together is fragmentary and not
entirely satisfactory, but it is sufficient upon which to base some
generalizations of scientific value. The sources of these data are
largely American. Little attempt is made to study European material,
or to discuss phases of the problem which are only of local concern.
Some topics, therefore, which have frequently been treated in
connection with the general subject of consanguineous marriages are
here ignored as having no scientific interest, as for instance that of
the so-called "marriages of affinity," which has been so warmly
debated for the past fifty years in the British Parliament.

For obvious reasons it will often b