RATIO OF THE CONSANGUINEOUS TO ALL MARRIAGES Previous Estimates--Mayo-Smith--Mulhall--Darwin--Application of Darwin's Method to American Data--Direct Method--Consanguineal Attraction--Same-name and Different-name Cousin Marriages--Summary CHAPTER III MASCULINITY Constancy of the Sex-ratio--Consanguinity and Masculinity--Theory of Westermarck and Thomas--Duesing--Gache--Negroes in the United States--Genealogical Material--Other Compilations--Summary CHAPTER IV CONSANGUINITY AND REPRODUCTION Theories of the Effect of Consanguinity upon Offspring--Comparative Fertility--Statistics from Darwin and Bemiss--Genealogical Statistics--Youthful Death-rate--Degeneracy--Fallacies in the Work of Bemiss--Isolated Communities--_The Jukes_--Other Degenerate Families--Scrofula CHAPTER V CONSANGUINITY AND MENTAL DEFECT Idiocy and Insanity--Inheritability of Mental Defect--Intensified Heredity--Barr's Investigations--Other American and English Data--Mayet's Prussian Statistics--Genealogical Data CHAPTER VI CONSANGUINITY AND THE SPECIAL SENSES United States Census Data--The Blind--Consanguinity of Parents--Blind Relatives--Degree 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.