It is, natural for contributors to overlook many of the more fortunate results of family intermarriage, and furnish those followed by defective offspring and sterility.  The mere existence of either of these conditions would prompt inquiry, while the favorable cases might pass unnoticed.  Contributors have been particularly requested to furnish without prejudice or selection all instances of the marriage of consanguinity within their various circles of observation, whatever their results.[51]

[Footnote 51:  Bemiss. see Trans. of Am.  Med.  Asso., vol. xi, 1858, p. 323.]

Yet he does not seem to believe that this bias seriously affects his conclusions.

In order as far as possible to avoid this bias, I sent my own circulars to genealogists and others who would naturally be more interested in the relationships than in pathological conditions.  I asked, however, that all such results be noted.  Among 722 children of first cousins I found 95 or 13 per cent who were defective in the sense in which Bemiss used the term.  This is much nearer the actual percentage, but I have reason to believe, as will be seen hereafter, that even this percentage is far too high.  A good illustration of the unconscious bias, which I tried to avoid is afforded by the reports on the cause of death among children of first cousins.  Only 58 replies were given to this question, and of the 58 deaths 14 or one-fourth were either accidental or otherwise violent, while only one person was reported to have succumbed to pneumonia.

Many efforts have been made to investigate the occurrence of degeneracy in the offspring of consanguineous marriages, by studying communities in which such unions have been frequent, but the results are untrustworthy.  Huth[52] quotes a number of instances where communities have lived for generations without crosses and with no apparent degeneracy, while other writers tell of high percentages of degeneracy.  Smith’s Island, Maryland, as has been said, seems absolutely free from serious congenital abnormalities, in spite of the great frequency of consanguineous marriages.

[Footnote 52:  Marriage of Near Kin, chap. iv.]

The causes of degeneracy are so varied, complicated, and obscure that even if consanguinity is a cause, there can be but few cases in which it is not complicated by other factors.  But for the same reason that it is so difficult to prove any connection between consanguinity and degeneracy, it is equally difficult to disprove such a connection.  It is very probable that from the mere operation of the law of heredity, there must be a comparatively large percentage of degenerates among the offspring of related parents, for defects which tend to be bred out by crossing are accentuated by inbreeding.  This may be the reason for the disagreement among investigators of isolated communities.  If an island, for instance, were settled by a small group of families in even one of which some hereditary