The collection of data from personal sources is likewise open to grave objections.  Not only is the informant likely to be biassed, but the cases which he will remember will be those in which something unusual has occurred.  Herein lay the fallacy in the conclusions of Dr. Bemiss.  I have endeavored to overcome this bias by restricting my requests for information to genealogists and others who would more naturally appeal to records, but my efforts have been only partially successful.

The number of cases of consanguineous marriage, embracing all degrees of consanguinity, which I have collected from these two sources, genealogies and correspondence, is 723, a number too small in itself to establish any definite conclusions; but by using this material in connection with other related data, I trust I may be able to add something to the comparatively small amount of real knowledge which the world already possesses in regard to the marriage of kin.

In the course of my investigations I visited Smith’s Island, in the Chesapeake Bay, about twelve miles across Tangier Sound, from Crisfield, Maryland, and nearly opposite the mouth of the Potomac.  Here is a community of about seven hundred people, who are principally engaged in the sea-food industry.  Their ancestors have lived on the island for many generations and there have been comparatively few accessions to the population from the mainland.  As a natural consequence the population is largely a genetic aggregation.  Consanguineous marriages have been very frequent, until now nearly all are more or less interrelated.  Out of a hundred or more families of which I obtained some record, at least five marriages were between first cousins.  All of these were fertile, and all the children were living and apparently healthy.  Since over thirty per cent of the inhabitants bear one surname (Evans), and those bearing the first four surnames in point of frequency (Evans, Brad-shaw, Marsh, and Tyler) comprise about fifty-nine per cent of the population, it will readily be seen that comparatively few absolutely non-related marriages take place.  Yet in this community from September, 1904, to October, 1907, or during the residence there of the present physician, Dr. P.H.  Tawes, there have been 87 births and but 30 deaths, the latter from the usual causes.  During this period there has not been a single case of idiocy, insanity, epilepsy, deaf-mutism or even of typhoid fever on the island.

The evidence gathered from various other isolated communities is very conflicting.  Huth describes a great many of them which have existed for many generations without crosses without ill results.  Other writers quote instances where whole communities have become degenerate.  Until the antecedents of a community are known it is of course impossible to estimate the effect of consanguinity.  The exceptionally high percentage of deaf-mutism on Martha’s Vineyard may to some extent be due to a high percentage of consanguineous marriage, but that inbreeding is not the primary cause is revealed by the records showing that among the first settlers were two deaf-mutes, whose defect has been inherited from generation to generation for two hundred and fifty years.[13]