This monograph does not claim to treat exhaustively, nor to offer a
final solution of all the problems which have been connected with the
marriage of kin. The time has not yet come for a final work on the
subject, for the systematic collection of the necessary statistics,
which can only be done by governmental authority, has never been
attempted. The statistics which have been gathered, and which are
presented in the following pages, are fragmentary, and usually bear
upon single phases of the subject, but taken together they enable us
better to understand many points which have long been in dispute.

The need for statistics of the frequency of occurrence of
consanguineous marriages has been strongly felt by many far-sighted
men. G.H. Darwin and A.H. Huth have tried unsuccessfully to have the
subject investigated by the British Census, and Dr. A.G. Bell has
recently urged that the United States Census make such an
investigation.[1] Another motive for undertaking this present work,
aside from the desire to study the problems already referred to, has
been to test the widely prevalent theory that consanguinity is a
factor in the determination of sex, the sole basis of which seems to
be the Prussian birth statistics of Düsing, which are open to other
interpretations.

[Footnote 1: Cf. Bell, "A Few Thoughts Concerning Eugenics." In
_National Geographic Magazine_, March, 1908.]

The stock illustrations from isolated communities have been omitted as
too difficult to verify, and little space has been given to the
results of the inbreeding of domestic animals, for although such
results are of great value to Biology, they are not necessarily