thirty, and when men had done the greater part of their procreation
before thirty-five; this is still the case in many tropical climates,
and I do not think I favour my case unduly by assuming that the average
parent must be about, or even less than, five and twenty. This gives
four generations to a century. At that rate and DISREGARDING
INTERMARRIAGE OF RELATIONS the ancestors living a thousand years ago
needed to account for a living person would be double the estimated
population of the world. But it is obvious that if a person sprang from
a marriage of first cousins, the eight ancestors of the third generation
are cut down to six; if of cousins at the next stage, to fourteen in the
fourth. And every time that a common pair of ancestors appears in any
generation, the number of ancestors in that generation must be reduced
by two from our original figures, or if it is only one common ancestor,
by one, and as we go back that reduction will have to be doubled,
quadrupled and so on. I daresay that by the time anyone gets to the 8916
names of his Elizabethan ancestors he will find quite a large number
repeated over and over again in the list and that he is cut down to
perhaps two or three thousand separate persons. But this does not
effectually invalidate my assumption that if we go back only to the
closing years of the Roman Republic, we go back to an age in which
nearly every person living within the confines of what was then the
Roman Empire who left living offspring must have been ancestral to every
person living within that area to-day. No doubt they were so in very
variable measure. There must be for everyone some few individuals in
that period who have so to speak intermarried with themselves again and
again and again down the genealogical series, and others who are
represented by just one touch of their blood. The blood of the Jews, for
example, has turned in upon itself again and again; but for all we know
one Italian proselyte in the first year of the Christian era may have
made by this time every Jew alive a descendant of some unrecorded