There may have been a time in the past when most women were mothers
in their early teens and bore most or all of their children before
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.
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