Each tribe had thus its central column of descent; and the more remote the
progenitor, the more numerous the tribes ranging under his name. This central
column was termed by traditionists, "The Genealogical Tree," Amûd
al nasab): and with this stem, every clan of the race supposed to spring
from the common patriarch was connected, by assigning its descent from some of
the successive progenitors; the common appellation of the group of sub-tribes
thus affiliated together being generally assumed as the name of such
progenitor. It became necessary, therefore, to provide that the number of
links in the tree of a sub-tribe reaching up to the progenitor under whom it
branched off from the main tribe, should correspond with the number of links
in the parent stem. For example, as there are eighteen generations between
Mahomet and Modhar, it follows that in the family tree of the Beni Suleim
descended from Aylân son of Modhar, there must be seventeen links. These
removes are termed Cúdûd (close relations) in the technical language
of the genealogists; and as they were drawn out merely to square with a
theory, so they were no doubt filled up generally in an arbitrary manner. If
real names were not forthcoming for a gap, names were invented, and so the
synchronism maintained.