422 Marc Ereshefsky
promote a ‘rankless taxonomy.’ They offer a couple of alternative methods for
indicating the hierarchical relations among taxa. Here we will consider just one
suggested method.
According to Hennig [1969], Linnaean ranks should be replaced with positional
numbers. Consider a standard Linnaean classification.
Subclass Reptilomorpha
Infraclass Aves
Infraclass Mammalia
Division Monotremata
Division Theria
Cohort Metaheria
Cohort Eutheria.A classification of the same taxa using Hennig’s positional numbers is the following:
2.4. Reptilomorpha
2.4.1. Aves
2.4.2. Mammalia
2.4.2.1 Monotremata
2.4.2.2. Theria
2.4.2.2.1. Metaheria
2.4.2.2.2. Eutheria.Positional numbers indicate a couple of things. First, positional numbers in-
dicate hierarchical relations among taxa. Eurtheria is a part of Mammalia and
this is shown by Eurtheria’s positional number containing the positional number
of the more inclusive Mammalia. Second, positional numbers indicate the degree
of inclusiveness of a taxon: the fewer the digits in a taxon’s positional number,
the more inclusive the taxon.
Hennig’s positional number system overcomes the problems associated with Lin-
naean ranks and names. Recall that a problem with the Linnaean ranks is the
assumption that taxa of the same Linnaean rank are comparable across classifi-
cations (Section 4.1). Positional numbers carry no such assumption; positional
numbers are merely notational devices to indicate the hierarchical relations of
taxa within specific classifications. Positional numbers have no meaning outside
of particular classifications. Consequently, no suspect ontological categories are
associated with positional numbers, as is the case with the Linnaean ranks.
Another feature of Hennig’s positional number system is that it avoids the
naming problems facing the Linnaean rules of nomenclature. Recall that under
the Linnaean rules, a taxon’s name is not merely a name but also a device for
indicating a taxon’s classification. Taxon names in Hennig’s system do not play
that dual role: taxon names are merely names; positional numbers perform the
task of indicating a taxon’s placement in a classification. In Hennig’s system, the
activity of naming taxa is divorced from the activity of classifying taxa. By keeping