Again it is not permissible to break up a natural
group, Birds for instance, by putting its members under different
bifurcations, as is done in the published dichotomies, where some
birds are ranked with animals of the water, and others placed in a
different class. The group Birds and the group Fishes happen to be
named, while other natural groups have no popular names; for instance,
the groups that we may call Sanguineous and Bloodless are not known
popularly by any designations. If such natural groups are not to be
broken up, the method of Dichotomy cannot be employed, for it
necessarily involves such breaking up and dislocation. The group of
the Many-footed, for instance, would, under this method, have to be
dismembered, and some of its kinds distributed among land animals,
others among water animals.
3
Again, privative terms inevitably form one branch of dichotomous
division, as we see in the proposed dichotomies. But privative terms
in their character of privatives admit of no subdivision. For there
can be no specific forms of a negation, of Featherless for instance or
of Footless, as there are of Feathered and of Footed. Yet a generic
differentia must be subdivisible; for otherwise what is there that
makes it generic rather than specific? There are to be found
generic, that is specifically subdivisible, differentiae; Feathered
for instance and Footed.
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