On The Parts Of Animals   

Horses, Oxen, Dogs in India, Pigs, Goats, Sheep; groups which, if
double, ought to have what they have not, namely, different
appellations; and which, if single, prove that Wildness and Tameness
do not amount to specific differences. And whatever single element we
take as a basis of division the same difficulty will occur.
The method then that we must adopt is to attempt to recognize the
natural groups, following the indications afforded by the instincts of
mankind, which led them for instance to form the class of Birds and
the class of Fishes, each of which groups combines a multitude of
differentiae, and is not defined by a single one as in dichotomy. The
method of dichotomy is either impossible (for it would put a single
group under different divisions or contrary groups under the same
division), or it only furnishes a single ultimate differentia for each
species, which either alone or with its series of antecedents has to
constitute the ultimate species.
If, again, a new differential character be introduced at any stage
into the division, the necessary result is that the continuity of the
division becomes merely a unity and continuity of agglomeration, like
the unity and continuity of a series of sentences coupled together by
conjunctive particles. For instance, suppose we have the bifurcation
Feathered and Featherless, and then divide Feathered into Wild and
Tame, or into White and Black. Tame and White are not a
differentiation of Feathered, but are the commencement of an
independent bifurcation, and are foreign to the series at the end of
which they are introduced.
As we said then, we must define at the outset by multiplicity of
differentiae. If we do so, privative terms will be available, which
are unavailable to the dichotomist.
The impossibility of reaching the definition of any of the ultimate
forms by dichotomy of the larger group, as some propose, is manifest
also from the following considerations. It is impossible that a single
differentia, either by itself or with its antecedents, shall express
the whole essence of a species. (In saying a single differentia by
itself I mean such an isolated differentia as Cleft-footed; in saying
a single differentia with antecedent I mean, to give an instance,
Manycleft-footed preceded by Cleft-footed. The very continuity of a
series of successive differentiae in a division is intended to show
that it is their combination that expresses the character of the
resulting unit, or ultimate group. But one is misled by the usages of
language into imagining that it is merely the final term of the
series, Manycleft-footed for instance, that constitutes the whole
differentia, and that the antecedent terms, Footed, Cleft-footed, are
superfluous. Now it is evident that such a series cannot consist of
many terms. For if one divides and subdivides, one soon reaches the
final differential term, but for all that will not have got to the
ultimate division, that is, to the species.) No single differentia, I
repeat, either by itself or with its antecedents, can possibly express
the essence of a species. Suppose, for example, Man to be the animal
to be defined; the single differentia will be Cleft-footed, either by
itself or with its antecedents, Footed and Two-footed. Now if man was
nothing more than a Cleft-footed animal, this single differentia would
duly represent his essence. But seeing that this is not the case, more
differentiae than this one will necessarily be required to define him;
and these cannot come under one division; for each single branch of a
dichotomy ends in a single differentia, and cannot possibly include
several differentiae belonging to one and the same animal.
It is impossible then to reach any of the ultimate animal forms by
dichotomous division.
Part 4
It deserves inquiry why a single name denoting a higher group was not
invented by mankind, as an appellation to comprehend the two groups of
Water animals and Winged animals. For even these have certain
attributes in common. However, the present nomenclature is just.
Groups that only differ in degree, and in the more or less of an