V. CLASSIFICATION
Animals were traditionally grouped into Birds,
Beasts, and Fishes, air,
land, and sea creatures, and
some of them into subgroups like
cloven-hoofed,
horned, carnivorous. Plants were grouped as trees,
shrubs, fruiting, leguminous, and others that Theoph-
rastus reports. Popular classification did not go far,
and
was unsystematic in that some classes cut across each
other.
Plato developed in several dialogues the logical
technique of
“Division,” whereby a large concept is
analyzed into
its varieties—for example, the concept
“living on
land” is divisible into walking and flying,
and walking is
further divisible into quadruped, biped,
and polypod. With it he combined
the complementary
technique of “Collection,” whereby
data are sorted
into groups by common concepts, and these groups are
further grouped under more general concepts; for ex-
ample, biped and quadruped share the concept walk-
ing, and walking, flying, and swimming share the con-
cept locomotion. Collection is the inductive
process
which Division follows and confirms deductively.
Plato's aim
was to “track down” and identify concepts
by showing
their relationships in a Division.
Plato intended his method to reveal the actual divi-
sions in nature, but when applied to animal characters
it split
up natural groups. Aristotle therefore took as
initial data not characters
but animal types (which have
multiple characters). Class the species by
likeness, then
class the classes by more general likenesses, until you
arrive at the class Animal itself. Then reconsider what
are the most
significant distinguishing characters, and
re-divide the classes
accordingly down to the individual
species again. You now have an orderly
hierarchy of
genera and species, enabling you to define species by
group-characters, to classify new species as they ap-
pear, and to predict characters not yet observed. This
is the same conception that Ray and Linnaeus success-
fully developed, but Aristotle
lacked enough data to
make it workable. He also handicapped it
unnecessarily
by laying down that within a genus the member-species
must have characters differing only in degree (for ex-
ample, wings of various shapes), whereas between
genera the
comparison must be one of “analogy” (as
wing to fin).
He began with a division of Animal into
“blooded”
(that is, red-blooded) and “bloodless,” and
divided
these into (1) man, viviparous quadrupeds,
oviparous quadrupeds, cetaceans,
birds, fishes and (2)
crustaceans, testaceans, mollusks, insects. But he
points
out that this scheme omits some types (snakes, sponges),
and
that below this level the natural subgroups cut
across each other: for
example, the classes solid-hoofed,
hornless, ruminant are defeated by the
“overlapping”
types pig and camel.
He therefore preferred to arrange the species in an
order of
“perfection.” This too was inspired by Plato,
who had
set out a rough scala naturae running down-
wards from man, through bipeds,
quadrupeds,
polypods, serpents, to fishes; his indices were intelli-
gence and posture. Aristotle took
vital heat as his index
of perfection, as shown not only by intelligence
and
posture but also by respiration, method of repro-
duction, and state at birth. This method better
suited
his view that nature is continuous from plant to animal
(the
testaceans and sponges being in both categories).
In his actual practice he
often argues from this order
of perfection, but makes almost no use of
genus-species
classification.
