varying one) to the subject. Commonly, the names of any two classes (or,
popularly, the classes themselves), one of which includes all the other
and more, are called respectively _genus_ and _species_. But the
Aristotelians, i.e. the schoolmen, meant by _differences in kind_
(_genere_ or _specie_) something which was in its nature (and not merely
with reference to the connotation of the name) distinct from
_differences_ in the _accidents_. Now, it is the fact that, though a
fresh class may be founded on the smallest distinction in attributes,
yet that some classes have, to separate them from other classes, no
common attributes except those connoted by the name, while others have
innumerable common qualities (from which we have to select a few samples
for connotation) not referrible to a common source. The ends of language
and of classification would be subverted if the latter (not if the
former) sorts of _difference_ were disregarded. Now, it was these only
that the Aristotelians called _kinds_ (_genera_ or _species_), holding
_differences_ made up of _certain_ and _definite_ properties to be
_differences_ in the _accidents_ of things. In conformity with this
distinction--and it is a true one--any class, e.g. negro as opposed to
white man, may, according as physiology shall show the _differences_ to
be infinite or finite, be discovered to be a distinct _kind_ or
_species_ (though not according to the naturalist's construction of
_species_, as including all descended from the same stock), or merely a
subdivision of the _kind_ or _species_, Man. Among _kinds_, a _genus_ is
a class divisible into other _kinds_, though it may be itself a species
in reference to higher _genera_; that which is not so divisible, is an
individual's _proximate kind_ or _infima species_ (_species
praedicabilis_ and also _subjicibilis_), whose common properties must
include all the common properties of every other real _kind_ to which
the individual can be referred.

The Aristotelians said that the _differentia_ must be of the _essence_
of the subject. They vaguely understood, indeed, by the _essence_ of a
thing, that which makes it the _kind_ of thing that it is. But, as a
_kind_ is such from innumerable qualities not flowing from a common
source, logicians selected the qualities which make the thing be what it
is called, and termed these the essence, not merely of the _species_,
but, in the case of the _infima species_, of the individual also. H