ses, enclosing walls, a roof, a means
of exit and entrance. For the full concept of the natural kinds,
_men_, _dogs_, _mice_, etc., we should have to go to the natural
historian.

DEGREES OF GENERALITY. One class is said to be of higher generality
than another when it includes that other and more. Thus animal
includes man, dog, horse, etc.; man includes Aryan, Semite, etc.;
Aryan includes Hindoo, Teuton, Celt, etc.

The technical names for higher and lower classes are GENUS and
SPECIES. These terms are not fixed as in Natural History to certain
grades, but are purely relative one to another, and movable up and
down a scale of generality. A class may be a species relatively to one
class, which is above it, and a genus relatively to one below it. Thus
Aryan is a species of the genus man, Teuton a species of the genus
Aryan.

In the graded divisions of Natural History genus and species are fixed
names for certain grades. Thus: Vertebrates form a "division"; the
next subdivision, _e.g._, Mammals, Birds, Reptiles, etc., is called a
"class"; the next, _e.g_., Rodents, Carnivora, Ruminants, an "order";
the next, _e.g._, Rats, Squirrels, Beavers, a "genus"; the next,
_e.g._, Brown rats, Mice, a "species".

Vertebrates (division).
|
Mammals, Birds, Reptiles, etc. (class).
|
Rodents, Ruminants, Carnivors, etc. (order).
|
Rats, Squirrels, Beavers, etc. (genus).
|
Brown rats, Mice, etc. (species).

If we subdivide a large class into smaller classes, and, again,
subdivide these subdivisions, we come at last to single objects.

Men
|
------------
Europeans, Asiatics, etc.
|
-------------
Englishmen, Frenchmen, etc.
|
---------------------
John Doe, Richard Roe, etc.

A table of higher and lower classes arranged in order has been known
from of old as a _tree_ of division or classification. The following
is Porphyry's "tree":--

Being
/ \
Corporeal Incorporeal
\ /
(Body)
/ \
Animate Inanimate
\ /
(Living Being)
/ \
Sensible Insensible
\ /
(Animal)
/ \
Rational Irrational
\ /
(Man)
|