Mineral--
Nonmetallic--
Glass.
Metallic--
Aluminum.
Zinc.
Nonmineral--
Vegetable--
Rubber.
Animal--
Ivory.

However, it is evident that indention carried to its full extant, useful
as it is in keeping analogous things together, would make the printing
of schedules complex and unwieldy. Nevertheless, in the generalizing
process necessary in logical division and arrangement, the divisions of
species should always be _mentally indented_, as it were, under their
_proximate_ genera. Thus, under a genus unnamed may be arranged several
species in juxtaposition, without actually printing the name of the
genus, so that the schedule above may read:

Balls--
Glass.
Aluminum
Zinc.
Rubber.
Ivory.

In an arrangement printed in idea-order, though relegating the genera
mineral, nonmetallic, metallic, nonmineral, vegetable, animal, to the
mind unaided by printed words, the different species of the same genus
may be kept together except that species for which no title has been
provided must go back to the subclass under which the named species are
indented. Thus the arrangement above necessitates placing in subdivision
"Balls" all _copper_ balls, whereas indention under proximate genus
"metal" would have brought all metal balls together. In a finely divided
classification, printing of titles for all genera is not practicable;
hence great care ought to be directed toward grouping species according
to the principles of arrangement herein outlined, noting that whenever a
change of basis is made, a new genus is implied, and that subclasses for
all other species of the same genus under whatever name, must be brought
into juxtaposition as if indented under the implied genus.[10]

_Bifurcate division._--Most discussions of classification make reference
to the so-called bifurcate scheme of division as the only one by which
exhaustive division can be surely achieved. This is commonly illustrated
by the ancient tree of Porphyry. By this method any subject it is
desired to subdivide is first divided by writing the name of one
selected species at one branch and writing at the other branch the name
of the same species prefixed by "Not." Thus the Agassiz classification
of living beings divides them first into sensible and not sensible
(plants). A botanical classification divides plants into flowering and
not flowering. A zoological classification divides anima