There are animals so like one another that they are given the same name.
Such animals are ranked in the same _species_. Animals which differ so
much that they have to be referred to different species, but which
notwithstanding agree in the majority of their characters, especially
the most important ones, are placed in the same _genus_. Hare and
rabbit, or horse and donkey, are reckoned as different species of the
same genus. Genera resembling one another are united into a _family_;
thus, the pine marten and the beech or stone marten both belong to the
_Marten genus_ (Martes), while the weasel and stoat are different
species of the _Weasel genus_ (Mustela); but these two genera are so
similar that they are both placed in the same family, _i.e._ the _Weasel
family_ (Mustelidæ). Nearly related families together build up an
_order_. Thus, the Weasel family, Dog family, Cat family, etc.,
collectively constitute the _order of Carnivora_, characterized,
speaking generally, by the same kind of teeth, claws, habits, and food.
Several related orders are united into a _class_. Thus, for example,
carnivorous animals (Carnivora), ruminating animals (Ruminantia),
gnawing animals (Rodentia), etc., constitute different orders of the
_class of Sucklers_ (Mammalia); while birds of prey (Raptores), pigeons
(Gyrantes), and poultry (Rasores), are included in a second class, that
of _Birds_ (Aves). But both Birds and Mammals have a skeleton, of which
the chief support is the backbone; on this account they are placed in a
larger subdivision, the _sub-kingdom_ of Backboned animals (Vertebrata);
while snails are grouped under the sub-kingdom of _Molluscs_, millipedes
and centipedes under that of _Jointed-limbed animals_ (Arthropods).
In this way the animal kingdom is divided into _sub-kingdoms_, the
sub-kingdoms into _classes_, the classes into _orders_, the orders into
_families_, the families into _genera_, and the genera into _species_.
Animals of the same species which differ from one another in more or
less constant characters, belong to different _races_ (domestic or
geographical races).
There are many species of animals the external features of which are
well known to ordinary folk, and which therefore possess a definite
English name, but a much larger number, of the smaller forms especially,
have no English name. It is, therefore, necessary to devise new names
for these species. The English names, however, are liable to cause great
confusion, since in different districts the same name is often applied
to widely different animals. Besides this, distinct names have usually
been given to successive stages in the life history of the same form:
“wireworms,” for example, are the young state of the “click beetle.”
By using the scientific method of naming invented by Linnæus, confusion
is made impossible. The Latin names of this naturalist have the great
advantage that they not only give a perfectly distinct name to any
particular species, but also at the same time show the genus to which it
belongs. Each kind of animal possesses, in fact, two names; just in the
same way as every person possesses at least two names, a Christian name
and a surname. The generic name comes first, and is, of course, common
to all animals of the same genus. The second name is the specific one,
and belongs exclusively to animals of the same species. The hare and
rabbit, for example, are both included in the genus _Lepus_. The Latin
name of the first is _Lepus timidus_; that of the second, _Lepus
cuniculus_. Horse = _Equus caballus_; ass = _Equus asinus_.